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Essential Pascal

1. 5 Acknowledgments tee 6 About the EU I e PERENNE QUERIT 6 Chapter 1 A Short History of the Pascal Language eeeeeeeereeeeeee nnne 7 Wirth s Past al ossi pdeptrek tossed uen EFa pep oquaaqU P REEF ENORMI E EPIBEN MM OD aU AERE V FRqu P LIBER ERERIERQUR 7 HUGE c v TC c M A 7 Delphi s le MM T rm 7 Chapter 2 Coding in Pascal 1 rasura dasn su iwiausurruv usu kv avEEruVFEn acc ExUyw EVER e aea FE VE uFE Un dd 9 uid TTE T 9 Use of DUDDELC USE cose patti da bobus Saa Teac uMtetatatapepboxR Rit un oStD CREAN RpRpFrDaMRAS CHE REEERE 10 Pretty PrintnNg TEE QU TU 11 Syntax HIGMIGHUNG sovesendite EET baee nb GER PRA Ub EU MUR UD a aea a n vin ida CR dna d a a aaaea 11 Using Code Templates assebebme iab bodi ERR ab M ppp PUN aa a RUP apa GPC pex mM oM CRN VE Da EPIS 12 Language irs irap rc em 13 REV s no c CL 13 Expressions and Operators uia Leice oni eru tip P Rd eub ean cosa KR C op Me RB Vega GRE Ra opNE RUD Pod iN Ed Uba dt 16 Bogsrc m 17 Chapter 3 Types Variables and Constants n eeeeeeeeeei eren nnne nnn 18 Wal IEE Co 18 ic ER RU qu PR 19 Resource Stri
2. list of exported functions and procedures Essential Pascal Copyright 1995 2003 Marco Cant www marcocantu com epascal 82 procedure MyProc implementation uses D E hidden global variable var PartialTotal Integer all the exported functions must be coded procedure MyProc begin code of procedure MyProc end initialization optional initialization part finalization optional clean up code end The uses clause at the beginning of the interface section indicates which other units we need to access in the interface portion of the unit This includes the units that define the data types we refer to in the definition of other data types such as the components used within a form we are defining The second uses clause at the beginning of the implementation section indicates more units we need to access only in the implementation code When you need to refer to other units from the code of the routines and methods you should add elements in this second uses clause instead of the first one All the units you refer to must be present in the project directory or in a directory of the search path you can set the search path for a project in the Directories Conditionals page of the project s Options dialog box C programmers should be aware that the uses statement does not correspond to an include directive The effect of a uses statement is to imp
3. Figure 6 1 THE OPENARR EXAMPLE WHEN THE PARTIAL SLICE BUTTON IS PRESSED Typed open arrays are fully compatible with dynamic arrays introduced in Delphi 4 and covered in Chapter 8 Dynamic arrays use the same syntax as open arrays with the difference that you can use a notation such as array of Integer to declare a variable not just to pass a parameter Essential Pascal Copyright 1995 2003 Marco Cant www marcocantu com epascal 51 Type Variant Open Array Parameters Besides these typed open arrays Delphi allows you to define type variant or untyped open arrays This special kind of array has an undefined number of values which can be handy for passing parameters Technically the construct array of const allows you to pass an array with an undefined number of elements of different types to a routine at once For example here is the definition of the Format function we ll see how to use this function in Chapter 7 covering strings function Format const Format string const Args array of const string The second parameter is an open array which gets an undefined number of values In fact you can call this function in the following ways N 20 S Total aabell Caption Format Total d N abel2 Caption Format Int d Float f N 12 4 abel3 Caption Format s d S N 2 Notice that you can pass a parameter as either a constant value the valu
4. function Min Double Double overload function Min Extended Extended overload When you call Min 10 20 the compiler easily determines that you re calling the first function of the group so the return value will be an Integer The basic rules are two Each version of the routine must be followed by the overload keyword The differences must be in the number or type of the parameters or both The return type instead cannot be used to distinguish among two routines Here are three overloaded versions of a ShowMsg procedure I ve added to the OverDef example an application demonstrating overloading and default parameters procedure ShowMsg str string overload begin MessageDlg str mtInformation mbOK 0 end A A procedure ShowMsg FormatStr string Params array of const overload begin MessageDlg Format FormatStr Params mtInformation mbOK 0 end procedure ShowMsg I Integer Str string overload begin ShowMsg IntToStr I Str end The three functions show a message box with a string after optionally formatting the string in different ways Here are the three calls of the program ShowMsg Hello ShowMsg Total d 100 ShowMsg 10 MBytes What surprised me in a positive way is that Delphi s Code Parameters technology works very nicely with overloaded procedures and functions As you type the open parenthesis after the Essential Pascal Cop
5. EWProc EnumWindowsProc begin ListBoxl Items Clear EWProc GetTitle EnumWindows EWProc 0 end could have called the function without storing the value in a temporary procedural type variable first but wanted to make clear what is going on in this example The effect of this program is actually quite interesting as you can see in Figure 9 2 The Callback example shows a list of all the existing main windows running in the system Most of them are hidden windows you usually never see and many actually have no caption Essential Pascal Copyright 1995 2003 Marco Cant www marcocantu com epascal 75 f Callback Function Call ioj Xx 2532 1544 Essential Pascal Microsoft Internet Explorer 832 2624 Open 1164 Debug 1160 384 This is a hidden form used to trap Dreamweaver mes 540 HomeSite 4 0 1316 1184 Replace 676 Allaire HomeSite 4 0 1 C WINDOWS Desktops Exploring C WINDOWS Desktop WebS ites marc FIGURE 9 2 THE OUTPUT OF THE CALLBACK EXAMPLE LISTING THE CURRENT MAIN WINDOWS VISIBLE AND HIDDEN A Minimal Windows Program To complete the coverage of Windows programming and the Pascal language want to show you a very simple but complete application built without using the VCL The program simply takes the command line parameter stored by the system in the cmdLine global variable and then extracts information from it with the ParamCount and ParamStr Pascal f
6. Marco Cantu Essential Pascal 2 7 Edition March 2003 version 2 01 APOLLO THE Gop WonsutrPED AT DELPHI IN AN ITALIAN 17TH CENTURY FRESCO Essential Pascal Copyright 1995 2003 Marco Cantu www marcocantu com epascal 1 I ntroduction he first few editions of Mastering Delphi the best selling Delphi book I ve written provided an introduction to the Pascal language in Delphi Due to space constraints and because many Delphi programmers look for more advanced information in the latest edition this material was completely omitted To overcome the absence of this information I ve started putting together this ebook titled Essential Pascal This is a detailed book on Pascal which for the moment will be available for free on my web site I really don t know what will happen next might even find a publisher This is a work in progress and any feedback is welcome The first complete version of this book dated July 99 has been published on the Delphi 5 Companion CD Note to the Second Edition After a few years in the early 2003 the book had a complete revision trying to refocus it even more on the core features of the Pascal language Alongside the book covers the language from the perspective of the Delphi for Windows programmer but also of the Kylix and Delphi for NET programmer Differences among these different versions of the language will be mentioned This change in focus not only Delphi with the VCL librar
7. character to indicate where the cursor should jump to after the operation that is where you start typing to complete the template with custom code Language Statements Once you have defined some identifiers you can use them in statements and in the expressions that are part of some statements Pascal offers several statements and expressions Let s look at keywords expressions and operators first Keywords Keywords are all the Object Pascal reserved identifiers which have a role in the language Delphi s help distinguishes between reserved words and directives Reserved words cannot be used as identifiers while directives should not be used as such even if the compiler will accept them In practice you should not use any keywords as an identifier In Table 2 1 you can see a complete list of the identifiers having a specific role in the Object Pascal language in Delphi 4 including keywords and other reserved words TaBLE 2 1 KEYWORDS AND OTHER RESERVED WORDS IN THE OBJECT PASCAL LANGUAGE ome edena abstract flrective method Essential Pascal Copyright 1995 2003 Marco Cant www marcocantu com epascal 12 assembler backward compatibility asm automated egin cded Es const constructor contains efault iv operator do atem O except rt y far backward compatibility class ile ype finally statement exceptions statement unction directive declaration Statement Statement
8. the Essential Pascal Copyright 1995 2003 Marco Cantu www marcocantu com epascal 63 memory location of the updated string will change This is the effect of the copy on write technique JF String References Oy Xx 512 2 Stri Str Address 12276540 Length 5 References 2 Value Hello Str2 Address 12276540 Lenath 5 References 2 Value Hello Str 2 a Stri Address 12276560 Length 5 References 1 Value Hallo Str2 Address 12276540 Length 5 References 1 Value Hello Figure 7 1 THE STRREF EXAMPLE SHOWS THE INTERNAL STATUS OF TWO STRINGS INCLUDING THE CURRENT REFERENCE COUNT We can actually produce this effect shown in the second part of the list box of Figure 7 1 by writing the following code for the OnClick event handler of the second button procedure TFormStrRef BtnChangeClick Sender TObject begin Strl 2 a ListBoxl Items Add Stri1 2 a ListBoxl Items Add Stri StringStatus Str1 ListBoxl Items Add Str2 StringStatus Str2 end Notice that the code of the BtnChangeClick method can be executed only after the BtnAssignClick method To enforce this the program starts with the second button disabled its Enabled property is set to False it enables the button at the end of the first method You can freely extend this example and use the StringStatus function to explore the behavior of long strings in many other circumstances Delphi
9. Borderlcons biSystemMenu biMinimize biM aximize biSystemMer True biMinimize True biMaximize Tre BorderStyle bsSizeable Another property based on a set type is the style of a font Possible values indicate a bold italic underline and strikethrough font Of course the same font can be both italic and bold have no attributes or have them all For this reason it is declared as a set You can assign values to this set in the code of a program as follows Font Style 1 no style Font Style fsBold bold style only Font Style fsBold fsItalic two styles You can also operate on a set in many different ways including adding two variables of the same set type or to be more precise computing the union of the two set variables Font Style OldStyle fsUnderline two sets Again you can use the OrdType examples included in the TOOLS directory of the book source code to see the list of possible values of many sets defined by the Delphi component library Array Types Array types define lists of a fixed number of elements of a specific type You generally use an index within square brackets to access to one of the elements of the array The square brackets are used also to specify the possible values of the index when the array is defined For example you can define a group of 24 integers with this code type DayTemperatures array 1 24 of Integer In the arra
10. Variants iade ecusees e ex inis uas UN eEV Yours aaia use Vus vXva vedi dV er OS Sea YE VeNVF ROME iiaeaa 76 Variants Have No TYPE viseri sitene er ii rab uL cet qua na cedes ee D Per eL Urea LEM ea EKARA ARTEEN ENSA 76 Variante no DepEDI cu rosa vs pet y sut ao D eo ce deris RR bd RE erba ooo COT EE NR A Re X dex aa sua 77 Varants Are SIW susanne RAE ia e E e AEAEE EE EERE EEES EA 78 Conclusione neeesa s e n EEE AEA EEE O EEEE AA E aide KUNKUNE DDE AAA AE ESEESE 79 Chapter 11 Program and Units 5 2 15 x imanan aana anana anaana 80 Wi c NR MEE 80 Unis and Scopes o eon ade e e tr e EXE DO EUR EE RU ORE NEN XE REX E DR E HR UE EC t 81 Units as Narmespaces crece er eer eee ex reni aa ke b a POR e ve x EU RR rr iC E ER 82 Units arid Progra MSs esee cierre rich aee dee Fe nae eere xxi nar e Wa e UE X XC dr e E 83 CONCTUSION D ee 83 Chapter 12 Files in the Pascal Language 1 leeeeeeeieeeee eene nnn 84 Routines for Working with PIGS sss oda S ox EYEE FE no Seir KEY RR SR EPEERME M a eer etd raa ERR a EEEE 84 Handling Text FIGS os osirius eaa QUE YLI Fahr ROT PERO Red sae e YI ERI ETSI ce Ud 85 A Text PIG SCONVENLCIE 4 ussecos YE p RR dip gst Fes p RO ER a Pee RM REPE ERES ne 86 SAVING Generic Datas sur veau esqbecou nte ong EESE sade FUA Med eque URN EROR Cote ds he da dal aca id Dd 87 Prom Files fo SETOSITIS occides procera i a dob bci E Medidor Da Fd Ga bii Ae NEN Mad de E aiie 88 COMCIUSI
11. such as DWORD or UINT many records or structures several pointer types and so on Among Windows data types the most important type is represented by handles discussed in Chapter 9 Typecasting and Type Conversions As we have seen you cannot assign a variable to another one of a different type In case you need to do this there are two choices The first choice is typecasting which uses a simple functional notation with the name of the destination data type var N Integer C Char B Boolean begin N Integer X C Char N B Boolean 0 Essential Pascal Copyright 1995 2003 Marco Cantu www marcocantu com epascal 26 You can typecast between data types having the same size It is usually safe to typecast between ordinal types or between real types but you can also typecast between pointer types and also objects as long as you know what you are doing Casting however is generally a dangerous programming practice because it allows you to access a value as if it represented something else Since the internal representations of data types generally do not match you risk hard to track errors For this reason you should generally avoid typecasting The second choice is to use a type conversion routine The routines for the various types of conversions are summarized in Table 3 4 Some of these routines work on the data types that we ll discuss in the following sections Notice that the table doesn t include
12. var UppLetter UpperCase begin UppLetter F But this one is not I var UppLetter UpperCase begin UppLetter e compile time error Writing the code above results in a compile time error Constant expression violates subrange bounds If you write the following code instead var UppLetter Uppercase Letter Char begin Letter e UppLetter Letter Delphi will compile it At run time if you have enabled the Range Checking compiler option in the Compiler page of the Project Options dialog box you ll get a Range check error message suggest that you turn on this compiler option while you are developing a program so it ll be more robust and easier to debug as in case of errors you ll get an explicit message and not an undetermined behavior You can eventually disable the option for the final build of the program to make it a little faster However the difference is really small and for this reason suggest you to leave all these run time checks turned on even in a shipping program The same holds true for other run time checking options such as overflow and stack checking Essential Pascal Copyright 1995 2003 Marco Cantu www marcocantu com epascal 30 Enumerated Types Enumerated types constitute another user defined ordinal type Instead of indicating a range of an existing type in an enumeration you list all of the possible values for the type In other words an enumerati
13. which is executed while the condition is true For this reason had to reverse the condition in the code above to obtain a similar statement An Example of Loops To explore the details of loops let s look at a small Delphi example The Loops program highlights the difference between a loop with a fixed counter and a loop with an almost random counter Start with a new blank project place a list box and two buttons on the main form and give the buttons a proper name BtnFor and BtnWhile by setting their Name property in the Object Inspector You can also remove the word Btn from the Caption property and eventually even add the amp character to it to activate the following letter as a shortcut key Here is a summary of the textual description of this form object Forml TForml Caption Loops object ListBoxl TListBox object BtnFor TButton Caption amp For OnClick BtnForClick end object BtnWhile TButton Caption While OnClick BtnWhileClick end end Figure 5 2 EACH TIME YOU PRESS THE FOR BUTTON OF THE LOOPS EXAMPLE THE LIST BOX IS FILLED WITH CONSECUTIVE NUMBERS i Loops Pul E3 Essential Pascal Copyright 1995 2003 Marco Cant www marcocantu com epascal 43 Now we can add some code to the OnClick events of the two buttons The first button has a simple for loop to display a list of numbers as you can see in Figure 5 2 Before executing this loop which adds a number of strings to the
14. you can easily change the code of the function but the calling code will remain unchanged although executing it will be faster The same principle can be applied to the Hello procedure We can modify the program output by changing the code of this procedure and the Button2Click method will automatically change its effect Here is how we can change the code procedure Hello begin MessageDlg Hello world mtInformation mbOK end When you call an existing Delphi function or procedure or any VCL method you should remember the number and type of the parameters Delphi editor helps you by suggesting the parameters list of a function or procedure with a fly by hint as soon as you type its name and the open parenthesis This feature is called Code Parameters and is part of the Code Insight technology Reference Parameters Pascal routines allow parameter passing by value and by reference Passing parameters by value is the default the value is copied on the stack and the routine uses and manipulates the copy not the original value Passing a parameter by reference means that its value is not copied onto the stack in the formal parameter of the routine avoiding a copy often means that the program executes faster Instead the program refers to the original value also in the code of the routine This allows the procedure or function to change the value of the parameter Parameter passing by reference is expressed by the var key
15. Essential Pascal in the subject and your request or comment in the text Acknowledgments If l m publishing a book on the web for free think this is mainly due to Bruce Eckel s experience with Thinking in J ava I m a friend of Bruce and think he really did a great job with that book and few others As mentioned the project to people at Borland got a lot of positive feedback as well And of course must thank the company for making first the Turbo Pascal series of compilers and now the Delphi series of visual IDEs I m starting to get some precious feedback The first readers who helped improving this material quite a lot are Charles Wood and Wyatt Wong Mark Greenhaw helped with some editing of the text Rafael Barranco Droege offered a lot of technical corrections and language editing Thanks About the Author Marco Cantu lives in Piacenza Italy After teaching the C language and writing C and Object Windows Library books and articles in 1995 he delved into Delphi programming He is the author of the Mastering Delphi book series published by Sybex as well as the advanced Delphi Developers Handbook which is hardly available any more Marco writes articles for many magazines including The Delphi Magazine speaks at Delphi and Borland conferences around the world and teaches Delphi classes at basic and advanced levels Lately he s getting more and more involved in XML related technologies although mostly from the Delphi pe
16. Strings and Windows PChars Another important point in favor of using long strings is that they are null terminated This means that they are fully compatible with the C language null terminated strings used by Windows A null terminated string is a sequence of characters followed by a byte that is set to zero or null This can be expressed in Delphi using a zero based array of characters the data type typically used to implement strings in the C language This is the reason null terminated character arrays are so common in the Windows API functions which are based on the C language Since Pascal s long strings are fully compatible with C null terminated strings you can simply use long strings and cast them to PChar when you need to pass a string to a Windows API function For example to copy the caption of a form into a PChar string using the API function GetWindowText and then copy it into the Caption of the button you can write the following code procedure TForml ButtonlClick Sender TObject var Essential Pascal Copyright 1995 2003 Marco Cantu www marcocantu com epascal 64 S1 String begin SetLength S1 100 GetWindowText Handle PChar S1 Length S1 Buttonl Caption 51 end You can find this code in the LongStr example Note that if you write this code but fail to allocate the memory for the string with SetLength the program will probably crash If you are using a PChar to pass a value and not to receive one as
17. TObject var P Integer begin P nil ShowMessage IntToStr P end You can see an example of the effect of this code by running the GPF example or looking at the corresponding Figure 4 4 The example contains also the code fragments shown above Figure 4 4 THE SYSTEM ERROR RESULTING FROM THE ACCESS TO A NIL POINTER FROM THE GPF EXAMPLE Essential Pascal Copyright 1995 2003 Marco Cant www marcocantu com epascal 36 J JBPF ial x Gpf x e Access violation at address 00438FF5 in module GPF EXE Read of address FFFFFFFF In the same program you can find an example of safe data access In this second case the pointer is assigned to an existing local variable and can be safely used but I ve added a safe check anyway procedure TFormGPF BtnSafeClick Sender TObject var P Integer X Integer begin P X X 100 if P lt gt nil then ShowMessage IntToStr P end Delphi also defines a Pointer data type which indicates untyped pointers such as void in the C language If you use an untyped pointer you should use GetMem instead of New The GetMem procedure is required each time the size of the memory variable to allocate is not defined The fact that pointers are seldom necessary in Delphi is an interesting advantage of this environment Nonetheless understanding pointers is important for advanced programming and for a full understanding of the Delphi object model w
18. a certainly a very common data type Although you can safely use them in most cases without understanding how they work this chapter should have made clear the exact behavior of strings making it possible for you to use all the power of this data type Strings are handled in memory in a special dynamic way as happens with dynamic arrays This is the topic of the next chapter Essential Pascal Copyright 1995 2003 Marco Cantu www marcocantu com epascal 68 Chapter 8 Memory Author s Note This chapter will cover memory handling discuss the various memory areas and introduce dynamic arrays Temporarily only this last part is available Dynamic Arrays declare a data type using the array construct you have to specify the number of elements of the array As expert programmers probably know there were a few techniques you could use to implement dynamic arrays typically using pointers and manually allocating and freeing the required memory Delphi 4 introduced a very simple implementation of dynamic arrays modeling them after the dynamic long string type I ve just covered As long strings dynamic arrays are dynamically allocated and reference counted but they do not offer a copy on write technique That s not a big problem as you can deallocate an array by setting its variable to nil You can simply declare an array without specifying the number of elements and then allocate it with a given size using the SetLength procedure The s
19. above placed three edit boxes on a new form added a couple of buttons and then wrote the following code for the OnClick event of the first button procedure TForml ButtonlClick Sender TObject var V Variant begin Vo 10 Editl Text V V Hello World Edit2 Text V V 45 55 Edit3 Text V end Funny isn t it Besides assigning a variant holding a string to the Text property of an edit component you can assign to the Text a variant holding an integer or a floating point number As you can see in Figure 10 1 everything works Essential Pascal Copyright 1995 2003 Marco Cant www marcocantu com epascal 78 f Variant Test IDE x Hello World Compute 45 55 Figure 10 1 THE OUTPUT OF THE VARITEST EXAMPLE AFTER THE ASSIGN BUTTON HAS BEEN PRESSED Even worse you can use the variants to compute values as you can see in the code related to the second button procedure TForml Button2Click Sender TObject var V Variant N Integer begin V Editl Text N Integer V 2 V N Edit1 Text V end Writing this kind of code is risky to say the least If the first edit box contains a number everything works If not an exception is raised Again you can write similar code but without a compelling reason to do so you shouldn t use the Variant type stick with the traditional Pascal data types and type checking approach In Delphi and in t
20. basic definition of an open array parameter is that of a typed open array This means you indicate the type of the parameter but do not know how many elements of that type the array is going to have Here is an example of such a definition function Sum const A array of Integer Integer var I Integer Essential Pascal Copyright 1995 2003 Marco Cantu www marcocantu com epascal 50 begin Result 0 for I Low A to High A do Result Result A I end Using High A we can get the size of the array Notice also the use of the return value of the function Result to store temporary values You can call this function by passing to it an array of Integer expressions x lt Sum 110 Y 27 11 Given an array of Integers of any size you can pass it directly to a routine requiring an open array parameter or instead you can call the Slice function to pass only a portion of the array as indicated by its second parameter Here is an example where the complete array is passed as parameter var List array 1 10 of Integer X I Integer begin initialize the array for I Low List to High List do List I I 2 call X Sum List If you want to pass only a portion of the array to the Slice function simply call it this way x Sum Slice List 5 You can find all the code fragments presented in this section in the OpenArr example see Figure 6 1 later on for the form Openar Fx
21. character from the source file until the program reaches the end of the file Eof Each single character is converted and copied to the output file As an alternative it is possible to read and convert one line at a time that is a string at a time using string handling routines This will make the program significantly faster The approach I ve used here is reasonable only for an introductory example The conversion procedure s actual code however is complicated by the fact that it has to update the dialog box s progress bar At each step of the conversion a long integer variable with the current position in the file is incremented This variable s value is used to compute the percentage of work completed as you can see in the code above The conversion procedure for removing symbols is very simple while not Eof FilelIn do begin Read FileIn Ch if Ch Chr 127 then Write FileOut Choose The procedure used to capitalize the text in contrast is really a complex piece of code which you can find in the code of the example The conversion is based on a case statement with four branches f the letter is uppercase and it is the first letter after an ending punctuation mark as indicated by the Period Boolean variable it is left as is otherwise it is converted to lowercase This conversion is not done by a standard procedure simply because there isn t one for single characters It s done with a low level function I ve w
22. generally lists the possible values of an enumeration As an alternative you can use the OrdType program available on www marcocantu com to see the list of the values of each Delphi enumeration set subrange and any other ordinal type You can see an example of the output of this program in Figure 4 2 FIGURE 4 2 DETAILED INFORMATION ABOUT AN ENUMERATED TYPE AS DISPLAYED BY THE ORDTYPE PROGRAM AVAILABLE ON MY WEB SITE Essential Pascal Copyright 1995 2003 Marco Cant www marcocantu com epascal 31 J Ordinal Types RTTI TBrushStyle Type Name TCloseAction TButtonLayout Type Kind tkEnumeration TButtonSet A Implement otUByte TButtonState Min Value 0 TCanvasState Max Value 3 TCanvasStates Base Type TCloseAction TCheckBoxState Values D caNone TColorDialogOption 1 caHide TColorDialogOptions 2 caFree TColumnButtonStyle 3 caMinimize TColumnValue Set Types Set types indicate a group of values where the list of available values is indicated by the ordinal type the set is based onto These ordinal types are usually limited and quite often represented by an enumeration or a subrange If we take the subrange 1 3 the possible values of the set based on it include only 1 only 2 only 3 both 1 and 2 both 1 and 3 both 2 and 3 all the three values or none of them A variable usually holds one of the possible values of the range of its type A set type variable instead can contain none one two three or more
23. modified version of the contents The program named Filter can convert all the characters in a text file to uppercase capitalize only the initial word of each sentence or ignore the characters from the upper portion of the ASCII character set The form of the program has two read only edit boxes for the names of the input and output files and two buttons to select input and output files using the standard dialog boxes The form s lower portion contains a RadioGroup component and a bitmap button named ConvertBitBtn to apply the current conversion to the selected files The radio group has three items as you can see from the following portion of the form s textual description object RadioGroupl TRadioGroup Caption Conversion Items Strings amp Uppercase Capitalize amp sentences Remove s amp ymbols The user can click on the two buttons to choose the names of the input and output files displayed in the two edit boxes procedure TForml ButtonlClick Sender TObject begin if OpenDialogl Execute then Editl Text OpenDialogl Filename end The second button activates the saveDialogi dialog box The real code of the example is in the three conversion routines that are called by the bitmap button s onclick event handler These calls take place inside a case statement in the middle of the ConvertBitBtn button s OnClick handler case RadioGroupl ItemIndex of 0 ConvUpper 1 ConvCapitalize 2 C
24. overload begin MessageDlg Str IntToStr I mtInformation mbOK 0 end then the compiler won t complain this is a legal definition However the call ShowMsg Hello is flagged by the compiler as Ambiguous overloaded call to ShowMsg Notice that this error shows up in a line of code that compiled correctly before the new overloaded definition In practice we have no way to call the ShowMsg procedure with one string parameter as the compiler doesn t know whether we want to call the version with only the string parameter or the one with the string parameter and the integer parameter with a default value When it has a similar doubt the compiler stops and asks the programmer to state his or her intentions more clearly Conclusion Writing procedure and functions is a key element of programming although in Delphi you ll tend to write methods procedures and functions connected with classes and objects Instead of moving on to object oriented features however the next few chapters give you some details on other Pascal programming elements starting with strings Essential Pascal Copyright 1995 2003 Marco Cantu www marcocantu com epascal 61 Chapter 7 Handling Strings tring handling in Delphi is quite simple but behind the scenes the situation is quite complex Pascal has a traditional way of handling strings Windows has its own way borrowed from the C language and 32 bit versions of Delphi include a powerf
25. particularly popular on the PC platform thanks to its balance of simplicity and power Turbo Pascal introduced an Integrated Development Environment IDE where you could edit the code in a WordStar compatible editor run the compiler see the errors and jump back to the lines containing those errors It sounds trivial now but previously you had to quit the editor return to DOS run the command line compiler write down the error lines open the editor and jump there Moreover Borland sold Turbo Pascal for 49 dollars where Microsoft s Pascal compiler was sold for a few hundred Turbo Pascal s many years of success contributed to Microsoft s eventual cancellation of its Pascal compiler product You can actually download a copy of the original version of Borland s Turbo Pascal from the Borland Developer s Network web site http bdn borland com in the Museum section Delphi s Pascal After 9 versions of Turbo and Borland Pascal compilers which gradually extended the language into the OOP realm Borland released Delphi in 1995 turning Pascal into a visual programming language Delphi extends the Pascal language in a number of ways including many Essential Pascal Copyright 1995 2003 Marco Cantu www marcocantu com epascal 6 object oriented extensions which are different from other flavors of Object Pascal including those in the Borland Pascal with Objects compiler the last incarnations of Turbo Pascal With Delphi 2 Borland brough
26. properly Delphi still can use this string for output and can figure out when it ends by looking for the null terminator but if you append further characters after the null terminator they will be skipped altogether How can we fix this problem The solution is to tell the system to convert the string returned by the GetWindowText API call back to a Pascal string However if you write the following code Sl String 51 the system will ignore it because converting a data type back into itself is a useless operation To obtain the proper long Pascal string you need to recast the string to a PChar and let Delphi convert it back again properly to a string S1 String PChar S1 Actually you can skip the string conversion because PChar to string conversions are automatic in Delphi Here is the final code procedure TForml Button3Click Sender TObject var Sls String begin SetLength S1 100 GetWindowText Handle PChar S1 Length S1 Essential Pascal Copyright 1995 2003 Marco Cant www marcocantu com epascal 65 Sl String PChar S1 S1 S1 is the title Button3 Caption S1 end An alternative is to reset the length of the Delphi string using the length of the PChar string by writing SetLength S1 StrLen PChar S1 You can find three versions of this code in the LongStr example which has three buttons to execute them However if you just need to access the title of a form you can simp
27. t generate any special code to support default parameters nor does it create multiple copies of the routines The missing parameters are simply added by the compiler to the calling code There is one important restriction affecting the use of default parameters You cannot skip parameters For example you can t pass the third parameter to the function after omitting the second one MessBox Hello mb OK error Essential Pascal Copyright 1995 2003 Marco Cantu www marcocantu com epascal 60 This is the main rule for default parameters In a call you can only omit parameters starting from the last one In other words if you omit a parameter you must omit also the following ones There are a few other rules for default parameters as well Parameters with default values must be at the end of the parameters list Default values must be constants Obviously this limits the types you can use with default parameters For example a dynamic array or an interface type cannot have a default parameter other than nil records cannot be used at all Default parameters must be passed by value or as const A reference var parameter cannot have a default value Using default parameters and overloading at the same time can cause quite a few problems as the two features might conflict For example if add to the previous example the following new version of the ShowMsg procedure procedure ShowMsg Str string I Integer 0
28. this actually makes them more readable Using Code Templates When writing Pascal language statements you often repeat the same sequence of keywords For this reason Borland has provided a new feature called Code Templates A code template is simply a piece of code related with a shorthand You type the shorthand then press Ctril and the full piece of code appears For example if you type arraya and then press Ctrl J the Delphi editor will expand your text into array 0 of Since the predefined code templates usually include several versions of the same construct the shortcut generally terminates with a letter indicating which of the versions you are interested in However you can also type only the initial part of the shortcut For example if you Essential Pascal Copyright 1995 2003 Marco Cantu www marcocantu com epascal 11 type ar and then press Ctrl J the editor will display a local menu with a list of the available choices with a short description as you can see in Figure 2 2 AMETE Iof x ravem C9 Form ES UT we TFoml E G3 Uses ar array declaration const arrayc array declaration var a Po Insert 2X 3 Modified FIGURE 2 2 CoDE TEMPLATES SELECTION You can fully customize the code templates by modifying the existing ones or adding your own common code pieces If you do this keep in mind that the text of a code template generally includes the
29. to the same array in memory At this point however if you modify one of the arrays the other is affected as well as they both refer to the same memory area procedure TForml btnAliasClick Sender TObject begin alias Array2 Arrayl change one both change Array2 99 1000 show the other Caption IntToStr Arrayl 99 The btnAliasClick method does two more operations The first is an equality test on the arrays This tests not the actual elements of the structures but rather the memory areas the arrays refer to checking whether the variables are two aliases of the same array in memory procedure TForml btnAliasClick Sender TObject begin Essential Pascal Copyright 1995 2003 Marco Cant www marcocantu com epascal 70 if Arrayl Array2 then Beep truncate first array Arrayl Copy Array2 0 10 end The second is a call to the Copy function which not only moves data from one array to the other but also replaces the first array with a new one created by the function The effect is that the Array1 variable now refers to an array of 11 elements so that pressing the Get value or Set value buttons produces a memory error and raises an exception unless you have range checking turned off in which case the error remains but the exception is not displayed The code of the Fill button continues to work fine even after this change as the items of the array to modify are determined using its current bo
30. to use a specific type you can simply add the type name in the declaration as in const Thousand Integer 1000 When you declare a constant the compiler can choose whether to assign a memory location to the constant and save its value there or to duplicate the actual value each time the constant is used This second approach makes sense particularly for simple constants The 16 bit version of Delphi allows you to change the value of a typed constant at run time as if it was a variable The 32 bit version still permits this behavior for backward compatibility when you enable the J compiler directive or use the corresponding Assignable typed constants check box of the Compiler page of the Project Options dialog box This was the default until Delphi 6 but in any case you are strongly advised not to use this trick as a general programming technique Assigning a new value to a constant disables all the compiler optimizations on constants In such a case simply declare a variable instead Resource String Constants When you define a string constant instead of writing const AuthorName Marco Cant starting with Delphi 3 you can write the following resourcestring AuthorName Marco Cant In both cases you are defining a constant that is a value you don t change during program execution The difference is only in the implementation A string constant defined with the resourcestring directive is stored in the r
31. type is quite easy because Delphi includes a number of functions that operate on this type You can find a list of these functions in Table 3 3 TABLE 3 3 System ROUTINES FOR THE TDATETIME TYPE Routine Description Now fetums the current date and time into a single TDateTime value Date Retums only the current date Time Returns only the current time Converts a date and time value into a string using default formatting to have more control on the conversion use the FormatDateTime function instead DateTimeToString Copies the date and time values into a string buffer with default formatting Converts the date portion of a TDateTime value into a string Converts the time portion of a TDateTime value into a string Formats a date and time using the specified format you can specify which values you want to see and which format to use providing a complex format string Converts a string with date and time information to a TDateTime value raising an exception in case of an error in the format of the string Converts a string with a date value into the TDateTime format Converts a string with a time value into the TDateTime format Returns the number corresponding to the day of the week of the TDateTime value passed as parameter Retrieves the year month and day values from a date value Retrieves out of a time value Turns year month and day values into a TDateTime value DateTimeToStr DateToStr TimeToStr Forma
32. unit structure implements directive property operator set project structure directive dipinterface Statement unit structure backward compatibility see asm forward unction oto index inherited inline interface lt E D operator RTTI declaration program structure directive method od operator math ibrary le 2 z S73 2 18121515 D ct 5 7 Uu o 2 a o N 3 Q D 2 D o cr ct o 2 3 Essential Pascal Copyright 1995 2003 Marco Cant www marcocantu com epascal 13 name directive function backward compatibility class directive property operator boolean backward compatibility class 2 2 o Q e w c statement case statement exceptions operator boolean 2 o ut irective parameters overload unction directive override unction directive ive d ackage packed ascal rogram roperty rotected ublic published raise statement exceptions read readonly type p overload override paced published rise read readony record record ype register unction calling convention reintroduce unction directive repeat Statement safecall function calling convention e et e Uu pm D Ww o c s a D A ct m E e yp unction calling convention yp Uu hl operator math h operator math s gJ Ea lt requires rogr
33. up to 2 GigaBytes of address space most of which can be used by the heap Stack Memory The term Stack indicates a portion of the memory available to a program which is dynamic but is allocated and deallocated following specific order The stack allocation is LIFO Last In First Out This means that the last memory object you ve allocated will be the first to be deleted Stack memory is typically used by routines procedure function and method calls When you call a routine its parameters and return type are placed on the stack unless you optimize the call as Delphi does by default Also the variables you declare within a routine using a var block before the begin statement are stored on the stack so that when the routine terminates they ll be automatically removed before getting back to the calling routine in LIFO order The stack is one of the three memory areas available to an application The other two are called global memory and heap See the heap entry in this glossary Delphi uses the stack for routine parameters and return values unless you use the default register calling convention for local routine variables for Windows API function calls and so on Windows applications can reserve a large amount of memory for the stack In Delphi you set this in the linker page of the project options however the default generally does it If you receive a stack full error message this is probably because you have a function recurs
34. values of the range It can even include all of the values Here is an example of a set Letters set of Uppercase Now can define a variable of this type and assign to it some values of the original type To indicate some values in a set you write a comma separated list enclosed within square brackets The following code shows the assignment to a variable of several values a single value and an empty value Lettersl Letters2 Letters3 Letters begin Lettersl1 A B C Letters2 K Letters3 l In Delphi a set is generally used to indicate nonexclusive flags For example the following two lines of code which are part of the Delphi library declare an enumeration of possible icons for the border of a window and the corresponding set type type TBorderIcon biSystemMenu biMinimize biMaximize biHelp TBorderIcons set of TBorderIcon In fact a given window might have none of these icons one of them or more than one When working with the Object Inspector see Figure 4 3 you can provide the values of a set by expanding the selection double click on the property name or click on the plus sign on its left and toggling on and off the presence of each value FiGURE 4 3 A sET TYPE PROPERTY IN THE OBJECT INSPECTOR Essential Pascal Copyright 1995 2003 Marco Cant www marcocantu com epascal 32 Object Inspector Lx Formt TFormi Properties Events AutoScroll
35. version OF x Type Integer Short Int Byte Size 2 Small Int Word Max 32767 Cardinal The size of the Integer type varies depending on the CPU and operating system you are using In 16 bit Windows an Integer variable is two bytes wide In 32 bit Windows an Integer is four bytes wide For this reason when you recompile the Range example you get a different output The two different representations of the Integer type are not a problem as long as your program doesn t make any assumptions about the size of integers If you happen to save an Integer to a file using one version and retrieve it with another though you re going to have some trouble In this situation you should choose a platform independent data type such as Longlnt or Smalll nt For mathematical computation or generic code your best bet is to stick with the standard integral representation for the specific platform that is use the Integer type because this is what the CPU likes best The Integer type should be your first choice when handling integer numbers Use a different representation only when there is a compelling reason to do so Min 32768 Ordinal Types Routines There are some system routines routines defined in the Pascal language and in the Delphi system unit that work on ordinal types They are shown in Table 3 2 C programmers should notice that the two versions of the Inc procedure with one or two parameters correspond to the
36. we cover arrays Besides calling TimeToStr and DateToStr you can use the more powerful FormatDateTime function as I ve done in the last method above see the Delphi Help file for details on the formatting parameters Notice also that time and date values are transformed into strings depending on Windows international settings Delphi reads these values from the system and copies them to a number of global constants declared in the SysUtils unit Some of them are DateSeparator Char ShortDateFormat string LongDateFormat string TimeSeparator Char TimeAMString string TimePMString string ShortTimeFormat string LongTimeFormat string ShortMonthNames array 1 12 of string LongMonthNames array 1 12 of string ShortDayNames array 1 7 of string LongDayNames array 1 7 of string More global constants relate to currency and floating point number formatting You can find the complete list in the Delphi Help file under the topic Currency and date time formatting variables Delphi includes a DateTimePicker component which provides a sophisticated way to input a date selecting it from a calendar Specific Windows Types The predefined data types we have seen so far are part of the Pascal language Delphi also includes other data types defined by Windows These data types are not an integral part of the language but they are part of the Windows libraries Windows types include new default types
37. you get when you try is Previous declaration of lt name gt was not marked with the overload directive However you can overload a routine that was originally declared in a different unit This is for compatibility with previous versions of Delphi which allowed different units to reuse the same routine name Notice anyway that this special case is not an extra feature of overloading but an indication of the problems you can face For example you can add to a unit the following code procedure MessageDlg str string overload Dialogs MessageDlg str mtInformation mbOK 0 end This code doesn t really overload the original MessageDlg routine In fact if you write MessageDlg Hello you ll get a nice error message indicating that some of the parameters are missing The only way to call the local version instead of the one of the VCL is to refer explicitly to the local unit something that defeats the idea of overloading lov rDefF MessageDlg Hello Essential Pascal Copyright 1995 2003 Marco Cant www marcocantu com epascal 59 Default Parameters A related new feature of Delphi 4 is that you can give a default value for the parameter of a function and you can call the function with or without the parameter Let me show an example We can define the following encapsulation of the MessageBox method of the Application global object which uses PChar instead of strings providing two default para
38. you lose these new items 7 Format Test CI x Floating point value Integer value 100 fi 23423412 sm Format string Format string zd Number ze Ready to use format strings Ready to use format strings Weight is d pounds Wmberzo 0000 Weight is 1d nounds Numher 2f Weight ic TN Hex valu d valu Number 1 2342341 2000000 006 FIGURE 7 2 THE OUTPUT OF A FLOATING POINT VALUE FROM THE FMrTTEST PROGRAM The code of this example simply uses the text of the various controls to produce its output This is one of the three methods connected with the Show buttons Essential Pascal Copyright 1995 2003 Marco Cantu www marcocantu com epascal 67 procedure TFormFmtTest BtnIntClick Sender TObject begin ShowMessage Format EditFmtInt Text StrToInt EditInt Text if the item is not there add it if ListBoxInt Items IndexOf EditFmtInt Text 0 then ListBoxInt Items Add EditFmtInt Text end The code basically does the formatting operation using the text of the EditFmtlnt edit box and the value of the Edit nt control If the format string is not already in the list box it is then added to it If the user instead clicks on an item in the list box the code moves that value to the edit box procedure TFormFmtTest ListBoxIntClick Sender TObject begin EditFmtInt Text ListBoxInt Items ListBoxInt ItemIndex end Conclusion Strings
39. 2 end The above formatting is based on pretty printing but programmers have different interpretations of this general rule Some programmers indent the begin and end statements to the level of the inner code some of them indent begin and end and then indent the internal code once more other programmers put the begin in the line of the if condition This is mostly a matter of personal taste There are Delphi add in programs you can use to convert an existing source code to the indentation format you prefer A similar indented format is often used for lists of variables or data types and to continue a statement from the previous line type Letters set of Char var Name string begin long comment and long statement going on in the following line and indented two spaces MessageDlg This is a message mtInformation mbOk 0 Of course any such convention is just a suggestion to make the code more readable to other programmers and it is completely ignored by the compiler I ve tried to use this rule consistently in all of the samples and code fragments in this book Delphi source code manuals and Help examples use a similar formatting style Syntax Highlighting To make it easier to read and write Pascal code the Delphi editor has a feature called color syntax highlighting Depending on the meaning in Pascal of the words you type in the editor they are displayed using different colors
40. By default keywords are in bold strings and comments are in color and often in italic and so on Essential Pascal Copyright 1995 2003 Marco Cantu www marcocantu com epascal 10 Reserved words comments and strings are probably the three elements that benefit most from this feature You can see at a glance a misspelled keyword a string not properly terminated and the length of a multiple line comment You can easily customize the syntax highlight settings using the Editor Colors page of the Environment Options dialog box see Figure 2 1 If you work by yourself choose the colors you like If you work closely with other programmers you should all agree on a standard color scheme I find that working on a computer with a different syntax coloring than the one am used to is really difficult Environment Options Ed Browser Code Insight Explorer Type Library Preferences Libray Editor Display Color Palette Color SpeedSetting Defaults Element 2 Text attributes Bold Comment Italic Reserved word ii dein Use defaults for v Foreground v Background Syntax Highlighting procedure TFormi ButtoniClick Sender TObjec var Number I X Integer hegin Wumber 12355 FIGURE 2 1 THE DIALOG BOX USED TO SET THE COLOR SYNTAX HIGHLIGHTING In this book I ve tried to apply a sort of syntax highlighting to the source code listings hope
41. FloatToStrF As the previous routine copies the floating point value to a string buffer using the specified formatting StrToFloat Converts the given Pascal string to a floating point value TextT oFloat Converts the given null terminated string to a floating point value Essential Pascal Copyright 1995 2003 Marco Cantu www marcocantu com epascal 27 FloatToTextFmt In recent versions of Delphi s Pascal compiler the Round function is based on the FPU processor of the CPU This processor adopts the so called Banker s Rounding which rounds middle values as 5 5 or 6 5 up and down depending whether they follow an odd or an even number Conclusion In this chapter we ve explored the basic notion of type in Pascal But the language has another very important feature It allows programmers to define new custom data types called user defined data types This is the topic of the next chapter Essential Pascal Copyright 1995 2003 Marco Cantu www marcocantu com epascal 28 Chapter 4 User Defined Data Types long with the notion of type one of the great ideas introduced by the Pascal language is the ability to define new data types in a program Programmers can define their own data types by means of type constructors such as subrange types array types record types enumerated types pointer types and set types The most important user defined data type is the class which is part of the object oriented extensions of Obje
42. Items property of the list box you need to clear the contents of the list box itself procedure TForml BtnForClick Sender TObject var I Integer begin ListBoxl Items Clear for I 1 to 20 do Listboxl Items Add String IntToStr 1 end The code associated with the second button is slightly more complex In this case there is a while loop based on a counter which is increased randomly To accomplish this I ve called the Randomize procedure which resets the random number generator and the Random function with a range value of 100 The result of this function is a number between 0 and 99 chosen randomly The series of random numbers control how many times the while loop is executed procedure TForml BtnWhileClick Sender TObject var I Integer begin ListBoxl Items Clear Randomize I 0 while I lt 1000 do begin I I Random 100 Listboxl Items Add Random Number IntToStr 1 end end Each time you click the While button the numbers are different because they depend on the random number generator Figure 5 3 shows the results from two separate button clicks Notice that not only are the generated numbers different each time but so is the number of items That is this while loop is executed a random numbers of times If you press the While button several times in a row you ll see that the list box has a different number of lines Essential Pascal Copyright 1995 2003 Marco Cant www marcocan
43. ON m T nT HH 88 Appendix A Glossary siii suus xa ice tu su qu Ead UFe auo ev EN Ra RUN ERE NIE AEE anae dus ae RD UNEI uS NR SV EREUS 90 Heap Memory orel enseris raan e n rn e Ea a anam EE un abad Daa uin Ife es 90 Stack Memory MN D E EA AAEE aaa eee oh haber E Na tale 90 M re Terms forthe Glossat Vissantn ts culiyadsi dean pb EEEE E EE e E a an 91 Appendix B Examples lt iaaea a Aaaa ar Aap aNEKSi aan EE NENH 92 Essential Pascal Copyright 1995 2003 Marco Cant www marcocantu com epascal 4 Source Code The source code of all the examples mentioned in the book is available The code has the same copyright as the book Feel free to use it at will but don t publish it on other documents or site Links back to the Essential Pascal site www marcocantu com epascal are welcomed If you go to the site you can download the source code in a single zip file EPasCode zip about 30 KB in size and check out the list of the examples in Appendix B Feedback Please let me know of any errors you find but also of topics not clear enough for a beginner I ll be able to devote time to the project depending also on the feedback receive Let me know also which other topics you d like to see here The preferred way of sending feedback is on my public newsgroup see my web site for login information in the area devoted to books If you have troubles using the newsgroups send an email at the address to marco marcocantu com putting
44. a ede PEEL HIER EE E dus 58 Goal o MET 59 Chapter 7 Handling Strings iiid soc ses nsne sun z wusunek uduV uio ava nuEG GE KVRYRxC vaxOE reFR SV EUN YN INE FRE MEM NEC 60 Types of SUIS aaia aeaa ERU CU a n Me vv xke ted IN eaaa bi da Dore ex eA 123 HEAR REIR S 60 Using LONG Stings eoi necat i aesan tur uet opidi M Maus an aii stets rco SEARA dae Heer amid di qu Eie Mes 60 Looking at Strings In Mermopyoedsesceteoco t Spa etel e eon a eb CDXS E OA NE RE duae DO Eben ead lafd 61 Delphi Strings and Windows P CHAI Sons tsesticenewnekensnsetias toner pen Qaa ouvre EP CU CE CE Et pace dab aet 62 Formatting signe Mb m Hn 64 COMIC MIS IO seo toe rid NE E FUOD pati coast o xr ates aD COD behbt bi er dnm ar erc E 66 Chapter B Memory isis phases eueses suse vasi cde een a0 ah kian re cena vua qi e e saa FaREREexew Koc Vase a ei dRe a eaae 67 by 67 CONCUSSION p 69 Chapter 9 Windows Programming 11e eeee eese sena e nennen annia anu a nna nana nan 70 Windows Handles estet ident sS n comp RI Aus RE Catania INL LM EIS T C ME MEE E E EE 70 ExtenialuDeclaratiofls use cc s esos tme iacu lae a md gie E eL A ee CP RES 71 A Windows Callback FUnGHDE asinus eh pe vex XV CEN UAR EIER vn o XE VEI Ud radii aL IM CLE E 72 A Minimal Windows PEOBESETISO seres Node APR EE Rue ve PLN betta sani caus rr pvFa DIU unus 74 eu PO a 75 Chapter L0
45. a reference only in terms of memory usage and speed of the operation But if you modify the value of the string the original value is not affected On the contrary if you pass the long string by reference you can alter the original value Delphi 3 introduced a new kind of parameter out An out parameter has no initial value and is used only to return a value These parameters should be used only for COM procedures and functions in general it is better to stick with the more efficient var parameters Except for not having an initial value out parameters behave like var parameters Constant Parameters As an alternative to reference parameters you can use a const parameter Since you cannot assign a new value to a constant parameter inside the routine the compiler can optimize parameter passing The compiler can choose an approach similar to reference parameters or a const reference in C terms but the behavior will remain similar to value parameters because the original value won t be affected by the routine In fact if you try to compile the following silly code Delphi will issue an error function DoubleTheValue const Value Integer Integer begin Value Value 2 compiler error Result Value end Open Array Parameters Unlike C a Pascal function or procedure always has a fixed number of parameters However there is a way to pass a varying number of parameters to a routine using an open array The
46. actually using the memory This technique is based on a feature of the Windows operating systems and is used by Delphi for all dynamic memory allocations For example when you request a very large array its memory is reserved but not allocated Setting the length of a string is seldom necessary The only case in which you must allocate memory for the long string using SetLength is when you have to pass the string as a parameter to an API function after the proper typecast as I ll show you shortly Looking at Strings in Memory To help you better understand the details of memory management for strings I ve written the simple StrRef example In this program declare two global strings Str1 and Str2 When the first of the two buttons is pressed the program assigns a constant string to the first of the two variables and then assigns the second variable to the first Strl Hello Strz se Stril Besides working on the strings the program shows their internal status in a list box using the following StringStatus function function StringStatus const Str string string begin Result Address IntToStr Integer Str Length IntToStr Length Str References IntToStr PInteger Integer Str 8 Value Str end It is vital in the StringStatus function to pass the string parameter as a const parameter Passing this parameter by copying will cause the side effect of having one extra reference
47. akes the Object Pascal language completely appropriate for the wide range of programs that require numerical computations If you are interested in this aspect you can look at the arithmetic functions provided by Delphi in the system unit see the Delphi Help for more details Essential Pascal Copyright 1995 2003 Marco Cant www marcocantu com epascal 23 Delphi also has a Math unit that defines advanced mathematical routines covering trigonometric functions such as the ArcCosh function finance such as the nterestPayment function and statistics such as the MeanAndStaDev procedure There are a number of these routines some of which sound quite strange to me such as the MomentSkewkurtosis procedure I ll let you find out what this is Date and Time Delphi uses real types also to handle date and time information To be more precise Delphi defines a specific TDateTime data type This is a floating point type because the type must be wide enough to store years months days hours minutes and seconds down to millisecond resolution in a single variable Dates are stored as the number of days since 1899 12 30 with negative values indicating dates before 1899 in the integer part of the TDateTime value Times are stored as fractions of a day in the decimal part of the value TDateTime is not a predefined type the compiler understands but it is defined in the system unit as type TDateTime type Double Using the TDateTime
48. al symbol is the caret type PointerToInt Integer Once you have defined a pointer variable you can assign to it the address of another variable of the same type using the 9 operator var P Integer X Integer Essential Pascal Copyright 1995 2003 Marco Cantu www marcocantu com epascal 35 begin P X change the value in two different ways X 10 P 20 When you have a pointer P with the expression P you refer to the address of the memory location the pointer is referring to and with the expression P you refer to the actual content of that memory location For this reason in the code fragment above P corresponds to X Instead of referring to an existing memory location a pointer can refer to a new memory block dynamically allocated on the heap memory area with the New procedure In this case when you don t need the pointer any more you ll also have to to get rid of the memory you ve dynamically allocated by calling the Dispose procedure var P Integer begin initialization New P operations P 20 ShowMessage IntToStr P termination Dispose P end If a pointer has no value you can assign the nil value to it Then you can test whether a pointer is nil to see if it currently refers to a value This is often used because dereferencing an invalid pointer causes an access violation also known as a general protection fault GPF procedure TFormGPF BtnGpfClick Sender
49. ally in Pascal the TFNWndEnumProc type is not properly defined it is simply a pointer This means we need to provide a function with the proper parameters and then use it as a pointer taking the address of Essential Pascal Copyright 1995 2003 Marco Cant www marcocantu com epascal 74 the function instead of calling it Unfortunately this also means that the compiler will provide no help in case of an error in the type of one of the parameters Windows requires programmers to follow the stdcall calling convention every time we call a Windows API function or pass a callback function to the system Delphi by default uses a different and more efficient calling convention indicated by the register keyword Here is the definition of a proper compatible function which reads the title of the window into a string then adds it to a ListBox of a given form function GetTitle Hwnd THandle Param i var Pointer Boolean stdcall Text string begin l SetLength Text 100 GetWindowText Hwnd PChar Text 100 FormCallBack ListBoxl Items Add IntToStr Hwnd Text Result True end The form has a ListBox covering almost its whole area along with a small panel on the top hosting a button When the button is pressed the EnumWindows API function is called and the GetTitle function is passed as its parameter procedure TFormCallback BtnTitlesClick Sender TObject var
50. am structure package resident directive functions stdcall unction calling convention stored directive property String then threadvar 0 type declaration unit unti Essential Pascal Copyright 1995 2003 Marco Cant www marcocantu com epascal 14 uses unit structure na Birectve methodi xor operator boolean Expressions and Operators There isn t a general rule for building expressions since they mainly depend on the operators being used and Pascal has a number of operators There are logical arithmetic Boolean relational and set operators plus some others Expressions can be used to determine the value to assign to a variable to compute the parameter of a function or procedure or to test for a condition Expressions can include function calls too Every time you are performing an operation on the value of an identifier rather than using an identifier by itself that is an expression Expressions are common to most programming languages An expression is any valid combination of constants variables literal values operators and function results Expressions can also be passed to value parameters of procedures and functions but not always to reference parameters which require a value you can assign to Operators and Precedence If you have ever written a program in your life you already know what an expression is Here I ll highlight specific elements of Pascal operators You can see a list of the
51. ame procedure can also be used to resize an array without losing its content There are also other string oriented procedures such as the Copy function that you can use on arrays Here is a small code excerpt underscoring the fact that you must both declare and allocate memory for the array before you can start using it T raditionally the Pascal language has always had fixed size arrays When you procedure TForml ButtoniClick Sender TObject var Arrayl array of Integer begin Arrayl 1 100 error SetLength Arrayl 100 Arrayl 99 100 OK end As you indicate only the number of elements of the array the index invariably starts from 0 Generic arrays in Pascal account for a non zero low bound and for non integer indexes two features that dynamic arrays don t support To learn the status of a dynamic array you can use the Length High and Low functions as with any other array For dynamic arrays however Low always returns 0 and High always returns the length minus one This implies that for an empty array High returns 1 which when you think about it is a strange value as it is lower than that returned by Low Grow Create alias and truncate FiGuRE 8 1 THE FORM OF THE DYNARR EXAMPLE Essential Pascal Copyright 1995 2003 Marco Cant www marcocantu com epascal 69 After this short introduction can show you a simple example called DynArr and shown in Figure 8 1 It is indeed simpl
52. and operators the same holds for the Dec procedure TABLE 3 2 System ROUTINES FOR ORDINAL TYPES Romme Pupxse Dec Decrements the variable passed as parameter by one or by the value of the optional second parameter linc Jincrements the variable passed as parameter by one or by the specified value dd Retumns True if the argument is an odd number Pred Returns the value before the argument in the order determined by the data type the predecessor Succ Returns the value after the argument the successor Ord Low Returns the lowest value in the range of the ordinal type passed as its parameter i Returns the highest value in the range of the ordinal data type Notice that some of these routines when applied to constants are automatically evaluated by the compiler and replaced by their value For example if you call High X where X is defined as an Integer the compiler can simply replace the expression with the highest possible value of the Integer data type Essential Pascal Copyright 1995 2003 Marco Cantu www marcocantu com epascal 22 Returns a number indicating the order of the argument within the set of values of the data Real Types Real types represent floating point numbers in various formats The smallest storage size is given by Single numbers which are implemented with a 4 byte value Then there are Double floating point numbers implemented with 8 bytes and Extended numbers implemented
53. arameters menu command Another technique is to open the Windows Explorer locate the directory that contains the executable file of the program and drag the file you want to run onto the executable file The Windows Explorer will start the program using the name of the dropped file as a command line parameter Figure 9 3 shows both the Explorer and the corresponding output StrParam Command Line x C dev essPascal StrParam Strparam exe C DEVSESSPAS 14S TRPARAMSSTRPARAM DPR FIGURE 9 3 YOU CAN PROVIDE A COMMAND LINE PARAMETER TO THE STRPARAM EXAMPLE BY DROPPING A FILE OVER THE EXECUTABLE FILE IN THE WINDOWS EXPLORER Conclusion In this chapter we ve seen a low level introduction to Windows programming discussing handles and a very simple Windows program For normal Windows programming tasks you ll generally use the visual development support provided by Delphi and based on the VCL But this is beyond the scope of this book which is the Pascal language Next chapter covers variants a very strange addition to Pascal type system introduced to provide full OLE support Essential Pascal Copyright 1995 2003 Marco Cant www marcocantu com epascal 77 Chapter 10 Variants To provide full OLE support the 32 bit version of Delphi includes the Variant data type Here want to discuss this data type from a general perspective The Variant type in fact has a pervasive effect on the whole language and the Delphi components
54. ation f floating point The corresponding floating point value is converted to the shortest possible decimal string using either floating point or exponential notation n number The corresponding floating point value is converted to a floating point string but also uses thousands separators The corresponding floating point value is converted to a string representing a currency m money amount The conversion is based on regional settings see the Delphi Help file under Currency and date time formatting variables g general The best way to see examples of these conversions is to experiment with format strings yourself To make this easier I ve written the FmtTest program which allows a user to provide formatting strings for integer and floating point numbers As you can see in Figure 7 2 this program displays a form divided into two parts The left part is for Integer numbers the right part for floating point numbers Each part has a first edit box with the numeric value you want to format to a string Below the first edit box there is a button to perform the formatting operation and show the result in a message box Then comes another edit box where you can type a format string As an alternative you can simply click on one of the lines of the ListBox component below to select a predefined formatting string Every time you type a new formatting string it is added to the corresponding list box note that by closing the program
55. ayTemperatures YearTemps array Jan Dec of MonthTemps This declaration inverts the order of the indexes as presented above but it also allows assignment of whole blocks between variables For example the following statement copies January s temperatures to February var ThisYear YearTemps begin ThisYear Feb ThisYear Jan You can also define a zero based array an array type with the lower bound set to zero Generally the use of more logical bounds is an advantage since you don t need to use the index 2 to access the third item and so on Windows however uses invariably zero based arrays because it is based on the C language and the Delphi component library tends to do the same If you need to work on an array you can always test its bounds by using the standard Low and High functions which return the lower and upper bounds Using Low and High when operating on an array is highly recommended especially in loops since it makes the code independent of the range of the array Later you can change the declared range of the array indices and the code that uses Low and High will still work If you write a loop hard coding the range of an array you ll have to update the code of the loop when the array size changes Low and High make your code easier to maintain and more reliable Incidentally there is no run time overhead for using Lowand High with arrays They are resolved at compile time into constant expressions n
56. creating problems code With a second comment identifier you can write the following code which is correct lC o code this comment is OK code Note that if the open brace or parenthesis star is followed by the dollar sign it becomes a compiler directive as in X4 Actually compiler directives are still comments For example SX This is a comment is legal It s both a valid directive and a comment although sane programmers will probably tend to separate directives and comments Essential Pascal Copyright 1995 2003 Marco Cantu www marcocantu com epascal 8 Use of Uppercase The Pascal compiler unlike those in other languages ignores the case capitalization of characters Therefore the identifiers Myname MyName myname myName and MYNAME are all exactly equivalent On the whole this is definitely a positive since in case sensitive languages many syntax errors are caused by incorrect capitalization There is only one exception to the case insensitive rule of Pascal the Register procedure of a components package must start with the uppercase H because of a C Builder compatibility issue There are a couple of subtle drawbacks however First you must be aware that these identifiers really are the same so you must avoid using them as different elements Second you should try to be consistent in the use of uppercase letters to improve the readability of the code A consistent
57. ct Pascal not covered in this book If you think that type constructors are common in many programming languages you are right but Pascal was the first language to introduce the idea in a formal and very precise way There are still few languages with so many mechanisms to define new types Named and Unnamed Types These types can be given a name for later use or applied to a variable directly When you give a name to a type you must provide a specific section in the code such as the following type subrange definition Uppercase A Z array definition Temperatures array 1 24 of Integer record definition Date record Month Byte Day Byte Year Integer end enumerated type definition Colors Red Yellow Green Cyan Blue Violet set definition Letters set of Char Similar type definition constructs can be used directly to define a variable without an explicit type name as in the following code var DecemberTemperature array 1 31 of Byte ColorCode array Red Violet of Word Palette set of Colors In general you should avoid using unnamed types as in the code above because you cannot pass them as parameters to routines or declare other variables of the same type The type compatibility rules of Pascal in fact are based on type names not on the actual definition of the types Two variables of two identical types are still not compatible unless their types ha
58. d else ShowMessage CheckBox2 is NOT checked ena Notice that you cannot have a semicolon after the first statement and before the else keyword or the compiler will issue a syntax error The if then else statement in fact is a single statement so you cannot place a semicolon in the middle of it An ff statement can be quite complex The condition can be turned into a series of conditions using the and or and not Boolean operators or the if statement can nest a second ff statement The last two buttons of the IfTest example demonstrate these cases procedure TForml Button3Click Sender TObject begin statement with a double condition if CheckBoxl Checked and CheckBox2 Checked then ShowMessage Both check boxes are checked end procedure TForml Button4Click Sender TObject begin compound if statement if CheckBoxl Checked then if CheckBox2 Checked then ShowMessage CheckBoxl and 2 are checked else ShowMessage Only CheckBoxl is checked else ShowMessage Checkboxl is not checked who cares for Checkbox2 end Look at the code carefully and run the program to see if you understand everything When you have doubts about a programming construct writing a very simple program such as this can help you learn a lot You can add more check boxes and increase the complexity of this small example making any test you like Case Statements If your if statements become very complex at times you can
59. d with a Boolean expression A simple Delphi example will demonstrate how to write conditional statements First create a new application and put two check boxes and four buttons in the form Do not change the names of buttons or check boxes but double click on each button to add a handler for its OnClick event Here is a simple if statement for the first button procedure TForml ButtonlClick Sender TObject begin if CheckBoxl Checked then ShowMessage CheckBoxl is checked end When you click on the button if the first check box has a check mark in it the program will show a simple message see Figure 5 1 I ve used the ShowMessage function because it is the simplest Delphi function you can use to display a short message to the user FiGuRE 5 1 THE MESSAGE DISPLAYED BY THE lFTEST EXAMPLE WHEN YOU PRESS THE FIRST BUTTON AND THE FIRST CHECK BOX IS CHECKED PES lol x v CheckBoxl Button2 CheckBox2 Button3 CheckBox 1 is checked Button4 EERE If you click the button and nothing happens it means the check box was not checked In a case like this it would probably be better to make this more explicit as with the code for the second button which uses an if then else statement procedure TForml Button2Click Sender TObject begin if then else statement if CheckBox2 Checked then Essential Pascal Copyright 1995 2003 Marco Cant www marcocantu com epascal 40 ShowMessage CheckBox2 is checke
60. dd a new form to a project with the corresponding toolbar button or the File New Form menu command Delphi actually adds a new unit which defines the class for the new form Units Although every form is defined in a unit the reverse is not true Units do not need to define forms they can simply define and make available a collection of routines By selecting the File gt New menu command and then the Unit icon in the New page of the Object Repository you add a new blank unit to the current project This blank unit contains the following code delimiting the sections a unit is divided into l unit Uniti interface implementation end The concept of a unit is simple A unit has a unique name corresponding to its filename an interface section declaring what is visible to other units and an implementation section with the real code and other hidden declarations Finally the unit can have an optional initialization section with some startup code to be executed when the program is loaded into memory it can also have an optional finalization section to be executed on program termination The general structure of a unit with all its possible sections is the following unit unitName interface other units we need to refer to uses A B C exported type definition type newType TypeDefinition exported constants const Zero 0 global variables var Total Integer
61. dea emphasized by Pascal is the concept of the routine basically a series of statements with a unique name which can be activated many times by using their name This way you avoid repeating the same statements over and over and having a single version of the code you can easily modify it all over the program From this point of view you can think of routines as the basic code encapsulation mechanism I ll get back to this topic with an example after introduce the Pascal routines syntax Pascal Procedures and Functions In Pascal a routine can assume two forms a procedure and a function In theory a procedure is an operation you ask the computer to perform a function is a computation returning a value This difference is emphasized by the fact that a function has a result a return value while a procedure doesn t Both types of routines can have multiple parameters of given data types In practice however the difference between functions and procedures is very limited you can call a function to perform some work and then skip the result which might be an optional error code or something like that or you can call a procedure which passes a result within its parameters more on reference parameters later in this chapter Here are the definitions of a procedure and two versions of the same function using a slightly different syntax procedure Hello begin ShowMessage Hello world end function Double Value Integ
62. e because there is nothing very complex about dynamic arrays I ll also use it to show a few possible errors programmers might make The program declares two global arrays and initializes the first in the OnCreate handler var Arrayl Array2 array of Integer procedure TForml FormCreate Sender TObject begin allocate SetLength Arrayl 100 end This sets all the values to zero This initialization code makes it possible to start reading and writing values of the array right away without any fear of memory errors Assuming of course that you don t try to access items beyond the upper bound of the array For an even better initialization the program has a button that writes into each cell of the array procedure TForml btnFillClick Sender TObject var I Integer begin for I Low Arrayl to High Arrayl do Arrayl I I end The Grow button allows you to modify the size of the array without losing its contents You can test this by using the Get value button after pressing the Grow button procedure TForml btnGrowClick Sender TObject begin grow keeping existing values SetLength Arrayl 200 end procedure TForml btnGetClick Sender TObject begin extract Caption IntToStr Arrayl 99 end The only slightly complex code is in the OnClick event of the Alias button The program copies one array to the other one with the operator effectively creating an alias a new variable referring
63. e couse 2d COpYL i md sybex ar tke authsr kereky grant to you a licease to use the ecttware zubject Le Llc Ceser Lhel Lz z 4 Tour pureduber US yade ef ube e Lie cellwoure wil eonstitute your amp cceptence of Buch terme The seurce cone te provided as is wiza ao yavancuses whatsoever Ite ory sczpe in le hel yet Tw SB Whe source cece Le protected by c2orcytilga to marco cant You sre hereby zrsrted a single Lee cerse Lo use compile aud 295 sy preyiams anc Le rc 66gs ary eode pe ticr r prasravs von ar davaloniag Yor ave net allowed te prb k ond veproduce the cece is it is Bese free ta direct others to the ccwraelcc pare The aubles is of rz way Lespousikls Lor providi any supperl ic Lhe ecurue xj 1 uj xi Int fal Insert Ae hrima ort Pikien S 4 FIGURE 12 1 THE RESULT OF RUNNING THE FILTER EXAMPLE S CAPITALIZE CONVERSION Saving Generic Data In addition to using text files you can save other data types to a file including integers real numbers arrays and records Using a custom file type instead of a text file may be an advantage because it might take less space the textual representation of a number usually takes much more space than its binary value but this approach won t let the user browse through the files using a text editor which might be an advantage too How do you save a series of integers to a file First you have to define a file as shown SaveFile file of Integer Then you need
64. e it is fully defined Here is a silly example just to give you the idea procedure DoubleHello forward procedure Hello begin if MessageDlg Do you want a double message mtConfirmation mbYes mbNo 0 mrYes then DoubleHello else ShowMessage Hello end procedure DoubleHello begin Hello Hello end This approach allows you to write mutual recursion DoubleHello calls Hello but Hello might call DoubleHello too Of course there must be a condition to terminate the recursion to avoid a stack overflow You can find this code with some slight changes in the DoubleH example Although a forward procedure declaration is not very common in Delphi there is a similar case that is much more frequent When you declare a procedure or function in the interface portion of a unit more on units in the next chapter it is considered a forward declaration even if the forward keyword is not present Actually you cannot write the body of a routine in the interface portion of a unit At the same time you must provide in the same unit the actual implementation of each routine you have declared The same holds for the declaration of a method inside a class type that was automatically generated by Delphi as you added an event to a form or its components The event handlers declared inside a TForm class are forward declarations the code will be provided in the implementation portion of the unit Here is an excerp
65. e of a variable or an expression Declaring a function of this kind is simple but how do you code it How do you know the types of the parameters The values of a type variant open array parameter are compatible with the TVarRec type elements Do not confuse the TVarRec record with the TVarData record used by the Variant type itself These two structures have a different aim and are not compatible Even the list of possible types is different because TVarRec can hold Delphi data types while TVarData can hold OLE data types The TVarRec record has the following structure type TVarRec record case Byte of vtInteger VInteger Integer VType Byte vtBoolean VBoolean Boolean vtChar VChar Char vtExtended VExtended PExtended vtString VString PShortString vtPointer VPointer Pointer vtPChar VPChar PChar vtObject VObject TObject vtClass VClass TClass vtWideChar VWideChar WideChar vtPWideChar VPWideChar PWideChar vtAnsiString VAnsiString Pointer vtCurrency VCurrency PCurrency vtVariant VVariant PVariant vtInterface VInterface Pointer end Each possible record has the VType field although this is not easy to see at first because it is declared only once along with the actual Integer size data generally a reference or a pointer Using this information we can actually write a function capable of operating on different data types In the SumAll funct
66. ebbr ia FERE dvd a PORPE VER Ra TERI aba bibi EEEE 39 Assignment Statements oi ne dscovesiseaa d avemo strae ibat d ridet MURS PhiPIS PPP MMUNENIP A MIL UL RUINA S 39 Conditional StatBrmepi Ese oso go xaX Ua pO EVE ERROR VEA TY IA ELE TMDI A Ret LEE ROM NA Ya Aen bA 40 Essential Pascal Copyright 1995 2003 Marco Cant www marcocantu com epascal 3 Loopsiin e MM T 42 The With Statements Pm 45 Conclusions iieaeoe cod a E EA EEEE OE EO direc E E LU NAASE AEETI 46 Chapter 6 Procedures and FunctionS s sssss 55555555150unnnn nn 47 Pastal Procedures andsPulicllollS soo cec s ee ERBEN ha bd eu PT rn n EnA AS Pa USO ENG AR CO UAE REIR cleus 47 Reference Paramebersss Sto erp t o E EP ar tially en asian sd avin EIC ECC U PLUIE SU US UUE 48 Constant Parameters iss voee cott ieee vt eb ed ae le dee ve ee eet os dus 49 Open Affay Parametels c c rc eeepc de oe d etd ene e anon eek icon eo rrt EU crest 49 Delphi Calling Cohnventlons i uc ie de rt ene c Dno xd Lie x d re AA CE pO Re E 52 What 1S a Metliogd i eoe tea e a da p e a te e XE EN d ERE EAE CE CE EE E 53 Forward Declarduolls eera anah eet ELEA LAEE aa EEEa AEEA PIE YR Ue Je ELIT CRETAE SERRE 53 Procedural TV DES acc cts rio ud I aE e EY BRE AEYELI E Ead PRU REIS IE EM Spe mu EE 54 Function Overloading s dogs t omes Vhs inetd YT ona IER Eas de rut AtA AREA reed de UE AT ces VE VM Spa PE eee 56 Default Parameters eeen HERI AER ERV E E rper E M
67. ed as numbers but they are not used as such In Windows a handle is a reference to an internal data structure of the system For example when you work with a window or a Delphi form the system gives you a handle to the window The system informs you that the window you are working with is window number 142 for example From that point on your application can ask the system to operate on window number 142 moving it resizing it reducing it to an icon and so on Many Windows API functions in fact have a handle as the first parameter This doesn t apply only to functions operating on windows other Windows API functions have as their first parameter a GDI handle a menu handle an instance handle a bitmap handle or one of the many other handle types In other words a handle is an internal code you can use to refer to a specific element handled by the system including a window a bitmap an icon a memory block a cursor a font a menu and so on In Delphi you seldom need to use handles directly since they are hidden inside forms bitmaps and other Delphi objects They become useful when you want to call a Windows API function that is not supported by Delphi To complete this description here is a simple example demonstrating Windows handles The WHandle program has a simple form containing just a button In the code respond to the OnCreate event of the form and the OnClick event of the button as indicated by the following textua
68. em is to place these variables inside the form declaration type TForml class TForm private Private declarations IP intbroc X Integer end We will see exactly what this means in the next chapter but for the moment you need to modify the code generated by Delphi for the class type as indicated above and add the definition Essential Pascal Copyright 1995 2003 Marco Cantu www marcocantu com epascal 57 of the procedural type I ve shown before To initialize these two variables with suitable values we can handle the OnCreate event of the form select this event in the Object Inspector after you have activated the form or simply double click on the form suggest you refer to the listing to study the details of the source code of this example You can see a practical example of the use of procedural types in Chapter 9 in the section A Windows Callback Function Function Overloading The idea of overloading is simple The compiler allows you to define two functions or procedures using the same name provided that the parameters are different By checking the parameters in fact the compiler can determine which of the versions of the routine you want to call Consider this series of functions extracted from the Math unit of the VCL function Min function Min Integer Integer overload function Min A Int64 Int64 overload A B A B B Single Single overload B B
69. er Integer begin Double Value 2 end or as an alternative function Double2 Value Integer Integer begin Result Value 2 end The use of Result instead of the function name to assign the return value of a function is becoming quite popular and tends to make the code more readable in my opinion Once these routines have been defined you can call them one or more times You call the procedure to make it perform its task and call a function to compute the value procedure TForml ButtonlClick Sender TObject begin Hello end procedure TForml Button2Click Sender TObject var Essential Pascal Copyright 1995 2003 Marco Cant www marcocantu com epascal 48 X Y Integer begin X Double StrToInt Editl Text Y Double X ShowMessage IntToStr Y end For the moment don t care about the syntax of the two procedures above which are actually methods Simply place two buttons on a Delphi form click on them at design time and the Delphi IDE will generate the proper support code Now you simply have to fill in the lines between begin and end To compile the code above you need to add also an Edit control to the form Now we can get back to the encapsulation code concept I ve introduced before When you call the Double function you don t need to know the algorithm used to implement it If you later find out a better way to double numbers
70. es must be declared using the far directive in order to be used as actual values of a procedural type Procedural types can be used for two different purposes you can declare variables of a procedural type or pass a procedural type that is a function pointer as parameter to another routine Given the preceding type and procedure declarations you can write this code var IP IntProc X Integer begin IP DoubleTheValue X 5 IP X end This code has the same effect as the following shorter version var X Integer begin X 5 DoubleTheValue X end The first version is clearly more complex so why should we use it In some cases being able to decide which function to call and actually calling it later on can be useful It is possible to build a complex example showing this approach However prefer to let you explore a fairly simple one named ProcType This example is more complex than those we have seen so far to make the situation a little more realistic Simply create a blank project and place two radio buttons and a push button as shown in Figure 6 3 This example is based on two procedures One procedure is used to double the value of the parameter This procedure is similar to the version I ve already shown in this section A Essential Pascal Copyright 1995 2003 Marco Cantu www marcocantu com epascal 56 second procedure is used to triple the value of the parameter and therefore is named TripleT
71. esources of the program in a string table To see this capability in action you can look at the ResStr example which has a button with the following code resourcestring AuthorName Marco Cant BookName Essential Pascal procedure TForml ButtonlClick Sender TObject begin ShowMessage BookName 13 AuthorName end The output of the two strings appears on separate lines because the strings are separated by the newline character indicated by its numerical value in the 13 character type constant Essential Pascal Copyright 1995 2003 Marco Cantu www marcocantu com epascal 18 Starting with Delphi 6 you can use the sLineBreak predefined string for Windows Linux compatibility The interesting aspect of this program is that if you examine it with a resource explorer there is one available among the examples that ship with Delphi you ll see the new strings in the resources This means that the strings are not part of the compiled code but stored in a separate area of the executable file the EXE file Finally notice that although this might sound odd resource strings are available also on Kylix In short the advantage of resources is in an efficient memory handling performed by Windows and in the possibility of localizing a program translating the strings to a different language without having to modify its source code Data Types In Pascal there are several predefined data types which can be div
72. form classes are declared in the same way so that you can refer to a form and its public fields methods properties and components from the code of any other form Of course it s poor programming practice to declare everything as global Besides the obvious memory consumption problems using global variables makes a program less easy to maintain and update In short you should use the smallest possible number of global variables Units as Namespaces The uses statement is the standard technique to access the scope of another unit At that point you can access the definitions of the unit But it might happen that two units you refer to declare the same identifier that is you might have two classes or two routines with the same name In this case you can simply use the unit name to prefix the name of the type or routine defined in the unit For example you can refer to the ComputeTotal procedure defined in the given Totals unit as Totals ComputeTotal This should not be required very often as you are strongly advised against using the same name for two different things in a program However if you look into the VCL library and the Windows files you ll find that some Delphi functions have the same name as but generally different parameters than some Windows API functions available in Delphi itself An example is the simple Beep procedure If you create a new Delphi program add a button and write the following code procedure TForml But
73. g of files using the traditional Pascal language approach is certainly still an interesting technique a strongly urge you to use streams the rst ream and derived classes to handle any complex files Streams represent virtual files which can be mapped to physical files to a memory block to a socket or any other continuous series of bytes You can find more on streams in the Delphi help file and in my Mastering Delphi book Conclusion At least for the moment this chapter on files is the last of the book Feel free to send me feedback as suggested in the introduction newsgroup or email sending me your comment and requests If after this introduction on the Pascal language you want to delve into the object oriented elements of Object Pascal in Delphi you can refer to my printed book of the series Mastering Delphi Sybex As mentioned earlier buying one of my printed books possibly through the Amazon link on my web site and donating me some cash for this book is the best way to support its future development so that other programmers can benefit from this effort Helping me with the update providing corrections and suggestions is another very good way to contribute For more information on the latest edition of Mastering Delphi and more advanced books of mine and of other authors as well you can refer to my web site www marcocantu com The same site hosts updated versions of this book and its examples Keep also an eye for the co
74. ge ne of the peculiarities of Pascal compared with other programming languages is its built in support for files As you might recall from Chapter 2 the language has a file keyword which is a type specifier like array Or record You use file to define a new type and then you can use the new data type to declare new variables type IntFile file of Integers var IntFilel IntFile It is also possible to use the file keyword without indicating a data type to specify an untyped file Alternatively you can use the TextFile type defined in the System units to declare files of ASCII characters Each kind of file has its own predefined routines as we will see later in this chapter Routines for Working with Files Once you have declared a file variable you can assign it to a real file in the file system using the Assignriie method The next step is usually to call Reset to open the file for reading at the beginning Rewrite to open or create it for writing and Appena to add new items to the end of the file without removing the older items Once the input or output operations are done you should call closeFile As an example look at the following code which simply saves some numbers to a file type IntFile file of Integers var IntFilel IntFile begin AssignFile IntFilel c tmp test my Rewrite IntFilel Write IntFilel 1 Write IntFilel 2 CloseFile IntFilel end The CloseFile operation shou
75. he VCL Visual Component Library variants are basically used for OLE support and for accessing database fields Variants in Depth Delphi includes a variant record type TVarData which has the same memory layout as the Variant type You can use this to access the actual type of a variant The TVarData structure includes the type of the Variant indicated as VType some reserved fields and the actual value The possible values of the VType field correspond to the data types you can use in OLE automation which are often called OLE types or variant types Here is a complete alphabetical list of the available variant types varArray varBoolean varByRef varCurrency varDate varDispatch varDouble varEmpty varError varlInteger varNull varOleStr varSingle varSmallint varString varTypeMask varUnknown varVariant You can find descriptions of these types in the Values in variants topic in the Delphi Help system There are also many functions for operating on variants that you can use to make specific type conversions or to ask for information about the type of a variant see for example the VarType function Most of these type conversion and assignment functions are actually called automatically when you write expressions using variants Other variant support routines look for the topic Variant support routines in the Help file actually operate on variant arrays Essential Pascal Copyright 1995 2003 Marco Cant www marcocantu com epasca
76. heValue ima Proc Type FiGuRE 6 3 THE FORM OF THE PROC TYPE EXAMPLE procedure TripleTheValue var Value Integer begin Value Value 3 ShowMessage Value tripled IntToStr Value end Both procedures display what is going on to let us know that they have been called This is a simple debugging feature you can use to test whether or when a certain portion of code is executed instead of adding a breakpoint in it Each time a user presses the Apply button one of the two procedures is executed depending on the status of the radio buttons In fact when you have two radio buttons in a form only one of them can be selected at a time This code could have been implemented by testing the value of the radio buttons inside the code for the OnClick event of the Apply button To demonstrate the use of procedural types I ve instead used a longer but interesting approach Each time a user clicks on one of the two radio buttons one of the procedures is stored in a variable procedure TForml DoubleRadioButtonClick Sender TObject begin IP DoubleTheValue end When the user clicks on the push button the procedure we have stored is executed procedure TForml ApplyButtonClick Sender TObject begin IP X end To allow three different functions to access the IP and X variables we need to make them visible to the whole form they cannot be declared locally inside one of the methods A solution to this probl
77. hich uses pointers behind the scenes Although you don t use pointers often in Delphi you do frequently use a very similar construct namely references Every object instance is really an implicit pointer or reference to its actual data However this is completely transparent to the programmer who uses object variables just like any other data type Notice also that pointers support will be severely limited if not abolished in the NET version of the Delphi compiler due to the restrictions imposed by the platform to the so called safe code File Types Another Pascal specific type constructor is the file type File types represent physical disk files certainly a peculiarity of the Pascal language You can define a new file data type as follows i IntFile file of Integer Then you can open a physical file associated with this structure and write integer values to it or read the current values from the file Essential Pascal Copyright 1995 2003 Marco Cant www marcocantu com epascal 37 Files based examples are covered in a specific chapter The use of files in Pascal is quite straightforward but in Delphi there are also some components that are capable of storing or loading their contents to or from a file There is some serialization support in the form of streams and there is also database support Conclusion This chapter discussing user defined data types complete our coverage of Pascal type system Now we are read
78. hmetic data types are almost interchangeable and much stronger than in the original versions of BASIC which had no similar concept Variables Pascal requires all variables to be declared before they are used Every time you declare a variable you must specify a data type Here are some sample variable declarations var Value Integer IsCorrect Boolean A B Char The var keyword can be used in several places in the code such as at the beginning of the code of a function or procedure to declare variables local to the routine or inside a unit to declare global variables After the var keyword comes a list of variable names followed by a colon and the name of the data type You can write more than one variable name on a single line as in the last statement above Once you have defined a variable of a given type you can perform on it only the operations supported by its data type For example you can use the Boolean value in a test and the integer value in a numerical expression You cannot mix Booleans and integers as you can with the C language Using simple assignments we can write the following code value 10 IsCorrect True But the next statement is not correct as the two variables have different data types value IsCorrect error If you try to compile this code Delphi issues a compiler error with this description Incompatible types Integer and Boolean Usually errors like this are programming e
79. ibraries tend to use class types instead of record types but there are many record types defined by the Windows API Record types can also have a variant part that is multiple fields can be mapped to the same memory area even if they have a different data type This corresponds to a union in the C language Alternatively you can use these variant fields or groups of fields to access the same memory location within a record but considering those values from different perspectives The main uses of this type were to store similar but different data and to obtain an effect similar to that of typecasting something less useful now that typecasting has been introduced also in Pascal The use of variant record types has been largely replaced by object oriented and other modern techniques although Delphi uses them in some peculiar cases The use of a variant record type is not type safe and is not a recommended programming practice particularly for beginners Expert programmers can indeed use variant record types and the core of the Delphi libraries makes use of them You won t need to tackle them until you are really a Delphi expert anyway Pointers A pointer type defines a variable that holds the memory address of another variable of a given data type or an undefined type So a pointer variable indirectly refers to a value The definition of a pointer type is not based on a specific keyword but uses a special character instead This speci
80. ic notation when indicating letters digits or symbols When referring to special characters instead you ll generally use the numeric notation The following list includes some of the most commonly used special characters carriage return enter key Essential Pascal Copyright 1995 2003 Marco Cant www marcocantu com epascal 20 The Range Example To give you an idea of the different ranges of some of the ordinal types I ve written a simple Delphi program named Range The Range program is based on a simple form which has Six buttons each named after an ordinal data type and some labels for categories of information as you can see in Figure 3 1 Some of the labels are used to hold static text others to show the information about the type each time one of the buttons is pressed FIGURE 3 1 THE RANGE EXAMPLE DISPLAYS SOME INFORMATION ABOUT ORDINAL DATA TYPES INTEGERS IN THIS CASE ai Range L OL x Type Integer Short Int Byte Size 4 Small Int Word Max 2147483647 Min 2147483648 eee eee Every time you press one of the buttons on the right the program updates the labels with the output Different labels show the data type number of bytes used and the maximum and minimum values the data type can store Each button has its own OnClick event response method because the code used to compute the three values is slightly different from button to button For example here is the source code of the OnClick event for
81. ic subtraction set difference pointer offset subtraction Testwhetherequal S O Set Operators The set operators include union difference intersection membership test in plus some relational operators To add an element to a set you can make the union of the set with another one that has only the element you need Here s a Delphi example related to font styles Style Style fsBold Style Style fsBold fsItalic fsUnderline As an alternative you can use the standard Include and Exclude procedures which are much more efficient but cannot be used with component properties of the set type because they require an l value parameter Include Style fsBold Conclusion Now that we know the basic layout of a Pascal program we are ready to start understanding its meaning in detail We ll start by exploring the definition of predefined and user defined data types then we ll move along to the use of the keywords to form programming statements Essential Pascal Copyright 1995 2003 Marco Cant www marcocantu com epascal 16 Chapter 3 Types Variables and Constants he original Pascal language was based on some simple notions which have now become quite common in programming languages The first is the notion of data type The type determines the values a variable can have and the operations that can be performed on it The concept of type is stronger in Pascal than in C where the arit
82. ided into three groups ordinal types real types and strings We ll discuss ordinal and real types in the following sections while strings are covered later in this chapter In this section I ll also introduce some types defined by the Delphi libraries not predefined by the compiler which can be considered predefined types Delphi also includes a non typed data type called variant and discussed in Chapter 10 of this book Strangely enough a variant is a type without proper type checking It was introduced in Delphi 2 to handle OLE Automation Ordinal Types Ordinal types are based on the concept of order or sequence Not only can you compare two values to see which is higher but you can also ask for the value following or preceding a given value or compute the lowest or highest possible value The three most important predefined ordinal types are Integer Boolean and Char character However there are a number of other related types that have the same meaning but a different internal representation and range of values The following Table 3 1 lists the ordinal data types used for representing numbers TaBLE 3 1 ORDINAL DATA TYPES FOR NUMBERS 8 bits Shortlnt 128 to 127 Byte 0 to 255 Signed Range Unsigned Range 16 bits Smalllnt 32768 to 32767 Word 0 to 65 535 32 bits LongInt 2 147 483 648 to 2 147 483 647 LongWord since Delphi 4 0 to 4 294 967 295 As you can see these types correspond to different representations
83. in the code above the code is even simpler because there is no need to define a temporary string and initialize it The following line of code passes the Caption property of a label as a parameter to an API function simply by typecasting it to PChar SetWindowText Handle PChar Labell Caption When you need to cast a WideString to a Windows compatible type you have to use PWideChar instead of PChar for the conversion Wide strings are often used for OLE and COM programs Having presented the nice picture now want to focus on the pitfalls There are some problems that might arise when you convert a long string into a PChar Essentially the underlying problem is that after this conversion you become responsible for the string and its contents and Delphi won t help you anymore Consider the following limited change to the first program code fragment above Button1Click procedure TForml Button2Click Sender TObject var Sl String begin SetLength S1 100 GetWindowText Handle PChar S1 Length S1 S1 S1 is the title this won t work Buttonl Caption S1 end This program compiles but when you run it you are in for a surprise The Caption of the button will have the original text of the window title without the text of the constant string you have added to it The problem is that when Windows writes to the string within the GetWindowText API call it doesn t set the length of the long Pascal string
84. ion example want to be able to sum values of different types transforming strings to integers characters to the corresponding order value and adding 1 for Essential Pascal Copyright 1995 2003 Marco Cant www marcocantu com epascal 52 True Boolean values The code is based on a case statement and is quite simple although we have to dereference pointers quite often function SumAll const Args array of const Extended var I Integer begin Result 0 for I Low Args to High Args do case Args I VType of vtInteger Result Result Args I VInteger vtBoolean if Args I VBoolean then Result Result 1 vtChar Result Result Ord Args I VChar vtExtended Result Result Args I VExtended vtString vtAnsiString Result Result StrToIntDef Args I VString 0 vtWideChar Result Result Ord Args I VWideChar vtCurrency Result Result Args I VCurrency end case end I ve added this code to the OpenArr example which calls the SumAll function when a given button is pressed procedure TForml Button4Click Sender TObject var X Extended Y Integer begin Y 10 X SumAll Y Y k True 10 34 99999 ShowMessage Format SumAll Y Y k True 10 34 99999 gt n X1 end You can see the output of this call and the form of the OpenArr example in Figure 6 2 Essen
85. is code with Forml Canvas Pen do begin Width 2 Color clRed end When you are writing complex code the with statement can be effective and spares you the declaration of some temporary variables but it has a drawback It can make the code less readable particularly when you are working with different objects that have similar or corresponding properties A further drawback is that using the with statement can allow subtle logical errors in the code that the compiler will not detect For example with Buttonl do begin Width 200 Caption New Caption Color clRed end This code changes the Caption and the Width of the button but it affects the Color property of the form not that of the button The reason is that the TButton components don t have the Color property and since the code is executed for a form object we are writing a method of the form this object is accessed by default If we had instead written Buttonl Width 200 Buttonl Caption New Caption Essential Pascal Copyright 1995 2003 Marco Cant www marcocantu com epascal 46 Buttonl Color clRed error the compiler would have issued an error In general we can say that since the with statement introduces new identifiers in the current scope we might hide existing identifiers or wrongfully access another identifier in the same scope as in the first version of this code fragment Even considering this kind of drawback suggest
86. ively calling itself forever not because the stack space is too limited Essential Pascal Copyright 1995 2003 Marco Cant www marcocantu com epascal 92 More Terms for the Glossary TODO Finish glossary It is really incomplete 4 Dynamic Static 4 Virtual memory leak 4 painting lt s literal es array API Class reference 4 class method parent zs owner a Self Essential Pascal Copyright 1995 2003 Marco Cant www marcocantu com epascal 93 Appendix B Examples This is a list of the examples which are part of Essential Pascal and available for download Chapter 3 ResStr resource strings Range ordinal types ranges TimeNow time manipulation Chapter 4 GPF general protection faults with null pointers Chapter 5 IfTest if statements Loops for and while statements Chapter 6 OpenArr open array parameters DoubleH simple procedures ProcType procedural types OverDef overloading and default parameters Chapter 7 StrRef strings reference counting LongStr using long strings FmtTest formatting examples Chapter 8 DynArr dynamic arrays WHandle Windows handles Callback Windows callback functions StrParam command line parameters Chapter 10 VariTest simple variant operations VariSpeed the speed of variants Essential Pascal Copyright 1995 2003 Marco Cant www marcocantu com epascal 94
87. l 79 Variants Are Slow Code that uses the Variant type is slow not only when you convert data types but also when you add two variant values holding an integer each They are almost as slow as the interpreted code of Visual Basic To compare the speed of an algorithm based on variants with that of the same code based on integers you can look at the VSpeed example This program runs a loop timing its speed and showing the status in a progress bar Here is the first of the two very similar loops based on integers and variants procedure TForml ButtonlClick Sender TObject var timel time2 TDateTime nl n2 Variant begin timel Now nl 0 n2 05 ProgressBarl Position 0 while n1 lt 5000000 do begin n2 n2 nl Inc n1 if nl mod 50000 0 then begin ProgressBarl Position nl div 50000 Application ProcessMessages end end we must use the result Total n2 time2 Now Labell Caption FormatDateTime n ss Time2 Timel seconds end The timing code is worth looking at because it s something you can easily adapt to any kind of performance test As you can see the program uses the Now function to get the current time and the FormatDateTime function to output the time difference asking only for the minutes n and the seconds ss in the format string As an alternative you can use the Windows API s GetTickCount function which returns a very precise indication of the mil
88. l definition of the main form object FormWHandle TFormWHandle Caption Window Handle OnCreate FormCreate object BtnCallAPI TButton Caption Call API OnClick BtnCallAPIClick end end As soon as the form is created the program retrieves the handle of the window corresponding to the form by accessing the Handle property of the form itself We call IntToStr Essential Pascal Copyright 1995 2003 Marco Cant www marcocantu com epascal 72 to convert the numeric value of the handle into a string and we append that to the caption of the form as you can see in Figure 9 1 procedure TFormWHandle FormCreate Sender TObject begin Caption Caption IntToStr Handle end Because FormCreate is a method of the form s class it can access other properties and methods of the same class directly Therefore in this procedure we can simply refer to the Caption of the form and its Handle property directly J Window Handle 672 Iof x FiGuRE 9 1 THE WHANDLE EXAMPLE SHOWS THE HANDLE OF THE FORM WINDOW EVERY TIME YOU RUN THIS PROGRAM YOU LL GET A DIFFERENT VALUE If you run this program several times you ll generally get different values for the handle This value in fact is determined by Windows and is sent back to the application Handles are never determined by the program and they have no predefined values they are determined by the system which generates new values each time you run a program Whe
89. lass a topic related to object oriented programming and discussed in Mastering Delphi 4 The next three statements add three items to the ListBox component on the left of the form with the result you can see in Figure 3 3 The first line contains the time portion of the TDateTime value converted into a string the second the date portion of the same value At the end the code adds a simple reminder FIGURE 3 3 THE OUTPUT OF THE TIMENOW EXAMPLE AT STARTUP J Tinc Now Piel ES 308 41 PM i 2 15 99 i Elapsed j Press bullon for elapsed lime This third string is replaced by the program when the user clicks on the Elapsed button procedure TFormTimeNow ButtonElapsedClick Sender TObject var StopTime TDateTime begin StopTime Now ListBoxl Items 2 FormatDateTime hh nn ss StopTime StartTime end This code retrieves the new time and computes the difference from the time value stored when the program started Because we need to use a value that we computed in a different event handler we had to store it in a global variable There are actually better alternatives based on classes Essential Pascal Copyright 1995 2003 Marco Cant www marcocantu com epascal 25 The code that replaces the current value of the third string uses the index 2 The reason is that the items of a list box are zero based the first item is number 0 the second number 1 and the third number 2 More on this as
90. ld typically be done inside a finally block to avoid leaving the file open in case the file handling code generates an exception Actually file based operations generate exceptions or not depending on the 1 compiler settings In case the system doesn t raise exceptions you can check the IOResult global variable to see if anything went wrong Delphi includes many other file management routines some of which are includes in the list below Append FileClose Flush AssignFile FileCreate GetDir BlockRead FileDateToDateTime OResult BlockWrite FileExists MkDir Essential Pascal Copyright 1995 2003 Marco Cantu www marcocantu com epascal 86 ChangeFileExt FileGetAttr Read CloseFile FileGetDate Readln DateTimeToFileDate FileOpen Rename DeleteFile FilePos RenameFile DiskFree FileRead Reset DiskSize FileSearch Rewrite Eof FileSeek RmDir Eoln FileSetAttr Seek Erase FileSetDate SeekEof ExpandFileName FileSize SeekEoln ExtractFileExt FileWrite SetTextBuf ExtractFileName FindClose Truncate ExtractFilePath FindFirst Write FileAge FindNext Writeln Not all of these routines are defined in standard Pascal but many of them have been part of Borland Pascal for a long time You can find detailed information about these routines in Delphi s Help files Here I ll show you three simple examples to demonstrate how these features can be used Handling Text Files One of the most commonly used file formats is that of text files As mentioned befo
91. library also uses them in ways not related to OLE programming Variants Have No Type In general you can use variants to store any data type and perform numerous operations and type conversions Notice that this goes against the general approach of the Pascal language and against good programming practices A variant is type checked and computed at run time The compiler won t warn you of possible errors in the code which can be caught only with extensive testing On the whole you can consider the code portions that use variants to be interpreted code because as with interpreted code many operations cannot be resolved until run time This affects in particular the speed of the code Now that I ve warned you against the use of the Variant type it is time to look at what it can do Basically once you ve declared a variant variable such as the following var V Variant you can assign to it values of several different types V 10 V Hello World V 45 55 Once you have the variant value you can copy it to any compatible or incompatible data type If you assign a value to an incompatible data type Delphi performs a conversion if it can Otherwise it issues a run time error In fact a variant stores type information along with the data allowing a number of run time operations these operations can be handy but are both slow and unsafe Consider the following example called VariTest which is an extension of the code
92. liseconds elapsed since the operating system was started In this example the speed difference is actually so great that you ll notice it even without a precise timing Anyway you can see the results for my own computer in Figure 10 2 The actual values depend on the computer you use to run this program but the proportion won t change much Essential Pascal Copyright 1995 2003 Marco Cant www marcocantu com epascal 80 Variants Speed Test Figure 10 2 THE DIFFERENT SPEEDS OF THE SAME ALGORITHM BASED ON INTEGERS AND VARIANTS THE ACTUAL TIMING VARIES DEPENDING ON THE COMPUTER AS SHOWN BY THE VSPEED EXAMPLE Conclusion Variants are so different from traditional Pascal data types that I ve decided to cover them in this short separate chapter Although their role is in OLE programming they can be handy to write quick and dirty programs without having even to think about data types As we have seen this affects performance by far Now that we have covered most of the language features let me discuss the overall structure of a program and the modularization offered by units Essential Pascal Copyright 1995 2003 Marco Cantu www marcocantu com epascal 81 Chapter 11 Program and Units Delphi applications make intensive use of units or program modules Units in fact were the basis of the modularity in the language before classes were introduced In a Delphi application every form has a corresponding unit behind it When you a
93. ly use the Caption property of the form object itself There is no need to write all this confusing code which was intended only to demonstrate the string conversion problems There are practical cases when you need to call Windows API functions and then you have to consider this complex situation Formatting Strings Using the plus operator and some of the conversion functions such as IntToStr you can indeed build complex strings out of existing values However there is a different approach to formatting numbers currency values and other strings into a final string You can use the powerful Format function or one of its companion functions The Format function requires as parameters a string with the basic text and some placeholders usually marked by the 96 symbol and an array of values one for each placeholder For example to format two numbers into a string you can write Format First d Second d nl n2 where n1 and n2 are two Integer values The first placeholder is replaced by the first value the second matches the second and so on If the output type of the placeholder indicated by the letter after the symbol doesn t match the type of the corresponding parameter a runtime error occurs Having no compile time type checking is actually the biggest drawback of using the Format function The Format function uses an open array parameter a parameter that can have an arbitrary number of values something I ll discus
94. meters procedure MessBox Msg string Caption string Warning Flags LongInt mb OK or mb IconHand begin Application MessageBox PChar Msg PChar Caption Flags end With this definition we can call the procedure in each of the following ways MessBox Something wrong here MessBox Something wrong here Attention MessBox Hello Message mb OK In Figure 6 5 you can see that Delphi s Code Parameters properly use a different style to indicate the parameters that have a default value so you can easily determine which parameters can be omitted 3 OverDefF pas m xi sU Fin id Bad sh uil C9 Ritani pruvedur amp e TP r ButLurSllick erdzi TOL jzcL a Autant lick negin Y Buton 3h2vuMs3 EBgtzs a Butong lick cend Button3 T th Button3 lick primemlbuare TP cn cFetdbiur4T7 icki erd i TO jan is Buttons begin hh Buttond lick Sh2vMs3 Iotal amp d 1 U Ep Buttons ES amp Bulton3 lick i C9 Buttons uM 7 x M Button lick prneedurP Thorn Rrrrnrks2 iek er zr TOJNT lH Proced res eon f ow Usse McasaBoxk Bowcthinz cron kecel j end Msa String ephon Sting aming fleas Integer 1C procedure II rx Burcorsllick zerdsr Trcjzer xj n We 13 Modhcd Inscit vA Figure 6 5 DELPHI S CoDE PARAMETERS MARK OUT WITH SQUARE BRACKETS THE PARAMETERS THAT HAVE DEFAULT VALUES YOU CAN OMIT THESE IN THE CALL Notice that Delphi doesn
95. mpanion book Essential Delphi Happy coding Essential Pascal Copyright 1995 2003 Marco Cant www marcocantu com epascal 91 Appendix A Glossary his is a short glossary of technical terms used throughout the book They might also be defined elsewhere in the text but I ve decided to collect them here anyway to make it easier to find them Heap Memory The term Heap indicates a portion of the memory available to a program also called dynamic memory area The heap is the area in which the allocation and deallocation of memory happens in random order This means that if you allocate three blocks of memory in sequence they can be destroyed later on in any order The heap manager takes care of all the details for you so you simply ask for new memory with GetMem or by calling a constructor to create an object and Delphi will return you a new memory block optionally reusing memory blocks already discarded The heap is one of the three memory areas available to an application The other two are the global area this is where global variables live and the stack Contrary to the heap global variables are allocated when the program starts and remain there until it terminates For the stack see the specific entry in this glossary Delphi uses the heap for allocating the memory of each and every object the text of the strings for dynamic arrays and for specific requests of dynamic memory GetMem Windows allows an application to have
96. ms a given action over a number of internal elements of the system such as all of the windows of a certain kind Such a function also called an enumerated function requires as a parameter the action to be performed on each of the elements which is passed as a function or procedure compatible with a given procedural type Windows uses callback functions in other circumstances but we ll limit our study to this simple case Now consider the EnumWindows API function which has the following prototype copied from the Win32 Help file BOOL EnumWindows WNDENUMPROC lpEnumFunc address of callback function LPARAM lParam application defined value Of course this is the C language definition We can look inside the Windows unit to retrieve the corresponding Pascal language definition function EnumWindows lpEnumFunc TFNWndEnumProc lParam LPARAM BOOL stdcall Consulting the help file we find that the function passed as a parameter should be of the following type again in C BOOL CALLBACK EnumWindowsProc HWND hwnd handle of parent window LPARAM lParam application defined value This corresponds to the following Delphi procedural type definition type EnumWindowsProc function Hwnd THandle Param Pointer Boolean stdcall The first parameter is the handle of each main window in turn while the second is the value we ve passed when calling the EnumWindows function Actu
97. n be used inside loops or in other particular cases Although these final semicolons serve no purpose tend to use them and suggest you do the same Sometimes after you ve written a couple of lines you might want to add one more statement If the last semicolon is missing you should remember to add it so it might be better to add it in the first place Assignment Statements Assignments in Pascal use the colon equal operator an odd notation for programmers who are used to other languages The operator which is used for assignments in some other languages in Pascal is used to test for equality By using different symbols for an assignment and an equality test the Pascal compiler like the C compiler can translate source code faster because it doesn t need to examine the context in which the operator is used to determine its meaning The use of different operators also makes the code easier for people to read Essential Pascal Copyright 1995 2003 Marco Cantu www marcocantu com epascal 39 Conditional Statements A conditional statement is used to execute either one of the statements it contains or none of them depending on some test There are two basic flavors of conditional statements if statements and case statements If Statements The if statement can be used to execute a statement only if a certain condition is met if then or to choose between two different alternatives if then else The condition is describe
98. n its scope that is only within the block in which it is declared You cannot use an identifier outside its scope Here are some examples Global hidden variables If you declare an identifier in the implementation portion of a unit you cannot use it outside the unit but you can use it in any block and procedure defined within the unit The memory for this variable is allocated as soon as the program starts and exists until it terminates You can use the initialization section of the unit to provide a specific initial value Local variables f you declare a variable within the block defining a routine or a method you cannot use this variable outside that procedure The scope of the identifier spans the whole procedure including nested routines unless an identifier with the same name in the nested routine hides the outer definition The memory for this variable is allocated on the stack when the program executes the routine defining it As soon as the routine terminates the memory on the stack is automatically released Global variables f you declare an identifier in the interface portion of the unit its scope extends to any other unit that uses the one declaring it This variable uses memory and has the same lifetime as the previous group the only difference is in its visibility Any declarations in the interface portion of a unit are accessible from any part of the program that includes the unit in its uses clause Variables of
99. n the user presses the button the program simply calls a Windows API function SetWindowText which changes the text or caption of the window passed as the first parameter To be more precise the first parameter of this API function is the handle of the window we want to modify procedure TFormWHandle BtnCallAPIClick Sender TObject begin SetWindowText Handle Hi end This code has the same effect as the previous event handler which changed the text of the window by giving a new value to the Caption property of the form In this case calling an API function makes no sense because there is a simpler Delphi technique Some API functions however have no correspondence in Delphi as we ll see in more advanced examples later in the book External Declarations Another important element for Windows programming is represented by external declarations Originally used to link the Pascal code to external functions that were written in assembly language the external declaration is used in Windows programming to call a function from a DLL a dynamic link library In Delphi there are a number of such declarations in the Windows unit forward declaration function LineTo DC HDC X Y Integer BOOL stdcall external declaration instead of actual code function LineTo external gdi32 dll name LineTo This declaration means that the code of the function LineTo is stored in the GDI32 DLL dynamic library one of the mos
100. nes may get broken at any position even in the middle of a word when you print them When a function or procedure has several parameters it is common practice to place the parameters on different lines You can leave a line completely white blank before a comment or to divide a long piece of code in smaller portions Even this simple idea can improve the readability of the code both on screen and when you print it Use spaces to separate the parameters of a function call and maybe even a space before the initial open parenthesis Also keep operands of an expression separated know that some programmers will disagree with these ideas but insist Spaces are free you don t pay for them OK know that they use up disk space and modem connection time when you upload or download a file but this is less and less relevant nowadays Essential Pascal Copyright 1995 2003 Marco Cantu www marcocantu com epascal 9 Pretty Printing The last suggestion on the use of white spaces relates to the typical Pascal language formatting style known as pretty printing This rule is simple Each time you need to write a compound statement indent it two spaces to the right of the rest of the current statement A compound statement inside another compound statement is indented four spaces and so on lif then statement Dif then begin statementl statement2 end if then begin if then statementl statement
101. ng COBSEOEIES ua utumursvuso pp RpRUnd Qux E xe oto PEE tnt rt E AEEA E CREDAS SUA GC Doqp SE ER RdK a EU UU pM 19 bro e 20 Ordinal Types merean LUE TIE m 20 Real Type EU T m 24 Date and TIME Em 25 Specific Windows TY DGS snscuidsecehivehenssccinnnmasesibsctacaantneoamninelaaeh atatdenduaonsuandsianpeekodeenenindacdeias 27 Typecasting and Type CODVOP SIDES oasiesbpMev ay deu npn Nus plu ttnt rtrt inr n EEE E py pn Es UD PEU ED Np EEEE 27 CONCUSSION NET P H 29 Chapter 4 User Defined Data Types oisinsasausxu nua E nune xuEH RE a EE E NEN EUER ER YNRNENENEEAE RYREEXEER Aa 30 Named and Unnamed Types ssceseiuaxvuteddRu ent uruEE tnt t trt r ttnt ducis cREd cevet aei pP FERE MR UN DH EEEE 30 S brang TYPES TE 31 E merated Types ous dados eR uox a a a e TAEA AAEE 31 Set Type P a Eaa a 33 Amay TYPES ere enoni nanoa e aa E a aaea E aae aa 34 hen c srana aanraai iia EA iaaa pik EENE E Eaa EEEE NE Aaaa inii 35 PONCE Rc 36 File Type C M H 38 COMCIISION TTE L PH 38 Chapter 5 Statements voco cirirkxu se cis dade svsgudusnda saura F es nao EN Ea sE XR rA ET didide i Hikana nsspansi 39 Simple and Compound Staterientbs suu ios ense ierra prora rper ne d
102. nges and updates Donations Although the book is available for free to everyone if you benefit from reading it from a business perspective that is if Pascal programming is part of your job and if you can afford it because you live in a wealthy country ask yo to donate a small sum to me as live on writing and training on programming languages and tool Donation information is available on my web site The currently suggested price is 5 Euros which at the time I m writing this is just a little over 5 US Dollars An alternative way of supporting me is to buy one of my books but in that case I ll get only a limited fraction of the money you spend which is the beauty of publishing an ebook Coming to a seminar of mine or offering me consulting work is another interesting payback Essential Pascal Copyright 1995 2003 Marco Cantu www marcocantu com epascal 2 Notice that the money earned will allow me to pay for a professional editor the web site itself and of course the time devoted to update the book Table of Contents Hpsors Psi e 2 Note to the Second Edition uoctiebpt vs bise tb pepupra i DbbpgR biu Re aab Na MaPl ME mie PM aM CREDENS 2 COP VIII iann osn ea a a ai aeania monomer i anai 2 Donatos a ssesasi krenio eai o aana hae aaa a ii derie ia ianea ienna 2 The Book Str ct re aoussstansisicicsindsauvideauenaagedindst aaa aa E EEE aAa aE 3 cuis TM 5 Feedback
103. o refer to a record type variable or an object instead of repeating its name every time you can use a with statement For example while presenting the record type wrote this code type Date record Year Integer Month Byte Day Byte end var BirthDay Date begin BirthDay Year 1997 BirthDay Month 2 BirthDay Day 14 Using a with statement can improve the final part of this code as follows with BirthDay do Essential Pascal Copyright 1995 2003 Marco Cantu www marcocantu com epascal 45 begin Year 1995 Month 2 Day 14 end This approach can be used in Delphi programs to refer to components and other class types For example we can rewrite the final part of the last example Loops using a with statement to access the items of the list box procedure TForml WhileButtonClick Sender TObject var I Integer begin with ListBoxl Items do begin Clear shortcut Randomize I 0 while I lt 1000 do begin I I Random 100 shortcut Add Random Number IntToStr 1 end end end When you work with components or classes in general the with statement allows you to skip writing some code particularly for nested fields For example suppose that you need to change the Width and the Color of the drawing pen for a form You can write the following code Forml Canvas Pen Width 2 Forml Canvas Pen Color clRed But it is certainly easier to write th
104. of numbers depending on the number of bits used to express the value and the presence or absence of a sign bit Signed values can be positive or negative but have a smaller range of values because Essential Pascal Copyright 1995 2003 Marco Cant www marcocantu com epascal 19 one less bit is available for the value itself You can refer to the Range example discussed in the next section for the actual range of values of each type The last group marked as 16 32 indicates values having a different representation in the 16 bit and 32 bit versions of Delphi Integer and Cardinal are frequently used because they correspond to the native representation of numbers in the CPU I ntegral Types In Delphi 2 and 3 the 32 bit unsigned numbers indicated by the Cardinal type were actually 31 bit values with a range up to 2 gigabytes Delphi 4 introduced a new unsigned numeric type LongWord which uses a truly 32 bit value up to 4 gigabytes The Cardinal type is now an alias of the LongWord type LongWord permits 2GB more data to be addressed by an unsigned number as mentioned above Moreover it corresponds to the native representation of numbers in the CPU Another new type introduced in Delphi 4 is the Int64 type which represents integer numbers with up to 18 digits This new type is fully supported by some of the ordinal type routines such as High and Low numeric routines such as Inc and Dec and string conversion routines such as In
105. on is a list of values Here are some examples type Colors Red Yellow Green Cyan Blue Violet Suit Club Diamond Heart Spade Each value in the list has an associated ordinality starting with zero When you apply the Ord function to a value of an enumerated type you get this zero based value For example Ord Diamond returns 1 Enumerated types can have different internal representations By default Delphi uses an 8 bit representation unless there are more than 256 different values in which case it uses the 16 bit representation There is also a 32 bit representation which might be useful for compatibility with C or C libraries You can actually change the default behavior asking for a larger representation by using the Z compiler directive The Delphi VCL Visual Component Library uses enumerated types in many places For example the style of the border of a form is defined as follows type TFormBorderStyle bsNone bsSingle bsSizeable bsDialog bsSizeToolWin bsToolWindow When the value of a property is an enumeration you usually can choose from the list of values displayed in the Object Inspector as shown in Figure 4 1 FIGURE 4 1 AN ENUMERATED TYPE PROPERTY IN THE OBJECT INSPECTOR Object Inspector x Formi TFornt Properties Events BorderStyle bsSizeable Y bsDialog bsNone bsSinale Sizeable bsSizeT oolwin bsToolwindow Cursor The Delphi Help file
106. onvSymbols end Once again you can see the entire source code among the download files Before calling one of the conversion procedures the convertBitBtnciick method displays a dialog box ConvertForm with a ProgressBar component to show the user that the conversion is taking place This method does most of the work related to handling the files it opens the input file as a file of bytes a file storing data as plain bytes the first time so that it can use the FileSize procedure which is not available for text files Then this file is closed and reopened as a text file Since the program opens two files and each of these operations can fail it uses two nested try blocks to ensure a high level of protection although using the standard dialog boxes to select file names already provides a good confirmation of file selection Now let s take a look at one of the conversion routines in detail The simplest of the three conversion routines is ConvUpper Which converts every character in the text file to uppercase Here is its code Essential Pascal Copyright 1995 2003 Marco Cant www marcocantu com epascal 88 procedure TForml ConvUpper var Ch Char Position LongInt begin Position 0 while not Eof FileIn do begin Read FileIn Ch Ch UpCase Ch Write FileOut Ch Inc Position ConvertForm ProgressBarl Position Position 100 div FileLength Application ProcessMessages end end This method reads each
107. operators of the language grouped by precedence in Table 2 2 Contrary to most other programming languages the and and or operators have precedence compared to the relational one So if you write a b and c lt d the compiler will try to do the and operation first resulting in a compiler error For this reason you should enclose each of the expression in parentheses a lt b and c lt qd Some of the common operators have different meanings with different data types For example the operator can be used to add two numbers concatenate two strings make the union of two sets and even add an offset to a PChar pointer However you cannot add two characters as is possible in C Another strange operator is div In Pascal you can divide any two numbers real or integers with the operator and you ll invariably get a real number result If you need to divide two integers and want an integer result use the div operator instead TABLE 2 2 PAscAL LANGUAGE OPERATORS GROUPED BY PRECEDENCE Unary Operators Highest Precedence lo Address of the variable or function returns a pointer Boolean or bitwise not Essential Pascal Copyright 1995 2003 Marco Cantu www marcocantu com epascal 15 Multiplicative and Bitwise Operators Arithmetic multiplication or set intersection Floating point division Boolean or bitwise and Arithmetic addition set union string concatenation pointer offset addition arithmet
108. ort just the pre compiled interface portion of the units listed The implementation portion of the unit is considered only when that unit is compiled The units you refer to can be both in source code format PAS or compiled format DCU but the compilation must have taken place with the same version of the Delphi The interface of a unit can declare a number of different elements including procedures functions global variables and data types In Delphi applications the data types are probably used the most often Delphi automatically places a new class data type in a unit each time you create a form However containing form definitions is certainly not the only use for units in Delphi You can continue to have traditional units with functions and procedures and you can have units with classes that do not refer to forms or other visual elements Units and Scope In Pascal units are the key to encapsulation and visibility and they are probably even more important than the private and public keywords of a class In fact as we ll see in the next Essential Pascal Copyright 1995 2003 Marco Cant www marcocantu com epascal 83 chapter the effect of the private keyword is related to the scope of the unit containing the class The scope of an identifier such as a variable procedure function or a data type is the portion of the code in which the identifier is accessible The basic rule is that an identifier is meaningful only withi
109. ot actual function calls This compile time resolution of expressions and function calls happens also for many other simple system functions Delphi uses arrays mainly in the form of array properties We have already seen an example of such a property in the TimeNow example to access the Items property of a ListBox component I ll show you some more examples of array properties in the next chapter when discussing Delphi loops Delphi 4 introduced dynamic arrays into Object Pascal that is arrays that can be resized at runtime allocating the proper amount of memory Using dynamic arrays is easy but in this discussion of Pascal I felt they were not an proper topic to cover You can find a description of Delphi s dynamic arrays in Chapter 8 Essential Pascal Copyright 1995 2003 Marco Cant www marcocantu com epascal 34 Record Types Record types define fixed collections of items of different types Each element or field has its own type The definition of a record type lists all these fields giving each a name you ll use later to access it Here is a small listing with the definition of a record type the declaration of a variable of that type and few statements using this variable type Date record Year Integer Month Byte Day Byte end var BirthDay Date begin BirthDay Year 1997 BirthDay Month 2 BirthDay Day 14 Classes and objects can be considered an extension of the record type Delphi l
110. r var K I Integer begin K 0 for I 10 downto 1 do K K I The for loop in Pascal is less flexible than in other languages it is not possible to specify an increment different than one but it is simple and easy to understand If you want to test for a more complex condition or to provide a customized counter you need to use a while or repeat statement instead of a for loop The counter of a for loop doesn t need to be a number It can be a value of any ordinal type such as a character or an enumerated type While and Repeat Statements The difference between the while do loop and the repeat until loop is that the code of the repeat statement is always executed at least once You can easily understand why by looking at a simple example while I lt 100 and J lt 100 do begin use I and J to compute something I I 1 J Jg 41 end repeat use I and J to compute something Lss T 1 Essential Pascal Copyright 1995 2003 Marco Cant www marcocantu com epascal 42 J J 1 until I gt 100 or J gt 100 If the initial value of or J is greater than 100 the statements inside the repeat until loop are executed once anyway The other key difference between these two loops is that the repeat until loop has a reversed condition The loop is executed as long as the condition is not met When the condition is met the loop terminates This is the opposite from a while do loop
111. re Delphi has some specific support for text files most notably the TextFile data type defined by the System unit Ignoring the fact that Delphi s string lists can automatically save themselves to a file with the saveroriie method based on the use of streams you could easily save the content of a string list to a TextFile by writing the following code in which the file name is requested to the user using Delphi s SaveDialog component var OutputFile TextFile I Integer begin choose a file if SaveDialogl Execute then begin output the text to a file AssignFile OutputFile SaveDialogl FileName Rewrite OutputFile try save listbox data to file for I 0 to ListBoxl Items Count 1 do Writeln OutputFile ListBoxl Items I finally CloseFile OutputFile end end end Essential Pascal Copyright 1995 2003 Marco Cantu www marcocantu com epascal 87 Instead of being connected to a physical file a Pascal file type variable can be hooked directly to the printer so that the output will be printed instead of being saved to a file To accomplish this simple use the AssignPrn procedure For example in the code above you could replace the line AssignFile OutputFile SaveDialogl FileName withthe line AssignPrn OutputFile A Text File Converter Up to now we ve seen simple examples of creating new files In our next example we ll process an existing file creating a new one with a
112. replace them with case statements A case statement consists in an expression used to select a value a list of possible values or a range of values These values are constants and they must be unique and of an ordinal type Eventually there can be an else statement that is executed if none of the labels correspond to the value of the selector Here are two simple examples case Number of 1 Text One 2 Text Two 3 Text Three end case MyChar of Text Plus sign Text Minus sign tet Text Multiplication or division Q 9 Text i Number a z Text Lowercase character A Z Text Uppercase character Essential Pascal Copyright 1995 2003 Marco Cant www marcocantu com epascal 41 Text Unknown character Loops in Pascal The Pascal language has the typical repetitive statements of most programming languages including for while and repeat statements Most of what these loops do will be familiar if you ve used other programming languages so I ll cover them only briefly The For Loop The for loop in Pascal is strictly based on a counter which can be either increased or decreased each time the loop is executed Here is a simple example of a for loop used to add the first ten numbers var K I Integer begin K 0 for I 1 to 10 do K K I This same for statement could have been written using a reverse counte
113. rinciples you need to know regarding comments uppercase spaces and the so called pretty printing In general the goal of any coding style is clarity The style and formatting decisions you make are a form of shorthand indicating the purpose of a given piece of code An essential tool for clarity is consistency whatever style you choose be sure to follow it throughout a project Comments In Pascal comments are enclosed in either braces or parentheses followed by a star Delphi also accepts the C style comments which can span to the end of the line is a comment C this is another comment this is a comment up to the end of the line The first form is shorter and more commonly used The second form was often preferred in Europe because many European keyboards lacked the brace symbol The third form of comments has been borrowed from C and was added in Delphi 2 Comments up to the end of the line are very helpful for short comments and for commenting out a line of code In the listings of the book I ll try to mark comments as italic and keywords in bold to be consistent with the default Delphi syntax highlighting and that of most other editors Having three different forms of comments can be helpful for making nested comments If you want to comment out several lines of source code to disable them and these lines contain some real comments you cannot use the same comment identifier code comment
114. ritten called LowCase f the letter is lowercase it is converted to uppercase only if it was at the beginning of a new sentence f the character is an ending punctuation mark period question mark or exclamation mark Period is set to True f the character is anything else it is simply copied to the destination file and Period is set to False Figure 12 1 shows an example of this code s effect it shows a text file before and after the conversion This program is far from adequate for professional use but it is a first step toward building a full scale case conversion program Its biggest drawbacks are that it frequently Essential Pascal Copyright 1995 2003 Marco Cant www marcocantu com epascal 89 converts proper nouns to lowercase and capitalizes any letter after a period even if it s the first letter of a filename extension FmnTFXT nniTondo 7 mardmre mper nae irj x Gk Id voy Fak Tis Twos Sue ous Hei al Bj x 2E ES w Bs oF iB G eases maduu uppa td MasLeziig selpi aoaeeoo D i the source coce cf the kook ns Lr ats rou Fic lane publishcz web ritli ESI ulor drou FEON JA I ARS 3 and http www nybes 72m talpu He E de2zco zcv zcdro upat Change Yen cun eper gore Se Rapa cepe E i r r bat eh Seer tha consent She 2s Lipper acc cChaplez s elger A nau wanL ER Cere ju eed te downcled the wert bus a tar fv Captaits garlensee et P To z All bie sculc
115. routines for special types such as TDateTime or variant or routines specifically intended for formatting like the powerful Format and FormatFloat routines TABLE 3 4 System ROUTINES FOR TYPE CONVERSION Converts a string into a number raising an exception if the string does not represent a valid integer Converts a string into a number using a default value if the string is not correct Converts a string into a number traditional Turbo Pascal routine available for compatibility Converts a number into a string using formatting parameters traditional Turbo Pascal routine available for compatibility Converts a null terminated string into a Pascal style string This conversion is automatically done for AnsiStrings in 32 bit Delphi See the section on strings later in this chapter Copies a Pascal style string into a null terminated string This conversion is done with a simple PChar cast in 32 bit Delphi See the section on strings later in this chapter Copies a portion of a Pascal style string into a null terminated string Converts a floating point value to record including its decimal representation exponent digits sign FloatToStr Converts the floating point value to its string representation using default formatting Converts the floating point value to its string representation using the specified formatting FloatToText Copies the floating point value to a string buffer using the specified formatting
116. rrors because it does not make sense to assign a True or False value to a variable of the Integer data type You should not blame Delphi for these errors It only warns you that there is something wrong in the code Of course it is often possible to convert the value of a variable from one type into a different type In some cases this conversion is automatic but usually you need to call a specific system function that changes the internal representation of the data In Delphi you can assign an initial value to a global variable while you declare it For example you can write var Value Integer 10 Correct Boolean True This initialization technique works only for global variables not for variables declared inside the scope of a procedure or method Essential Pascal Copyright 1995 2003 Marco Cantu www marcocantu com epascal 17 Constants Pascal also allows the declaration of constants to name values that do not change during program execution To declare a constant you don t need to specify a data type but only assign an initial value The compiler will look at the value and automatically use its proper data type Here are some sample declarations const Thousand 1000 Pi 3 14 AuthorName Marco Cantu Delphi determines the constant data type based on its value In the example above the Thousand constant is assumed to be of type Smallrnt the smallest integral type which can hold it If you want to tell Delphi
117. rspective You can find more details about Marco and his work on his web site www marcocantu com Essential Pascal Copyright 1995 2003 Marco Cantu www marcocantu com epascal 5 Chapter 1 A Short History of the Pascal Language along with the Borland visual development environment It was simply extended from the Object Pascal language already in use in the Borland Pascal products But Borland didn t invent Pascal it only helped make it very popular and extended it a little Wirth s Pascal T he Object Pascal programming language we use in Delphi wasn t invented in 1995 The Pascal language was originally designed in 1971 by Niklaus Wirth professor at the Polytechnic of Zurich Switzerland Pascal was designed as a simplified version for educational purposes of the language Algol which dates from 1960 When Pascal was designed many programming languages existed but few were in widespread use FORTRAN C Assembler COBOL The key idea of the new language was order managed through a strong concept of data type and requiring declarations and structured program controls The language was also designed to be a teaching tool for students of programming classes Turbo Pascal Borland s world famous Pascal compiler called Turbo Pascal was introduced in 1983 implementing Pascal User Manual and Report by Jensen and Wirth The Turbo Pascal compiler has been one of the best selling series of compilers of all time and made the language
118. s toward the end of this chapter For the moment though notice only the array like syntax of the list of values passed as the second parameter Besides using 96d you can use one of many other placeholders defined by this function and briefly listed in Table 7 1 These placeholders provide a default output for the given data type However you can use further format specifiers to alter the default output A width specifier for example determines a fixed number of characters in the output while a precision specifier indicates the number of decimal digits For example Format 8d n1 converts the number n1 into an eight character string right aligning the text use the minus symbol to specify left justification filling it with white spaces TABLE 7 1 TYPE SPECIFIERS FOR THE FORMAT FUNCTION d decimal The corresponding integer value is converted to a string of decimal digits x hexadecimal The corresponding integer value is converted to a string of hexadecimal digits The corresponding pointer value is converted to a string expressed with hexadecimal p pointer e digits Essential Pascal Copyright 1995 2003 Marco Cantu www marcocantu com epascal 66 s string The corresponding string character or PChar value is copied to the output string The corresponding floating point value is converted to a string based on exponential The corresponding floating point value is converted to a string based on floating point not
119. sed on the ANSIChar type The WideString type is similar to the ANSI String type but is based on the WideChar type it stores Unicode characters Using Long Strings If you simply use the string data type you get either short strings or ANSI strings depending on the value of the H compiler directive H the default stands for long strings the ANSIString type which is what is used by the components of the Delphi library Delphi long strings are based on a reference counting mechanism which keeps track of how many string variables are referring to the same string in memory This reference counting is used also to free the memory when a string isn t used anymore that is when the reference count reaches zero If you want to increase the size of a string in memory but there is something else in the adjacent memory then the string cannot grow in the same memory location and a full copy of the string must therefore be made in another location When this situation occurs Delphi s run time support reallocates the string for you in a completely transparent way You simply set the Essential Pascal Copyright 1995 2003 Marco Cantu www marcocantu com epascal 62 maximum size of the string with the SetLength procedure effectively allocating the required amount of memory SetLength Stringl 200 The SetLength procedure performs a memory request not an actual memory allocation It reserves the required memory space for future use without
120. special kind of function or procedure that is related to a data type a class In Delphi every time we handle an event we need to define a method generally a procedure In general however the term method is used to indicate both functions and procedures related to a class We have already seen a number of methods in the examples in this and the previous chapters Here is an empty method automatically added by Delphi to the source code of a form procedure TForml ButtonlClick Sender TObject begin here goes your code end Essential Pascal Copyright 1995 2003 Marco Cant www marcocantu com epascal 54 Forward Declarations When you need to use an identifier of any kind the compiler must have already seen some sort of declaration to know what the identifier refers to For this reason you usually provide a full declaration before using any routine However there are cases in which this is not possible If procedure A calls procedure B and procedure B calls procedure A when you start writing the code you will need to call a routine for which the compiler still hasn t seen a declaration If you want to declare the existence of a procedure or function with a certain name and given parameters without providing its actual code you can write the procedure or function followed by the forward keyword procedure Hello forward Later on the code should provide a full definition of the procedure but this can be called even befor
121. t ToStr For the opposite conversion from a string to a number there are two new specific functions StrToInt 64 and StrToInt64Def Boolean Boolean values other than the Boolean type are seldom used Some Boolean values with specific representations are required by Windows API functions The types are ByteBool WordBool and LongBool In Delphi 3 for compatibility with Visual Basic and OLE automation the data types ByteBool WordBool and LongBool were modified to represent the value True with 1 while the value False is still 0 The Boolean data type remains unchanged True is 1 False is 0 Characters Finally there are two different representation for characters ANSI Char and WideChar The first type represents 8 bit characters corresponding to the ANSI character set traditionally used by Windows the second represents 16 bit characters corresponding to the new Unicode characters supported by Windows NT and only partially by Windows 95 and 98 Most of the time you ll simply use the Char type which in Delphi 3 corresponds to ANSIChar Keep in mind anyway that the first 256 Unicode characters correspond exactly to the ANSI characters Constant characters can be represented with their symbolic notation as in k or with a numeric notation as in 78 The latter can also be expressed using the Chr function as in Chr 78 The opposite conversion can be done with the Ord function It is generally better to use the symbol
122. t important Windows system libraries with the same name we are Essential Pascal Copyright 1995 2003 Marco Cant www marcocantu com epascal 73 using in our code Inside an external declaration in fact we can specify that our function refer to a function of a DLL that originally had a different name You seldom need to write declarations like the one just illustrated since they are already listed in the Windows unit and many other Delphi system units The only reason you might need to write this external declaration code is to call functions from a custom DLL or to call undocumented Windows functions In the 16 bit version of Delphi the external declaration used the name of the library without the extension and was followed by the name directive as in the code above or by an alternative index directive followed by the ordinal number of the function inside the DLL The change reflects a system change in the way libraries are accessed Although Win32 still allows access to DLL functions by number Microsoft has stated this won t be supported in the future Notice also that the Windows unit replaces the WinProcs and WinTypes units of the 16 bit version of Delphi A Windows Callback Function We ve seen in Chapter 6 that Objet Pascal supports procedural types A common use of procedural types is to provide callback functions to a Windows API function First of all what is a callback function The idea is that some API function perfor
123. t of the source code of an earlier example with the declaration of the Button1Click method type TForml class TForm ListBox1 TListBox Buttonl TButton procedure ButtonlClick Sender TObject end Essential Pascal Copyright 1995 2003 Marco Cantu www marcocantu com epascal 55 Procedural Types Another unique feature of Object Pascal is the presence of procedural types These are really an advanced language topic which only a few Delphi programmers will use regularly However since we will discuss related topics in later chapters specifically method pointers a technique heavily used by Delphi it s worth a quick look at them here If you are a novice programmer you can skip this section for now and come back to it when you feel ready In Pascal there is the concept of procedural type which is similar to the C language concept of function pointer The declaration of a procedural type indicates the list of parameters and in the case of a function the return type For example you can declare a new procedural type with an Integer parameter passed by reference with this code type IntProc procedure var Num Integer This procedural type is compatible with any routine having exactly the same parameters or the same function signature to use C jargon Here is an example of a compatible routine procedure DoubleTheValue var Value Integer begin Value Value 2 end In the 16 bit version of Delphi routin
124. t the Pascal compiler to the 32 bit world actually re engineering it to provide a code generator common with the C compiler This brought to the Pascal language many optimizations previously found only in C C compilers With Kylix Borland made a further step and opened to Pascal Delphi programmers the Linux operating system even if only in its Intel based incarnation Most of the examples of this book can be executed almost unchanged on Linux With the release of version 7 of Delphi and version 3 of Kylix Borland has formally started to names the Pascal or Object Pascal language as the Delphi language So Delphi 7 uses the Delphi language Kylix 3 supports both the Delphi and the C languages and Borland ships a Delphi language compiler for the Microsoft s NET architecture This is mainly a cosmetic and marketing change probably due to the fact that the Pascal language was never popular in the US as it used to be and still is in Europe and other areas of the world Essential Pascal Copyright 1995 2003 Marco Cantu www marcocantu com epascal 7 Chapter 2 Coding in Pascal efore we move on to the subject of writing Pascal language statements it is important to highlight a couple of elements of Pascal coding style The question I m addressing here is this Besides the syntax rules how should you write code There isn t a single answer to this question since personal taste can dictate different styles However there are some p
125. tDateTime StrToDateTime StrToDate StrToTime DayOfWeek DecodeDate EncodeDate Turns hour minute second and millisecond values into a TDateTime value To show you how to use this data type and some of its related routines I ve built a simple example named TimeNow The main form of this example has a Button and a ListBox Essential Pascal Copyright 1995 2003 Marco Cant www marcocantu com epascal 24 component When the program starts it automatically computes and displays the current time and date Every time the button is pressed the program shows the time elapsed since the program started Here is the code related to the OnCreate event of the form procedure TFormTimeNow FormCreate Sender TObject begin StartTime Now ListBoxl Items Add TimeToStr StartTime ListBoxl Items Add DateToStr StartTime ListBoxl Items Add Press button for elapsed time end The first statement is a call to the Now function which returns the current date and time This value is stored in the StartTime variable declared as a global variable as follows var FormTimeNow TFormTimeNow StartTime TDateTime l ve added only the second declaration since the first is provided by Delphi By default it is the following var ll Forml TForml Changing the name of the form this declaration is automatically updated Using global variables is actually not the best approach It should be better to use a private field of the form c
126. the Integer button Btnl nteger procedure TFormRange BtnIntegerClick Sender TObject begin jabelType Caption Integer aabelSize Caption IntToStr SizeOf Integer abelMax Caption IntToStr High Integer jabelMin Caption IntToStr Low Integer end If you have some experience with Delphi programming you can examine the source code of the program to understand how it works For beginners it s enough to note the use of three functions SizeOf High and Low The results of the last two functions are ordinals of the same type in this case integers and the result of the SizeOf function is always an integer The return value of each of these functions is first translated into strings using the IntToStr function then copied to the captions of the three labels The methods associated with the other buttons are very similar to the one above The only real difference is in the data type passed as a parameter to the various functions Figure 3 2 shows the result of executing this same program under Windows 95 after it has been recompiled with the 16 bit version of Delphi Comparing Figure 3 1 with Figure 3 2 you can see the difference between the 16 bit and 32 bit Integer data types FIGURE 3 2 THE OUTPUT OF THE 16 BIT VERSION OF THE RANGE EXAMPLE AGAIN SHOWING INFORMATION ABOUT INTEGERS Essential Pascal Copyright 1995 2003 Marco Cantu www marcocantu com epascal 21 ai Range 16 bit
127. the application the program In theory this is true In practice the program file is usually an automatically generated file with a limited role It simply needs to start up the program running the main form The code of the program file or Delphi project file DPR can be edited either manually or by using the Project Manager and some of the Project Options related to the application object and the forms The structure of the program file is usually much simpler than the structure of the units Here is the source code of a sample program file program Projectl uses Forms Unitl in Unitl1 PAS FormiDateForm begin Application Initialize Application CreateForm TForml Forml Application Run end As you can see there is simply a uses section and the main code of the application enclosed by the begin and end keywords The program s uses statement is particularly important because it is used to manage the compilation and linking of the application Conclusion Units were the Pascal actually Turbo Pascal technique for module programming Even if they were later followed b objects and classes they still play a key role for encapsulation for the definition of some sort of name spaces and for the overall structure of Delphi programs Also units have influence on scope and global memory allocations Essential Pascal Copyright 1995 2003 Marco Cant www marcocantu com epascal 85 Chapter 12 Files in the Pascal Langua
128. tial Pascal Copyright 1995 2003 Marco Cant www marcocantu com epascal 53 7 Open Array El x Plain Call Danie Slice Sum ll Y Y k True 10 34 999 gt 1 217 34 Partial Slice FiGURE 6 2 THE FORM OF THE OPENARR EXAMPLE WITH THE MESSAGE BOX DISPLAYED WHEN THE UNTYPED BUTTON IS PRESSED Delphi Calling Conventions The 32 bit version of Delphi has introduced a new approach to passing parameters known as fastcall Whenever possible up to three parameters can be passed in CPU registers making the function call much faster The fast calling convention used by default in Delphi 3 is indicated by the register keyword The problem is that this is the default convention and functions using it are not compatible with Windows the functions of the Win32 API must be declared using the stdcall calling convention a mixture of the original Pascal calling convention of the Win16 API and the cdecl calling convention of the C language There is generally no reason not to use the new fast calling convention unless you are making external Windows calls or defining Windows callback functions We ll see an example using the stdcall convention before the end of this chapter You can find a summary of Delphi calling conventions in the Calling conventions topic under Delphi help What Is a Method If you have already worked with Delphi or read the manuals you have probably heard about the term method A method is a
129. to assign the real file to the file variable open the file operate on it and close it For example the following code saves all the numeric data collected in a 5x4 string grid save to the current file AssignFile SaveFile CurrentFile Rewrit SaveFile try write the value of each grid element for I 1 to 5 do for J 1 to 4 do begin Value StrToIntDef Trim StringGridl Cells I J 0 Write SaveFile Value end finally CloseFil SaveFile Essential Pascal Copyright 1995 2003 Marco Cant www marcocantu com epascal 90 end To save the data to a file the program saves each value of the string grid after converting it into a number To accomplish this the program uses two nested for loops to scan the grid Notice the use of the temporary value variable the write and Read procedures require a parameter passed by reference vax so you cannot pass the property of a Delphi component since it doesn t correspond directly to a memory location Of course the data should be read in the same order it is written as you can see in the Openiclick method load from the current file AssignFile LoadFile CurrentFile Reset LoadFile try read the value of each grid element for I 1 to 5 do for J 1 to 4 do begin Read LoadFile Value StringGridl Cells I J IntToStr Value end finally CloseFile LoadFile end From Files to Streams Although direct handlin
130. to the string while the function is being executed By contrast passing the parameter via a reference var or constant const parameter doesn t imply a further reference to the string In this case I ve used a const parameter as the function is not supposed to modify the string To obtain the memory address of the string useful to determine its actual identity and to see when two different strings refer to the same memory area I ve simply made a hard coded typecast from the string type to the Integer type Strings are references in practice they re pointers Their value holds the actual memory location of the string To extract the reference count I ve based the code on the little known fact that the length and reference count are actually stored in the string before the actual text and before the position the string variable points to The negative offset is 4 for the length of the string a value you can extract more easily using the Length function and 8 for the reference count Keep in mind that this internal information about offsets might change in future versions of Delphi there is also no guarantee that similar undocumented features will be maintained in the future By running this example you should get two strings with the same content the same memory location and a reference count of 2 as shown in the upper part of the list box of Figure 2 1 Now if you change the value of one of the two strings it doesn t matter which one
131. tonlClick Sender TObject begin Beep end then as soon as you press the button you ll hear a short sound Now move to the uses statement of the unit and change the code from this Essential Pascal Copyright 1995 2003 Marco Cantu www marcocantu com epascal 84 uses Windows Messages SysUtils Classes to this very similar version simply moving the SysUtils unit before the Windows unit uses SysUtils Windows Messages Classes If you now try to recompile this code you ll get a compiler error Not enough actual parameters The problem is that the Windows unit defines another Beep function with two parameters Stated more generally what happens in the definitions of the first units you include in the uses statement might be hidden by corresponding definitions of later units The safe solution is actually quite simple procedure TForml ButtonlClick Sender TObject begin SysUtils Beep end This code will compile regardless of the order of the units in the uses statements There are few other name clashes in Delphi simply because Delphi code is generally hosted by methods of classes Having two methods with the same name in two different classes doesn t create any problem The problems arise only with global routines Units and Programs A Delphi application consists of two kinds of source code files one or more units and one program file The units can be considered secondary files which are referred to by the main part of
132. tu com epascal 44 A Luups n xj Sardom b mbe Eo Far dom bh be 7 33 Bardam h mha 271 Far Jom h omla 222 larem h mhe d E Sardom k mbe 232 larem h mbe ST Sardom h mbe 330 lardam h mhes I an i Sardom hk mbe 431 darda bombo 457 Tweaker 7S Rardombonbos4z2 Wmiomte SE arim h miro ka _ tarde Honto Se Fat dom b abes 635 m geris HE Aardam h mha 1117 Sa dom Hele SE Sacdam H mbes FAT Sa dom Henke 67 Bardon H mbes TEF 3e dom Hole 84 Tadam H mke AF 3a7dom M mbEe 36 lacdom H mbe HL ls FIGURE 5 3 THE CONTENTS OF THE LIST BOX OF THE LOOPS EXAMPLE CHANGE EACH TIME YOU PRESS THE WHILE BUTTON You can alter the standard flow of a loop s execution using the Break and Continue system procedures The first interrupts the loop the second is used to jump directly to the loop test or counter increment continuing with the next iteration of the loop unless the condition is zero or the counter has reached its highest value Two more system procedures Exit and Halt let you immediately return from the current function or procedure or terminate the program The With Statement The last kind of Pascal statement I ll focus on is the with statement which used to be peculiar to this programming language although it has been recently introduced also in J avaScript and Visual Basic and can be very useful in Delphi programming The with statement is nothing but shorthand When you need t
133. type As the name implies the Currency data type has been added to handle very precise monetary values with four decimal places We cannot build a program similar to the Range example with real data types because we cannot use the High and Low functions or the Ord function on real type variables Real types represent in theory an infinite set of numbers ordinal types represent a fixed set of values Let me explain this better when you have the integer 23 you can determine which is the following value Integers are finite they have a determined range and they have an order Floating point numbers are infinite even within a small range and have no order in fact how many values are there between 23 and 24 And which number follows 23 46 It is 23 47 23 461 or 23 4601 That s really hard to know For this reason it makes sense to ask for the ordinal position of the character w in the range of the Char data type but it makes no sense at all to ask the same question about 7143 1562 in the range of a floating point data type Although you can indeed know whether one real number has a higher value than another it makes no sense to ask how many real numbers exist before a given number this is the meaning of the Ord function Real types have a limited role in the user interface portion of the code the Windows side but they are fully supported by Delphi including the database side The support of IEEE standard floating point types m
134. ul long string data type which is the default string type in Delphi Types of Strings In Borland s Turbo Pascal and in 16 bit Delphi the typical string type is a sequence of characters with a length byte at the beginning indicating the current size of the string Because the length is expressed by a single byte it cannot exceed 255 characters a very low value that creates many problems for string manipulation Each string is defined with a fixed size which by default is the maximum 255 although you can declare shorter strings to save memory space A string type is similar to an array type In fact a string is almost an array of characters This is demonstrated by the fact that you can access a specific string character using the notation To overcome the limits of traditional Pascal strings the 32 bit versions of Delphi support long strings There are actually three string types The ShortString type corresponds to the typical Pascal strings as described before These strings have a limit of 255 characters and correspond to the strings in the 16 bit version of Delphi Each element of a short string is of type ANSIChar the standard character type The ANSIString type corresponds to the new variable length long strings These strings are allocated dynamically are reference counted and use a copy on write technique The size of these strings is almost unlimited they can store up to two billion characters They are also ba
135. unctions The first of these functions returns the number of parameters the second returns the parameter in a given position Although users seldom specify command line parameters in a graphical user interface environment the Windows command line parameters are important to the system For example once you have defined an association between a file extension and an application you can simply run a program by selecting an associated file In practice when you double click on a file Windows starts the associated program and passes the selected file as a command line parameter Here is the complete source code of the project a DPR file not a PAS file Program Strparam uses l Windows begin show the full string MessageBox 0 cmdLine StrParam Command Line MB OK show the first parameter if ParamCount 0 then MessageBox 0 PChar ParamStr 1 1st StrParam Parameter MB OK Essential Pascal Copyright 1995 2003 Marco Cantu www marcocantu com epascal 76 else MessageBox 0 PChar No parameters lst StrParam Parameter MB OK end The output code uses the MessageBox API function simply to avoid getting the entire VCL into the project A pure Windows program as the one above in fact has the advantage of a very small memory footprint The executable file of the program is about 16 Kbytes To provide a command line parameter to this program you can use Delphi s Run gt P
136. unds Conclusion This chapter temporarily covers only dynamic arrays certainly an important element for memory management but only a portion of the entire picture More material will follow The memory structure described in this chapter is typical of Windows programming a topic I ll introduce in the next chapter without going to the full extent of using the VCL though Essential Pascal Copyright 1995 2003 Marco Cant www marcocantu com epascal 71 Chapter 9 Windows Programming Delphi provides a complete encapsulation of the low level Windows API using Object Pascal and the Visual Component Library VCL so it is rarely necessary to build Windows applications using plain Pascal and calling Windows API functions directly Nonetheless programmers who want to use some special techniques not supported by the VCL still have that option in Delphi You would only want to take this approach for very special cases such as the development of new Delphi components based on unusual API calls and don t want to cover the details Instead we ll look at a few elements of Delphi s interaction with the operating system and a couple of techniques that Delphi programmers can benefit from Windows Handles Among the data types introduced by Windows in Delphi handles represent the most important group The name of this data type is THandle and the type is defined in the Windows unit as ee THandle LongWord Handle data types are implement
137. use of case isn t enforced by the compiler but it is a good habit to get into A common approach is to capitalize only the first letter of each identifier When an identifier is made up of several consecutive words you cannot insert a space in an identifier every first letter of a word should be capitalized MyLongIdentifier MyVeryLongAndAlmostStupidIdentifier Other elements completely ignored by the compiler are the spaces new lines and tabs you add to the source code All these elements are collectively known as white space White space is used only to improve code readability it does not affect the compilation Unlike BASIC Pascal allows you to write a statement on several lines of code splitting a long instruction on two or more lines The drawback at least for many BASIC programmers of allowing statements on more than one line is that you have to remember to add a semicolon to indicate the end of a statement or more precisely to separate a statement from the next one Notice that the only restriction in splitting programming statements on different lines is that a string literal may not span several lines Again there are no fixed rules on the use of spaces and multiple line statements just some rules of thumb The Delphi editor has a vertical line you can place after 60 or 70 characters If you use this line and try to avoid surpassing this limit your source code will look better when you print it on paper Otherwise long li
138. ve exactly the same name and unnamed types are given internal names by the compiler Get used to defining a data type each time you need a variable with a complicated structure and you won t regret the time you ve spent in it Essential Pascal Copyright 1995 2003 Marco Cant www marcocantu com epascal 29 But what do these type definitions mean I ll provide some descriptions for those who are not familiar with Pascal type constructs I ll also try to underline the differences from the same constructs in other programming languages so you might be interested in reading the following sections even if you are familiar with kind of type definitions exemplified above Finally I ll show some Delphi examples and introduce some tools that will allow you to access type information dynamically Subrange Types A subrange type defines a range of values within the range of another type hence the name subrange You can define a subrange of the Integer type from 1 to 10 or from 100 to 1000 or you can define a subrange of the Char type as in type Ten 1 10 OverHundred 100 1000 Uppercase A Z In the definition of a subrange you don t need to specify the name of the base type You just need to supply two constants of that type The original type must be an ordinal type and the resulting type will be another ordinal type When you have defined a subrange you can legally assign it a value within that range This code is valid
139. with 10 bytes These are all floating point data types with different precision which correspond to the IEEE standard floating point representations and are directly supported by the CPU numeric coprocessor for maximum speed In Delphi 2 and Delphi 3 the Realtype had the same definition as in the 16 bit version it was a 48 bit type But its usage was deprecated by Borland which suggested that you use the Single Double and Extended types instead The reason for their suggestion is that the old 6 byte format is neither supported by the Intel CPU nor listed among the official IEEE real types To completely overcome the problem Delphi 4 modifies the definition of the Real type to represent a standard 8 byte 64 bit floating point number In addition to the advantage of using a standard definition this change allows components to publish properties based on the Real type something Delphi 3 did not allow Among the disadvantages there might be compatibility problems If necessary you can overcome the possibility of incompatibility by sticking to the Delphi 2 and 3 definition of the type do this by using the following compiler option SREALCOMPATIBILITY ON There are also two strange data types Comp describes very big integers using 8 bytes which can hold numbers with 18 decimal digits and Currency not available in 16 bit Delphi indicates a fixed point decimal value with four decimal digits and the same 64 bit representation as the Comp
140. word This technique is available in most programming languages It isn t present in C but has been introduced in C where you use the amp pass by reference symbol In Visual Basic every parameter not specified as ByVal is passed by reference Here is an example of passing a parameter by reference using the var keyword procedure DoubleTheValue var Value Integer begin Value Value 2 end Essential Pascal Copyright 1995 2003 Marco Cant www marcocantu com epascal 49 In this case the parameter is used both to pass a value to the procedure and to return a new value to the calling code When you write var X Integer begin X 10 DoubleTheValue X the value of the X variable becomes 20 because the function uses a reference to the original memory location of X affecting its initial value Passing parameters by reference makes sense for ordinal types for old fashioned strings and for large records Delphi objects in fact are invariably passed by value because they are references themselves For this reason passing an object by reference makes little sense apart from very special cases because it corresponds to passing a reference to a reference Delphi long strings have a slightly different behavior they behave as references but if you change one of the string variables referring to the same string in memory this is copied before updating it A long string passed as a value parameter behaves as
141. y was another reason to change most of the examples from visual ones to console based ones something plan doing but that still haven t done Beside the theoretical advantage that the reader con focus even more on the language ignoring event handlers methods component and other more advance topics only using a console application the code will be easily portable among the different compilers Borland has made available for Delphi in the recent years Another change is that I ve gone from an HTML based format to a PDF based one With the increased bandwidth people generally have over a few years back this is really an advantage and distribution of a single file helps a lot By the way the book is now being written with OpenOffice org Copyright The text and the source code of this book is copyrighted by Marco Cant Of course you can use the programs and adapt them to your own needs only you are not allowed to use them in books training material and other copyrighted formats unless of course you use a reasonably limited amount and do mention the source As you can freely print this ebook as training material this doesn t sound like a real restriction to me Distributing the ebook is allowed only if no change is applied beyond the distribution cost Rather than placing a copy of it on your website though add a link to the main download site www marcocantu com epascal The reason is that expect the book to go over frequent cha
142. y definition you need to pass a subrange type within square brackets or define a new specific subrange type using two constants of an ordinal type This subrange specifies the valid indexes of the array Since you specify both the upper and the lower index of the array the indexes don t need to be zero based as is necessary in C C Java and other programming languages Since the array indexes are based on subranges Delphi can check for their range as we ve already seen An invalid constant subrange results in a compile time error and an out of range index used at run time results in a run time error if the corresponding compiler option is enabled Using the array definition above you can set the value of a DayTemp1 variable of the DayTemperatures type as follows type DayTemperatures array 1 24 of Integer var DayTempl DayTemperatures procedure AssignTemp Essential Pascal Copyright 1995 2003 Marco Cant www marcocantu com epascal 33 begin DayTempl 1 54 DayTempl 2 52 DayTempl 24 66 DayTempl 25 67 compile time error An array can have more than one dimension as in the following examples type MonthTemps array 1 24 1 31 of Integer YearTemps array 1 24 1 31 Jan Dec of Integer These two array types are built on the same core types So you can declare them using the preceding data types as in the following code type MonthTemps array 1 31 of D
143. y to look into the statements the language provides to operate on the variables we ve defined Essential Pascal Copyright 1995 2003 Marco Cantu www marcocantu com epascal 38 Chapter 5 Statements f the data types are one of the foundations of Pascal programming the other are statements Statements of the programming language are based on keywords and other elements which allow you to indicate to a program a sequence of operations to perform Statements are often enclosed in procedures or functions as we ll see in the next chapter Now we ll just focus on the basic types of commands you can use to create a program Simple and Compound Statements A Pascal statement is simple when it doesn t contain any other statements Examples of simple statements are assignment statements and procedure calls Simple statements are separated by a semicolon X Y Z assignment Randomize procedure call Usually statements are part of a compound statement marked by begin and end brackets A compound statement can appear in place of a generic Pascal statement Here is an example begin A B GC A 2 end The semicolon after the last statement before the end isn t required as in the following begin A B C A 2 end Both versions are correct The first version has a useless but harmless semicolon This semicolon is in fact a null statement that is a statement with no code Notice that at times null statements ca
144. you get used to with statements because they can be really very handy and at times even make the code more readable You should however avoid using multiple with statements such as with ListBoxl Buttonl do The code following this would probably be highly unreadable because for each property defined in this block you would need to think about which component it refers to depending on the respective properties and the order of the components in the with statement Speaking of readability Pascal has no endif or endcase statement If an if statement has a begin end block then the end of the block marks the end of the statement The case statement instead is always terminated by an end All these end statements often found one after the other can make the code difficult to follow Only by tracing the indentations can you see which statement a particular end refers to A common way to solve this problem and make the code more readable is to add a comment after the end statement indicating its role as in end if Conclusion This chapter has described how to code conditional statements and loops Instead of writing long lists of such statements programs are usually split in routines procedures or functions This is the topic of the next chapter which introduces also some advanced elements Essential Pascal Copyright 1995 2003 Marco Cant www marcocantu com epascal 47 Chapter 6 Procedures and Functions nother important i
145. yright 1995 2003 Marco Cant www marcocantu com epascal 58 routine name all the available alternatives are listed As you enter the parameters Delphi uses their type to determine which of the alternatives are still available In Figure 6 4 you can see that after starting to type a constant string Delphi shows only the compatible versions omitting the version of the ShowMsg procedure that has an integer as first parameter 2 OverDefF pas Iof Xx E OverDetF e P Form B i TForm1 ET Button procedure TFormi Button3Click Sender TObje fe ButtoniClick begin C9 Button2 ShowMsg 10 MBytes Button2Click end C9 Button3 oe Button3Click procedure TFormi Buttond4Click Sender TObje C9 Button begin po tps ShowMsg Tptal d 100 Sy z x pam k end str String O ds FormatStr String Params array of const 9 Bu z i Button amp Click procedure TFormi Button5Click Sender sese H C Procedures Begin H Uses MessBox Something wrong here end rocedure TFormi Button6Click Sender E b 103 14 Modified Insert Ui FiGURE 6 4 THE MULTIPLE ALTERNATIVES OFFERED BY CODE PARAMETERS FOR OVERLOADED ROUTINES ARE FILTERED ACCORDING TO THE PARAMETERS ALREADY AVAILABLE The fact that each version of an overloaded routine must be properly marked implies that you cannot overload an existing routine of the same unit that is not marked with the overload keyword The error message

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