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35) p. 66. This is embarassing but I`m not sure I`ve ever understood

1. The next Cecil facility enables the C side to access fields of complex objects corresponding to attributes of the generating classes To obtain a field of an object use the macro eif_field You may use the result of eif_field in two different ways as an expression or r value in C terminology or as a writable entity or l value which may then be the target of an assignment Such an assignment will re attach the corresponding object field Function eif_field takes three arguments The first is a value of type EJF_OBJ representing an object do not forget to protect it by eif_access The second is a string giving the name of the desired attribute The third is one of the following values describing the type of the attribute EIF_BOOLEAN EIF_CHARACTER EIF_INTEGER EIF_REAL EIF_ DOUBLE EIF_POINTER EIF_REFERENCE The result of eif_field is undefined if the object does not have a field with the given name and type To call an Eiffel function returning a Bit_type result use this scheme EIF_BIT your_bit EIF_FN_BIT your_function your_function eif_fn_bit function_name type_id your_bit your_function eif_access object actual_l actual_2 The last instruction assigns to your_bit a reference to the bit sequence returned by the function EIF_BIT describes a pointer type Similarly you may access and modify Bit_type fields as follows EIF_BIT your_bitl your_bit2 your_bitl eif_bit_field eif_acc
2. A Simple_string a specimen of Simple_string is a String which consists of at most one line that is to say has no embedded new line character Do not confuse String or Simple_string with Manifest_string seen in the discussion of expressions A specimen of Manifest_string a non terminal construct is a Simple_ string enclosed in double quotes as in SOME STRING Ww In the definition of String a character is a legal Eiffel character as defined later in this chapter This includes in particular e Any keyboard key other than e Any special character described as for some appropriate letter for example B representing the Backspace character e A character given by its numerical code under the form code for example 35 for the sharp sign which is the character of ASCII code 35 25 12 INTEGERS Definition integer Integer a variable lexical construct describes unsigned integer constants in decimal notation Except for underscores no intervening characters such as blanks are permitted between digits 86 25 9 function may be used in lieu of a local entity for example as target of an assignment and the basic type INTEGER may appear at a position where a type is expected No intervening blanks or other characters are permitted between the letters of a reserved word Letter case is not significant for reserved words so that CLASS result or even rEsULt are permissible forms o
3. Definition origin seed Every feature of a class C has a seed which is a feature and an origin which is a class defined as follows le Any immediate feature of C in other words any feature introduced in C rather than inherited is its own seed and has C as its origin 2e An inherited feature of C with two or more precursors all of which have recursively the same seed s also has s as its seed This is the case of sharing under repeated inheritance 3 If C joins a set of inherited deferred features yielding as explained above a single feature of C to which case 2 does not apply that feature is its own seed and its origin is C 4e Any feature of C to which none of the previous cases applies is inherited and has exactly one precursor then its seed and origin are recursively the seed and origin of that precursor 10 22 REDECLARATION RULES Definition redeclaration redefinition effecting A class C redeclares an inherited feature f if and only if one of the following two conditions holds e C contains a Feature_declaration for a feature g with the same final name as f e C inherits f as deferred and inherits as effective another feature g with the same final name as f A redefinition is a redeclaration which is not an effecting 38 10 17 10 17 THE JOIN MECHANISM If C inherits and joins two or more deferred features the net result for C is as if it had inherited a single deferred feature I
4. I gt I gt I gt l gt I gt I gt Parenthesized_qualifier Call_chain Parenthesized Q e Unqualified_call e Entity Actuals Actual_list Actual Expression Address Address_mark Feature_name Current Result 21 12 EXECUTING THE BODY To execute or run a system on a machine means to get the machine to apply a creation instruction to the system s root class Definition current object In all but trivial cases the root s creation procedure will create more objects and execute more calls This extremely simple semantic definition of system execution has as its immediate consequence to yield a precise definition of the current object and current routine At any time during execution the current object CO is the object to which the latest non completed routine call applies and the current routine cr is the feature of that call They may be defined precisely as follows At the start of the execution of a system CO is the root object the instance of the root class whose creation is the first act of system execution and cr is the selected creation procedure If the root class has no creation procedure execution terminates immediately 20 14 RULES ON ASSIGNMENT ATTEMPT 69 WALIGITY VJRV Definition dynamic type set dynamic class set The set of possible dynamic types for an entity or expression x is called the dynamic type
5. The present book will mostly be useful as a summary of Eiffel The Language for readers who are familiar with that earlier book Must it be assumed that because we are engineers beauty is not our concern and that while we make our constructions robust and durable we do not also strive to make them elegant Is it not true that the genuine conditions of strength always comply with the secret conditions of harmony The first principle of architectural esthetics is that the essential lines of a monument must be determined by a perfect adaptation to its purpose Gustave Eiffel 1887 From his response in the newspaper Le Temps to a petition by members of the literary and artistic Establishment protesting his project of elevating a tower of iron in Paris Book identification Eiffel The Reference ISE Technical Report TR EI 41 ER Publication history First published as Eiffel The Language TR EI 2 BR in 1988 Successive versions appeared in subsequent years and were replaced by TR EI 17 RM Eiffel The Language in 1991 second revised printing 1992 also available as a book published by Prentice Hall ISBN 0 13 245 925 7 Bisst edition ol Eifel Hie Reference der the presen reper puma ber appedced o present pays bois oe eoprespendine to seputen 2 ef be Te ellen ronnent his een thec to tne Lance Cop witiee of the Sonpee tt buepagonel Consoris ea as pester 2 ot the prepesed batted Peene The majority of the material in th
6. The printable characters include letters digits underscore the four characters permitted as first character of a Free_operator and other special characters such as star and They exclude Blank New Line Backspace and other characters with no external representation When used in expressions the standard operators have various precedence levels as given in the discussion of expressions free operators all have the same precedence higher than that of the standard operators Examples of free operators used as function names were given in the discussion of features A simple one is used in infix form as a synonym for item for array access a iis the i th element of i The Basic Libraries contain only a small number of uses of free operators for element access in arrays and strings for rotation in bit sequences Free operators are mostly intended for developers in application areas that have a tradition of specialized notations such as physics and mathematics The form of a free operator should attempt to suggest its meaning just as with a well chosen identifier The first character of a Free_operator must be one of only four possibilities not used by any other construct of the language As a result any free operator will stand out clearly from its context and no confusion or ambiguity is possible 25 11 STRINGS Definition string A String a specimen of construct String is an arbitrary sequence of characters
7. operator The names that may come in double quotes after infix or prefix and will be used in the corresponding operator expressions are called operators There are two kinds of operators standard and free A Free_operator is not as free as the name would seem to suggest Definition free operator A Free_operator is sequence of one or more characters whose first character is any one of I amp and whose subsequent characters if any may be any printable characters VIRW 25 5 TEXT LAYOUT 85 In both cases the form is the same a comment is made of one or more line segments each beginning with two consecutive dash characters and extending to the end of the line We Comment Simple_string Comment_break Comment_break New_line Blanks_or_tabs 25 5 TEXT LAYOUT An Eiffel text is a sequence each of the elements of the sequence is a break a comment or a token You may always insert a break between two sequence elements without affecting the semantics of the text A break is not required between two adjacent elements if one is a comment and the other a token or another comment Between two successive tokens a break may be required or not depending on the nature of the tokens We may divide tokens into two categories Definition symbol word 66599 e A symbol is either a special symbol of the language such as the semicolon and the e of dot notation
8. to Z yields X VNCN 12 15 ANCHORED TYPES 49 The base type BT of an anchored type like anchor appearing in a class C is determined as follows Definition base class base type anchored le If anchor is the final name of some feature of C then BT is the declared type of that feature in C 2e If anchor is a formal routine argument then BT is the type declared for that argument in the Formal_arguments list 3 If anchor is Current then BT is C followed by its Formal_generics if any with any Constraint removed 48 12 12 12 12 RULES ON EXPANDED TYPES Definition expanded type reference type A type T is expanded if and only if one of the following conditions holds le Tis aClass_type whose base class C is an expanded class 2e Tis of the form expanded CT As noted it is redundant but not erroneous for the base class of CT to be an expanded class 3 Tis of the form BIT M for some non negative integer M T is a reference type if it is not a Formal_generic_name and none of the above condition applies Expanded Type rule It is valid to use an expanded type of base class C in the text of a class B if and only if it satisfies the following two conditions le Cis not a deferred class 2e C either has no Creators part or has a Creators part containing exactly one creation procedure with no argument available to B for creation Any entity declared of an expanded type has
9. 2 be parents of D having a common ancestor A Let fi fa be features of these respective parents all having as their seed the same feature f of A Then le Any subset of these features inherited by D under the same final name yields a single feature of D 2e Any two of these features inherited under a different name yield two features of D Definition shared replicated Features will be said to be shared if case 1 of the Repeated Inheritance rule applies and replicated if case 2 applies 10 23 RULES ON JOINING FEATURES 41 Join Semantics rule le 2 o 36 4e 5 6 Joining deferred features with the same final name yields a non obsolete deferred feature defined as follows Its name is the final name of all its precursors Its signature is the precursors signature which the Join rule indicates must be the same for all precursors after possible redeclaration Its precondition is the or of all the precursors preconditions Its postcondition is the and of all the precursors postconditions Its Header_comment is the concatenation of those of all precursors It is not obsolete even if some of the precursors are obsolete 40 10 23 Redeclaration rule Let C be a class and g a feature of C It is valid for g to be a redeclaration of a feature f inherited from a parent B of C if and only if the following conditions are satisfied le No effective feature of C other than f and
10. 2e The dynamic type set for T of an occurrence of Current in the text of a routine of C is T 3 For any entity or expression e of expanded type appearing in the text of C if the type ET of e is expanded the dynamic type set of e for Tis ET 7 Rules 4 to 7 when used to determine elements of the dynamic type set of some e assume that e s type is not expanded 72 21 14 effect of a qualified call of the form seu where s is an expression and u is an Unqualified_call is defined recursively as the effect of a call of the form veu executed after the assignment v targetes v being a Writable entity used only for this definition D5 If dfis a function the call is syntactically an expression the value of that expression is the value of the entity Result on termination of the function s execution D6 If the values of any local entities have been saved under D2 restore these entities to their earlier values This terminates the execution 21 14 SEMANTICS OF CALLS 71 2 If crexecutes any construct other than a call the current object and current routine remain the same 3 If crexecutes a qualified call of the form targete fname where the value target_ value of target is attached to an object OD then for the duration of the call OD becomes the new current object and df the routine deduced from fname as discussed earlier becomes the new current routine When the qualified call termina
11. Ifthe value of y is attached to an object OY the call returns a newly created object of the same type as OY initialized by applying copy to that object with OY as source 19 7 OBJECT EQUALITY Here is the precise result that the standard version of equal must return when applied to two values x and y le If any one of x and y is void the result is true if the other is also void too and false otherwise Cases 2 to 6 assume that both arguments are attached to respective objects OX and OY of types TX and TY 2 If OX and OY are bit sequences the result is true if and only if the two sequences with the shorter one being extended with zeros on the left to match the length of the longer one if necessary are bit by bit identical Cases 3 to 6 assume that OX and OY are not bit sequences 18 13 DEFAULT INITIALIZATION VALUES 65 e For BOOLEAN the boolean value false e For CHARACTER the null character e For INTEGER REAL or DOUBLE the integer single precision or double precision Zero e For POINTER a null pointer e For a Bit_type of the form BIT N a sequence of N zeros 64 18 10 2 Cisan effective class 3 Ifthe Type part is present the type that it contains which is T conforms to the type of x and is a reference type 4e If C does not have a Creators part there is no Creation_call part 5e If C has a Creators part there is a Creation_call part and the call would be argument valid i
12. T is a valid Class_type generically derived or not expanded T is a valid Class_type_ expanded and the possible values for entities of that type are instances of T 46 12 4 12 Types 12 4 HOW TO DECLARE A TYPE Type 2 Class_type Actual_generics Type_list Class_type_expanded Bit_type Anchored Anchor l gt We We We We We We Class_type Class_type_expanded Formal_generic_name Anchored Bit_type Class_name Actual_generics Type_list Epe 4c expanded Class_type BIT Constant like Anchor Identifier Current 12 5 BASE CLASS BASE TYPE Class Type rule An Identifier CC is valid as the Class_name part of a Class_type if and only if it is the name of a class in the surrounding universe 12 7 UNCONSTRAINED GENERICITY Definition base class base type class Type The base class of a generically derived type is the class used to derive it by providing actual generic parameters Definition constrained unconstrained generic The syntax for Class_declaration includes an optional Constraint part after every formal generic parameter If present this part makes the parameter constrained if not the parameter is unconstrained A generic class is constrained if it has at least one constrained parameter unconstrained otherwise SYNTAX 5 3 VTCT 11 13 THE REPEATED INHERITANCE CONSISTENCY CONSTRAINT 45 2e If the Repeated Inheritance rule im
13. We We We I gt We Wo We We external Language_contribution Language File_list Eiffel Ada Pascal Fortran C Object Make Name generate Language_generation Language Generate_option Target Generate_option_value yes no Directory File Visible Class_visibility Visibility_adaptation External_class_rename External_feature_rename Creation_restriction Export_restriction External_feature_rename External_rename_list External_rename_pair We We We We We We We We Ne We visible Class_visibility Class_name Visibility_adaptation External_class_rename Creation_restriction Export_restriction External_feature_rename end as Name as Name creation Feature_name export Feature_name rename External_rename_list External_rename_pair Feature_name as Name STANT AR STATA D 11 SPECIFYING OPTIONS 101 SEMANTICS t Option assertion collect debug optimize trace Governs Level of assertion monitoring and execution of Check instructions Garbage collection Execution of Debug instructions Optimization of generated code Generation of run time tracing information for every call to and return from routines of classes to which the option applies Possible values no require ensure invariant loop check all
14. a non empty Actual_generics part whose base class C is not a constrained generic class CT is valid if and only if C satisfies the following two conditions le Cis a generic class 2e The number of Type components in CT s Actual_generics list is the same as the number of Formal_generic parameters in the Formal_generic_list of C s declaration A generically derived type is expanded if its base class is an expanded class otherwise it is a reference type 12 8 CONSTRAINED GENERICITY The effect of a Constraint if present is to restrict allowable actual generic parameters to types that conform to the Class_type given Constrained Genericity rule Let C be a constrained generic class A Class_type CT having C as base class is valid if and only if CT satisfies the two conditions of the Unconstrained Genericity rule VFUG page n Ot and in addition 3 For any Formal_generic parameter in the declaration of C having a constraint of the form gt D the corresponding Type in the Actual_generics list of CT conforms to D 12 9 USING FORMAL GENERIC PARAMETERS AS TYPES Definition base class base type constrained generic We consider the base type of a constrained generic parameter to be its constraining type with the associated base class Definition base class base type unconstrained generic ANY serves as both the base type and the base class of any unconstrained Formal_ generic_name 12 11 CLASS TYPES EXPANDED If
15. all Free_value require ensure invariant loop check Free_value File_name Directory_name Name A Target_list may only appear in an Options paragraph not in a Defaults paragraph A System_tag may only appear in an Ace level Defaults clause ALIIT VDOC D 8 STORING PROPERTIES WITH A CLUSTER 99 STANT AR VDUC STATA STATA Defaults Options Visible end cluster Cluster _name D 8 STORING PROPERTIES WITH A CLUSTER use File Name To keep things simple the Cluster_properties part contained in a Use file may not itself contain a Use paragraph D 9 EXCLUDING AND INCLUDING SOURCE FILES Include include File_list Exclude exclude File_list File_list Siler ay D 10 ADAPTING CLASS NAMES Name_adaptation 2 adapt Cluster_adaptation_list Cluster_adaptation_list A Cluster_adaptation Cluster_adaptation a Cluster_ignore Cluster_rename_clause Cluster_ignore a Cluster_name ignore Cluster_rename_clause 2 Cluster_name rename Class_rename_list Class_rename_list A Class_rename_pair Class_rename_pair Class_name as Class _name 98 D 6 D 6 ACE STRUCTURE Ace System System_name Root Class_name Cluster_mark Cluster_name Creation_procedure I We We We l gt We Wo We System Root Defaults Clusters Externals Generation end system Sy
16. an unsigned integer representing the character of code code For example in ASCII 59 represents the character of code 59 which is the semicolon The table of special character codes for form C2 is the following _ Special character Character Mnemonic name Code TA At sign BS B Backspace A C Circumflex D Dollar EE APF Form feed H backslasH tilda L tiLda NL LF JN Newline O back Quote CR R carriage Return S Sharp HT T horizontal Tab NUL U nUll character TV Vertical bar o percent single quote k To double quote opening bracket closing bracket lt opening brace gt closing brace 88 25 13 An Integer is a sequence of characters each of which must be either e A decimal digit 0 to 9 e An underscore _ which may not be the first character If any underscore is present then there must be three consecutive digits to the right of every underscore and there must not be any consecutive group of four digits Underscores if any have no effect on the integer value associated with the integer 25 13 REAL NUMBERS A real number is made of the following elements e An optional Integer writen backwards giving the integral part If this is absent the integral part is 0 e A required e dot e An optional Integer which gives the fractional part If this is absent the fractional part is 0 e An optional
17. class C is a client of a class B if C is a client of a type whose base class is 8 A class C is a direct or indirect client of a type S of base type B if there is a sequence of classes G A Co Gy B such that n gt 1 and every C is a client of CH 1 for Heit n The direct or indirect forms of the simple client expanded client and generic client relations are defined similarly 7 4 SIMPLE CLIENTS Definition simple client A class C is a simple client of a type S if some entity or expression of C is of type S 7 5 EXPANDED CLIENTS Definition expanded client A class C is an expanded client of a type S if S is an expanded type and some attribute of C is of type S Expanded Client rule It is valid for a class C to be an expanded client of a class SC if and only if SC is not a direct or indirect expanded client of C VALIDITY 7 6 GENERIC CLIENTS VLEC Definition generic client generic supplier A class C is a generic client of a type S if for some generically derived type T of the form B S one of the following holds le Cisaclient of T 2e One of the Parent clauses of C or of a proper ancestor of C lists T as parent 26 6 12 6 12 ANY Any class other than GENERAL and ANY which does not include an explicitly written Inheritance clause is considered to have an implicit clause of the form inherit ANY 6 15 PROVIDING YOUR OWN UNIVERSAL CLASS Whether you use the
18. corresponding to attribute a is an instance of the expanded type TA In other words the field is itself an object called a sub object of the enclosing object Depending on the precise nature of TA the sub object may be of various forms TA may be a basic type in which case the sub object is a basic object of the corresponding type the figure shows fields of type INTEGER DOUBLE and Bit_ type Definition composite object e If TA is anon basic expanded type the sub object is itself a complex object In this case the enclosing complex object is said to be composite 17 9 EXPRESSIONS AND ENTITIES Definition entity le 2 o 3o In a class text four kinds of entity may appear Final names of attributes of the class Local entities of routines including the predefined entity Result for functions Formal routine arguments 60 17 2 17 Objects values and entities 17 2 OBJECTS Definition standard object special object There are two kinds of object standard and special e A standard object is the direct result of a Creation instruction or clone operation executed by the system e A special object is a sequence of values all compatible with a given type It may be a String or an array In a string the values are all characters in an array they are either all references or all direct instances of a single type 17 4 INSTANCES OF A CLASS Definition instance of a class An instance of a class C i
19. declared in Eiffel as expanded class To force a result describing an expanded type apply eif_expanded to the result of either function the result is another type id All these functions return as result the error code EJF_NO_ TYPE if they cannot compute a type id no class with the given name in the universe more than one class wrong number of generic parameters It is possible to create an object from C using the function eif_create which takes an EJF_TYPE_ID argument and returns an EJF_OBJ Here is the list of functions used for calling the various kinds of Eiffel features from C with their types and template arguments EIF_PROC eif_proc routine_name type_id EIF_FN_BOOL eif_fn_bool routine_name type_id EIF_FN_CHAR eif_fn_char routine_name type_id EIF_FN_INT eif_fn_int routine_name type_id EIF_FN_REAL eif_fn_real routine_name type_id EIF_FN_DOUBLE eif_fn_double routine_name type_id EIF_FN_POINTER eif_fn_pointer routine_name type_id EIF_FN_REF eif_fn_ref routine_name type_id In all cases the arguments are a string representing a routine name and a type id obtained through eif_type_id or eif_generic_ic char starroutine_name EIF_TYPE_ID type_id These functions look for a routine of name routine_name in the base class of the type corresponding to type_id If no such routine exists the result is a null pointer Otherwise it is a pointer to a C function representing the desired routine you may then call that f
20. default ANY or another one any system will need to have a class of name ANY This is a constraint on any valid universe 6 10 FEATURES AND THEIR NAMES 25 6 10 FEATURES AND THEIR NAMES Definition name of a feature in a class Within the text of a class C any feature f of C is accessible through a feature name known as the name of f in C As this expression suggests the association between a feature and a feature name is not absolute but relative to a class The same feature may well be denoted by different names in different classes Definition original name The original name of a feature is the name under which it is declared in its class of origin Definition final name Every feature f of a class C has a final name in C defined as follows e If fis immediate in C its final name is its original name e If fis inherited fis obtained from a feature of a parent B of C Let parent_name be recursively the final name of that feature in B Then e If the Parent clause for B in C contains a Rename_pair of the form rename parent_ name as new_name the final name of fin C is new_name Otherwise the final name is parent_name Definition final name set The final names of all the features of a class constitute the final name set of a class Definition inherited name The inherited name of a feature obtained from a feature f of a parent B is the final name of fin B Definition name of a feature In this bo
21. exception in the current routine which becomes the recipient of the exception Depending on how the software has been written the exception will be handled through one of the three techniques listed above 15 8 SYNTAX AND VALIDITY OF THE EXCEPTION CONSTRUCTS SYNTAX Rescue 2 rescue Compound Retry retry VALIDITY It is valid for a Routine to include a Rescue clause if and only if its Routine_body is of VXRC the Internal form 56 14 8 computation unchanged if the Boolean_expression of its Exit clause evaluates to false otherwise it is the effect of executing the Compound clause followed recursively by the effect of executing the Loop_body again in the resulting state 14 8 THE DEBUG INSTRUCTION Debug A debug Debug_keys Compound end Debug_keys Debug_key_list Debug_key_list 4 Debug key Debug_key a Manifest_string The effect of a Debug instruction depends on the mode that has been chosen for the enclosing class e If the debug option is on generally or if the instruction includes a Debug_key_list and the option is on for at least one Debug_key in the list the effect of the Debug instruction is the same as that of its Compound e Otherwise the effect is that of a Null instruction 14 7 LOOP 55 Character_constant e Character_ constant Character_interval a Definition unfolded form of a multi _branch To discuss the constraints and th
22. feature will have a version df applicable to the current object this is the version of f for the current object s generator taking into account possible renaming and redefinition The value passed for attachment to the corresponding formal argument is the address of df This applies to both routines and variable attributes for an attribute the call will pass the address of the field corresponding to df in the current object 24 7 THE CECIL LIBRARY This section describes a library for interfacing with C software Cecil The Cecil library contains macros functions types and error codes All have names beginning with either eif_ functions and macros or E F_ types and error codes examples are the function eif_type_id and the type EJF_PROC explained below Their declarations appear in a C header file cecileh which you may add to a C program through the C preprocessor directive include lt cecileh gt Let us now look at the principal facilities declared in cecile h First of all the C side will need to refer to Eiffel types It will know a type through a type id of type EIF_ TYPE_ID To obtain a type id for a non generic class of name CLASSNAME and record WALIDITY VZAA 23 21 STRIP EXPRESSIONS 79 to attributes a d appearing in an order which only depends on class C that is to say is the same for all possible values of the object CO 78 23 17 Bit_constant Bit_seq
23. is clear making it easy to find the corresponding place in Eiffel The Language if when examining some part of the edifice you want to retrieve the scaffolding in other words some of the supporting comments and examples Using a different section numbering scheme for the present book would have caused endless misunderstandings Unlike the sections the pages have been renumbered since consecutive numbering facilitates searching for specific information In contrast with Eiffel The Language this book avoids repetition of any kind This is why you will find no syntax summary or syntax diagram Such elements which are present in the complete description will be easy to add if readers feel they are necessary There is currently no index but this omission will be repaired in a future version Finally please remember that the present edition is only the second release of Eiffel The Reference In particular some errors may remain in the software that served to extract the information and produce the index Indulgence is thus requested from the reader ACKNOWLEDGMENTS In addition to the acknowledgments included at the end ef the preface to Eiffel The Language it is appropriate to thank the various readers who have contributed comments since the book s publication most remarkably David Hollenberg from the Information Systems Institute of the University of Southern California and Helmut Weber from Austria formerly from IBM THE
24. is to execute that routine on the actual arguments given if any according to the rules of the language in which it is written There remains to cover the case in which dfis a non external non once routine with a Routine_body beginning with the keyword do Then the effect of the call is the effect of the following sequence of steps D1 If dfhas arguments attach every formal argument to the value of the corresponding actual argument applying the semantics of direct reattachment D2 If df has any local entities save the current values of these entities if any call to df has been started but not yet terminated then initialize each local entity to the default value of its base type D3 If df is a function initialize the predefined entity Result again according to the default initialization rules D4 Execute the body of df In this execution evaluation of the entity Current will return target_value the effect of an Unqualified_call u where u is neither Current nor Result is defined recursively as the effect of the qualified call targeteu and the 70 21 3 21 Feature call 21 3 e If the feature is an attribute or a function the Call is syntactically an expression USES OF CALLS e If the feature is a procedure the Call is an instruction 21 6 FORM OF A CALL Call Parenthesized_qualifier Call_chain Unqualified_call Actuals Actual_list Actual Address Address_mark I gt We Ne
25. lt P Other than implied by these rules no type conforms directly to a Bit_type and a Bit_type conforms directly to no type 13 7 FORMAL GENERIC PARAMETERS 51 VNCF VALIDITY VNCH VNCX VNCE 13 7 FORMAL GENERIC PARAMETERS Let G be a formal generic parameter of a class which in the class may be used as a type of the Formal_generic_name category No type conforms directly to G If G is not constrained it conforms directly to the type ANY based on the corresponding universal class and to no other type If G is constrained by CT G conforms directly to CT and to no other type 13 8 ANCHORED TYPES In a class C type like Current conforms directly to its base type CT where CT is C followed by its Formal_generic_list if any with any Constraint removed Type like anchor where anchor is a feature of C or a formal argument of a routine of C conforms directly to the type of anchor in C An anchored type conforms directly to no type other than implied by these rules No type conforms directly to an anchored type 13 9 EXPRESSION CONFORMANCE An expression v of type VT conforms to an expression t of type TT if and only if they satisfy any one of the following four conditions le VT conforms to TT 2e TT is like v v in this case must be an entity 3 VT and TT are both of the form like x for the same x 4e TT is like x where x is a formal argument to a routine r v is an actual argument in a c
26. text for a routine r and the Local_declarations part for r contains an Entity_ declaration_list including e as part of its Identifier_list 3 The occurrence is in a Feature_declaration for a routine r and the Formal_ arguments part for r contains an Entity_declaration_list including e as part of its Identifier_list 4e eis Current 17 7 FIELDS OF COMPLEX OBJECTS 61 e For BIT n with n 2 0 all the sequences of n binary zero or one values none if n 0 17 7 FIELDS OF COMPLEX OBJECTS Definition complex class complex type field 2 o 3o Every class other than BOOLEAN CHARACTER INTEGER REAL DOUBLE and POINTER is said to be a complex class Any type whose base class is complex is itself a complex type and its instances are complex objects Consider a class type TC of base class C and an attribute a of class C with TA being the type of a The possible values for the field corresponding to attribute a in a direct instance of TC depend on the nature of TA There are three possible cases for TA Reference type This also covers the case of an anchored type of the form like x which has a class type as base type Expanded type Formal generic parameter of class C In case 1 the field corresponding to attribute a is a reference That reference may be void or it may be attached to an instance of TA s base type not necessarily a direct instance Definition sub object In case 2 the field
27. 110 H H 109 108 H H 107 106 G 4 G 4 105 lt lt gt gt for manifest arrays semicolon as separator between an Assertion_clause and the next 104 G 2 G 2 RESERVED WORDS alias all and as BIT BOOLEAN CHARACTER check class creation Current debug deferred do DOUBLE else elseif end ensure expanded export external false feature from frozen if implies indexing infix inherit inspect INTEGER invariant is like local loop NONE not obsolete old once or POINTER prefix REAL redefine rename require rescue Result retry select separate STRING strip then true undefine unique until variant when xor G 4 OPERATORS AND THEIR PRECEDENCE Level 12 11 10 Symbol e Dot notation for Unqualified_call expressions old in postconditions strip not unary unary All free unary operators All free binary operators power integer division integer remainder binary binary not equal w AIAI AINI oO oO and and then implies G VISIBLE FEATURES 103 G Reserved words special symbols operator precedence 102 D 12 D 12 SPECIFYING EXTERNAL ELEMENTS Externals Language_contribution Language D 13 GENERATION Generation Language_generation Generate_option Generate_option_value Target D 14 VISIBLE FEATURES
28. 5 32 Basic classes The basic class BOOLEAN CHARACTER DOUBLE INTEGER and REAL describe arithmetic objects class POINTER describes opaque objects representing addresses to be passed to non Eiffel software The specification of these classes may be found in the Proposed Eiffel Library Kernel Standard 94 25 15 31 Input and output Classes FILE and STANDARD_FILES provide input and output facilities Their specification may be found in the Proposed Eiffel Library Kernel Standard 25 15 CHARACTERS 93 30 Persistence and environments Class STORABLE provides a mechanism for storing and retrieving object structures Its specification may be found in the Proposed Eiffel Library Kernel Standard 92 25 15 29 Exception facilities The Kernel Library class EXCEPTIONS provides a number of features for fine control of the exception handling mechanism Their specification may be found in the Proposed Eiffel Library Kernel Standard 25 15 CHARACTERS 91 28 Arrays and strings Arrays and strings are instances of the Kernel Library classes ARRAY and STRING The specification of these classes may be found in the Proposed Eiffel Library Kernel Standard 90 25 15 27 Universal features The universal features are those of class GENERAL They are specified in the Proposed Eiffel Library Kernel Standard 25 15 CHARACTERS 89 C3 The sequence code where code is
29. CH CHOICE Definition inspect expression A Multi_branch instruction contains a Expression called the inspect expression appearing after the keyword inspect The inspect expression last_input in the example may only be of type IVTEGER or as here CHARACTER It includes one or more When_part each of which indicates a list of one or more Choice separated by commas and a Compound to be executed when the value of the Expression is one of the given Choice values Definition inspect constant Every Choice specifies one or more values called inspect constants More precisely a Choice is either a single constant Manifest_constant or constant attribute or an interval of consecutive constants yielding all the interval s elements as inspect constants If present the instruction s optional Else_part is executed when the inspect expression is not equal to any of the inspect constants Multi_branch inspect Expression When_part_list Else_part end When_part_list A when When_part when aa When_part 4 Choices then Compound Choices 2 Choice Choice Constant Interval Interval A Integer_interval Character_interval Integer_interval Integer_constant ee Integer_constant 14 2 COMPOUND 53 14 Control structures 14 2 COMPOUND SYNTAX Compound A Instruction The effect of executing a Compound may be defined as follows e If the Compound has zero instructions the ef
30. RSTUVWXYZ A decimal digit is one of the ten characters 0123456789 An alphabetic character is a letter a digit or an underscore _ A printable character is any one of the characters listed as printable in the definition of the ASCII character set 25 3 BREAKS Definition break character break A break is made of a sequence of one or more of the following characters known as break characters e Blank also known as space e Tab e New Line also known as Line Feed 25 4 COMMENTS Definition comment Comments may be expected or free e An expected comment is a specimen of the construct Comment appearing in the syntax as an optional component of some construct An example is the Header_ comment of a Routine e Free comments on the other hand may appear at almost any position in a class text and are not covered by the syntax productions 24 7 THE CECIL LIBRARY 83 Here position is an integer index which must be in the range of the bit sequence counted from 1 otherwise the first two calls have undefined results and eif_bit_ith returns EJF_NO_BIT Routines eif_bit_set and eif_bit_clear set the element of index position to and 0 respectively Function eif_bit_ith returns an integer the value of the element of index position Function eif_bit_length returns an integer the length of the sequence Function eif_bit_clone returns an EJF _BIT a fresh copy of the bit sequence passed as argument 82 24 7
31. SHORT FORM The worst that could happen to the description of Eiffel is to follow the fate of so many earlier languages the emergence of several descriptions each slightly incompatible with the others Eiffel The Language went to considerable length to prevent this from happening By threading several levels of discourse into a single cloth the book was able to forestall eventual divergence In particular the book relied on an extensive system of road signs to identify each of the interwoven threads syntax validity semantics comment caveat preview reminder As this system is directly reflected in the source electronic form of the original document it is possible to use software tools aided by a little human intervention to extract one or several of the threads This approach is what made the present book possible It is not strictly speaking a new book but an extract of the relevant parts of Eiffel The Language Another way of expressing this observation is to claim that producing this book was to a certain extent a software project writing the tools that would extract the essential parts of the complete document and ignore the rest This effort required some massaging as programmers say of the original text to mark some parts as retained and others as discarded But the extent of that massaging was remarkably limited the text was organized in such a systematic way that most of the extraction could be done automat
32. a routine r of a class C is valid if and only if every feature whose final name appears in any Assertion_clause is available to every class to which r is available 32 8 9 TYPES OF INSTRUCTIONS Instruction A Creation Call Assignment Assignment_attempt Conditional Multi_branch Loop Debug Check Retry 8 6 ROUTINE BODY 31 SYNTAX J SYNTAX VRLE 2 Inthe other cases where the Routine_body is External or Deferred there is neither a Local_declarations part nor a Rescue part 8 6 ROUTINE BODY Routine_body 2 Effective Deferred Effective Internal External Internal 2 Routine _mark Compound Routine _mark dolonce Deferred deferred The introductory keywords do or once of an Internal body correspond to different semantics for calls to the routine le For a do body as indicated above the initialization and body are executed anew on each call 2e If routine o of class C has a once body o is then called a once routine the initialization and body of o are executed only for the first call to o applied to an instance of C during any given session For every subsequent call to o applied to an instance of C during the same session the routine call has no effect if the routine is a function the value it returns is the same as the value returned by the first call 8 7 LOCAL ENTITIES AND Result Local_declarations local Entity_declaratio
33. all to r and VT conforms to the type of the actual argument corresponding to x in the call 5e visa call to some function f of type like x where x is a formal argument of f and the type of the actual argument corresponding to x in the call conforms to TT 13 10 EXPANDED TYPES Definition heavier arithmetic type Any arithmetic type conforms to heavier ones where DOUBLE is heavier than REAL and INTEGER and REAL is heavier than INTEGER Let T be an expanded type other than a Bit_type A type U conforms directly to T if and only if they satisfy any one of the following three conditions le Tis of the form expanded BT and U is BT 2 Tis REAL and U is INTEGER 3 Tis DOUBLE and U is REAL or INTEGER In case 1 T also conforms directly to U An expanded type conforms directly to no type other than implied by this rule and the rules of 13 5 and 13 6 50 13 3 13 Conformance 13 3 SIGNATURE CONFORMANCE A signature t lt B B gt lt S gt conforms to a signature s lt A A gt lt R gt if and only if it satisfies the following conditions l Each of the two sequence components of t has the same number of elements as the corresponding component of s 2e Every type T in each of the two sequence components of t conforms to the corresponding type S in the corresponding component of s 13 4 DIRECT AND INDIRECT CONFORMANCE Let T and V be two types V conforms to T if and only if one of t
34. at lengths to include every relevant validity and semantic property it may of course have left an occasional one out Such an oversight might be a case of incompleteness a missing validity constraint or semantic specification or inconsistency ambiguous or contradictory answers If you run into such a case while trying to produce language processing tools please contact the language committee of NICE by sending electronic mail to lt language chair nice twr com gt 10 2 7 Every validity constraint has a four character code beginning with V shown as VVVV in the above fictitious example 2 7 INTERPRETING THE CONSTRAINTS General Validity rule Every validity constraint relative to a construct is considered to include an implicit supplementary condition stating that every component of the construct satisfies every validity constraint applicable to the component 2 8 SEMANTICS A construct specimen which is syntactically legal and valid has an associated semantics specifying its run time effect in a system in which the specimen appears That semantics may include executing certain actions producing a value or both It is defined by the paragraphs For specimens made of further components the specification usually refers recursively to the components own semantics It is important to remember that the SEMANTICS paragraphs only apply to valid specimens In many cases the semantic rules would not even make sense
35. ation_call e Unqualified_call Definition creation Type To discuss the validity and semantics of the Creation instruction it is useful to introduce the notion of creation type of a Creation instruction The creation type is the optional Type appearing in the instruction between exclamation marks if present otherwise it is the base type of the target 18 9 CREATION VALIDITY CLASS LEVEL Creation Instruction rule Consider a Creation instruction appearing in a class X Let x be the target of the instruction T its creation type and C the base class of T The instruction is class valid if and only if it satisfies the following conditions le Tis not a Formal_generic_name that is to say a formal parameter of the class where the instruction appears 62 17 9 4e Current the predefined entity used to represent a reference to the current object the target of the latest not yet completed routine call Unique 2 unique Entity rule An occurrence of an entity e in the text of a class C other than as feature of a qualified call is valid if and only if it satisfies one of the following conditions WALIDITY le eis the final name of an attribute of C 2A The occurrence is in a Local_declarations Routine_body Postcondition or Rescue VEEN part of a Routine text for a function and e is the Local entity Result 2B The occurrence is in a Local_declarations Routine_body or Rescue part of a Routine
36. atisfies either of the following two conditions le The call is an Unqualified_call and fname is the final name of a feature of C 2e The call has at least one dot D has a feature of name fname which is available to C and the call s target is either a valid entity of C or recursively a call which is export valid for D Consider an export valid call of target target and feature name fname appearing in a class C For an Unqualified_call take target to be Current Let ST be the type of target S the base class of ST and sf the feature of final name fname in S Let D be a descendant of S and df the version of sfin D The call is argument valid for D if and only if it satisfies the following four conditions le The number of actual arguments is the same as the number of formal arguments declared for df 2e Every actual argument if any conforms to the the corresponding formal argument of df 3 If target is itself a Call it is recursively argument valid for D 4e If any of the actual arguments is of the Address form fn fn is the final name of a feature of C which is not a constant attribute The dynamic type sets of the expressions entities and functions of a system result from performing all possible applications of the following rules to every Class_type T of base class C used in the system le Ifa routine of C contains a creation instruction with target x and creation type U the dynamic type set of x for Tis Uy
37. ature_declaration is a routine declaration if and only if it satisfies the following condition e There is a Constant_or_routine part whose Feature_value is a Routine In this case the Formal_arguments and Type_mark parts may or may not be present If the Type_mark is present the declaration describes a function otherwise it describes a procedure SYANTAR 5 3 IMMEDIATE AND INHERITED FEATURES 17 STANT AR 5 Features 5 3 IMMEDIATE AND INHERITED FEATURES Definitions features of a class inherited origin introduced le 2 o The features of a class C include its inherited features and its immediate features defined as follows The features obtained by C from its parents if any are its inherited features In the Features part of C consider a declaration describing a feature f If f is inherited the declaration is in fact a redeclaration of f giving f new properties in C If this is not the case fis a new feature said to be immediate in C C is then the class of origin or simply origin of f which is also said to be introduced in C 5 6 FEATURES PART SYNTAX I gt Features feature Feature_clause feature I gt Feature_clause Clients Header_comment Feature_declaration_list Feature_declaration_list Feature_declaration We Header_comment Comment 5 7 FORMS OF FEATURES Every feature of a class is either an attribute or a routine Defin
38. d B depends on A Root Class rule A class C may be used as root of a system if and only if it satisfies the following three conditions VALIDITY le Cis not generic VSRC 2e Cis not deferred 3 Any creation procedure of C has either no formal argument or a single formal argument of type ARRAY STRING 3 5 CLUSTERS Definition Cluster A cluster is a set of classes all with different names No two classes in a given cluster may have the same class name 3 6 UNIVERSES VSCN Definition Universe A universe is a set of clusters 12 3 The architecture of Eiffel software 3 1 OVERVIEW The constituents of Eiffel software are called classes By extracting classes from a given universe you may assemble them into executable systems To keep your classes and your development organized it is convenient to group classes into clusters These four concepts provide the basis for structuring Eiffel software e A class is a modular unit e A system results from the assembly of one or more classes to produce an executable unit e A cluster is a set of related classes e A universe is a set of clusters out of which developers will pick classes to build systems Of these only class describing the basic building blocks corresponds directly to a construct of the language To build systems out of classes you will use not a language mechanism but tools of the supporting environment As to clusters and
39. d form and C is effective VCCH 2 e There is a Header_mark of the deferred form and C is deferred 14 4 2 4 Classes 4 2 OBJECTS Definitions object instance Viewed as a type a class describes the properties of a set of possible data structures or objects which may exist during the execution of a system that includes the class these objects are called the instances of the class 4 3 FEATURES Definitions feature attribute routine Viewed as a module a class introduces through its class text a set of features Some features called attributes represent fields of the class s direct instances others called routines represent computations applicable to those instances tY Since there is no other modular facility than the class building a software system in Eiffel means analyzing the types of objects the system will manipulate and writing a class for each of these types 3 4 SYSTEMS 13 case independent names even if the names are written with different case conventions in the class texts 3 4 SYSTEMS Definition System Root A system is a set of classes one of which has been designated as the root of the system such that all the classes on which the root depends belong to the system Definition Dependency Here a class C is said to depend on a class A if one of the following holds e Cis an heir of A e Cis a client of A e Recursively there is a class B such that C depends on B an
40. dynamic type set of e for T If a routine of C contains an Assignment_attempt of target x with type U and source e the dynamic type set of x for T includes recursively every type conforming to Uy which is also a member of the dynamic type set of e for T If a routine of C contains a call h of target ta U is recursively a member of the dynamic type set of ta for T and f1s the version of the call s feature in the base class of U then the dynamic type set for U of any formal argument of tf includes every member of the dynamic type set for Uy of the corresponding actual argument in A If h tf and U are as in case 6 and ff 1s an attribute or function the dynamic type set of h for T includes recursively every member of the dynamic type set for Uy of the Result entity in tf 22 9 THE CALL VALIDITY RULE 73 VUGV VUCS WALIDITY VUEX AALIDE VUAR 22 Type checking 22 9 THE CALL VALIDITY RULE Call rule A call is valid if and only if it is both class valid and system valid Consider a single dot call with target x appearing in a class C Let S be the type of x Then le The call is class valid if it is export valid and argument valid for S 2e The call is system valid if for any element D of the dynamic class set of x it is export valid and argument valid for D A call appearing in a class C having fname as the feature of the call is export valid for a class D if and only if it s
41. e present book is excerpted from Eiffel The Language Author Bertrand Meyer Software credits See Credits at the end of the Preface and in Eiffel The Language Cover design Rich Ayling Copyright notice and proprietary information Copyright Bertrand Meyer 1992 1995 The material from Eiffel The Language is reproduced here under permission from copyright holder and the publisher for the exclusive benefit of users of ISE Eiffel 3 In addition the copyright holder grants permission to members in good standing of the Nonprofit International Consortium for Eiffel to make any use of this document that does not infringe on the rights of other parties in particular the publisher Eiffel The Reference
42. e semantics it is convenient to consider the unfolded form of a Multi_branch To obtain it just replace any integer or character Interval in the Choices of a When_part by a Choices list made up of all constants between the interval s bounds or empty if the second bound is smaller than the first Integer order is used for an Integer_interval and character code order for a Character_interval Multi_branch rule A Multi_branch instruction is valid if and only if its unfolded form satisfies the following conditions le The inspect expression is of type INTEGER or CHARACTER VOMB 2 Any inspect constant any value in one of the Choices parts is a Constant of the same type as the inspect expression 3 Any two non Unique inspect constants have different values 4e Any two Unique inspect constants have different names 5e If any inspect constant is Unique then every other inspect constant in the instruction is either Unique or has a negative or zero value 6 All Unique inspect constants if any have the same class of origin the enclosing class or a proper ancestor The effect of executing a Multi_branch instruction is defined as the effect of executing its unfolded form as follows The value of the inspect expression is computed Because of the above validity constraint that value may be equal to at most one of the inspect constants If there is indeed one such constant the effect of the Multi_branch is that of the Compound ap
43. eature may be available to specific classes only In that case it is also available to the descendants of all these classes Such a feature is said to be selectively available to the given classes and their descendants 3 The feature may be available to no classes Then it is said to be secret Definition available A feature of a class S is said to be available to a class C if and only if it is either selectively available to amp or generally available 7 13 ADAPTING THE EXPORT STATUS We Clients Class_list SYNTAX gt Class_list Class_name A Clients part is valid if and only if every Class_name appearing in its Class_list is the name of a class in the surrounding universe VALIDITY VLCP New_exports export New_export_list New_export_list New_export_item New_export_item 2 Clients Feature_set Feature_set Feature_list all Feature_list Feature_name Export List rule A New_exports parent appearing in class C in a Parent clause for a parent B of the form export class_list feature_list class_list feature_list is valid if and only if for i in the interval 1 n 7 3 CONVENTIONS 27 7 Clients and exports 7 3 CONVENTIONS Definition client A class C is a client of S if some ancestor of C is a simple client an expanded client or a generic client of S Definition client relation between classes A
44. eritance Consistency constraint seen below states that exactly one of them must appear under its final D name as part of a Select clause in D then df is that feature Definition potentially ambiguous Let D be a repeated descendant of a class A A feature f of A is potentially ambiguous in D if and only if one of the following two conditions holds le fis an attribute 2e D has two or more potential versions of f 11 13 THE REPEATED INHERITANCE CONSISTENCY CONSTRAINT Select A select Feature_list It is valid for a class D to be a repeated descendant of a class A if and only if D satisfies the following two conditions for every feature f of A le If the Repeated Inheritance rule implies that f will be shared in D then all the inherited versions of f are the same feature VMRC 11 10 NAME CLASHES 43 WALIDITY VMEN 11 10 NAME CLASHES Definition name clash A name clash occurs for a certain feature name fname in a class C if for two different parents A and B of C both A and B have a feature of name fname 11 11 THE INHERITED FEATURES OF A CLASS Definition inherited features Let D be a class The list inherited of inherited features of D is obtained as follows Let precursors be the list obtained by concatenating the lists of features of every parent of D this list may contain duplicates in the case of repeated inheritance Then inherited is obtained from precursors as follows le In list p
45. es and formal generic parameters other than G itself 4 10 OBSOLETE CLAUSE Obsolete obsolete Message We Message Manifest_string Declaring a class as Obsolete does not affect its semantics But some language processing tools should produce a warning when they process a class that relies as client or descendant on an obsolete class The warning should include the Message 4 11 ENDING COMMENT If present the ending comment must repeat the Class_name given at the head of the class ALIGI VCRN 4 5 CLASS TEXT STRUCTURE 15 4 5 CLASS TEXT STRUCTURE svnrax Class_declaration 2 Indexing Class_header Formal_generics Obsolete Inheritance Creators Features Invariant end class Class_name 4 7 INDEXING A CLASS sywtax Indexing indexing Index_list Index_list Index_clause Index_clause Index Index_terms Index Identifier n yi Index_terms Index_value We Ne We We We le Index_value Identifier Manifest_constant The Indexing clause has no effect on the execution semantics SEMANTICS 4 8 CLASS HEADER Syntax Class_header Header_mark class Class_name Header_mark 2 deferred expanded Class_name A Identifier A Class_header appearing in the text of a class C is valid if and only if it satisfies either of the following two conditions 1 There is no header mark of the deferre
46. es the following two conditions le It appears in a Postcondition clause of a Routine r 2e Transforming r into a function with result type TE by adding a result type if r is idia procedure or changing its result type if it is already a function and replacing its entire Routine part by do Result e end would yield a valid routine The value of an Old expression old e is defined only at the end of the execution of a call to r just before the call returns it is the result that would have been produced by evaluating e just before the call s execution began 9 11 CLASS INVARIANTS Definition invariant of a class The invariant of a class C is an assertion obtained by concatenating the following assertions omitting any one which is absent or empty le The invariants of all parents determined recursively through the present rule in the order of the corresponding Parent clauses 2e The postconditions of any inherited functions which C redefines as an attribute with every occurrence of Result replaced by the attribute s final name If there are two or more such redefinitions include them in the order in which their new declarations appear in C 3 The Assertion in C s own Invariant clause if any 9 12 CONSISTENCY OF A CLASS Definition consistency A class C is consistent if and only if it satisfies the following two conditions le For every creation procedure p of C pre do INVc posty 2e For every
47. ess your_object some_bit_attribute eif_bit_set_field eif_access your_object some_bit_attribute your_bit2 Here two primitives are needed Function eif_bit_field takes two arguments an object pointer returned by eif_access and an attribute name In contrast with eif_field the result of eif_bit_field may only be used as an expression r value not as a writable variable To change the value of a Bit_type field use eif_bit_set_field whose last argument is the new value of type E F_BIT If the object has no appropriate Bit_type field eif_bit_field returns the value EJF_NO_BFIELD and the effect of eif_bit_set_field is undefined The following primitives are applicable to your_bit of type E F_BIT oe a 24 7 THE CECIL LIBRARY 1 itin a C variable your_id use the function eif_type_id which returns a result of type EJF_ TYPE _ID EIF_TYPE_ID your_id your_id eif_type_id CLASSNAME If the class is generic replace the last instruction by your_id eif_generic_id CLASSNAME genl gen2 where genl gen2 are type ids corresponding to the desired actual generic parameters Function eif_generic_id has a variable number of arguments the number of arguments following the first one CLASSNAME must match the number of formal generic parameters of the class of name CLASSNAME The result returned by eif_type_id or eif_generic_id describes a type which is expanded if and only if CLASSNAME is
48. exponent which is the letter e or E followed by an optional sign or and an Integer The Integer is required if the e or E is present This indicates that the value appearing before the e or E must be scaled by 10 sup n where n is the given integer No intervening character blank or otherwise is permitted between these elements The integral and fractional parts may not both be absent If underscores are used in either the integral part or the fractional part they must also appear in the other part unless it has three digits or less Underscores if any have no effect on the integer value associated with the integer 25 14 BIT SEQUENCES A Bit_sequence is a sequence of digits O or 1 followed by a b or B with no other intervening characters 25 15 CHARACTERS Definition character A character is one of the following Cle Any key associated with a printable character except for the percent key The key plays a special role for cases 2 and 3 C2 The sequence k where kis a one key code taken from the table given below This is used to represent special characters such as the Backspace represented as B or characters which are not available on all keyboards or have different codes on different keyboards An example is the opening bracket when supported by the keyboard this character may be entered using form C1 as it may also in all cases be represented as 25 11 STRINGS 87
49. f it appeared in the text of C 6 If case 5 holds and f is the feature of the Creation_call then f is available for creation to X 18 10 CREATION VALIDITY SYSTEM LEVEL A Creation instruction is system valid if and only if it satisfies one of the following two conditions le The creation type is explicit in other words the instruction begins with T for some type 7 2e The creation type is implicit in other words the instruction begins with and every possible dynamic type T for x with base class C satisfies conditions 1 to 6 of the Creation Instruction rule pagetnOs In applying conditions 5 and 6 the feature of the call f must be replaced by its version in C A Creation instruction is valid if and only if it is both class valid and system valid 18 11 CREATION SEMANTICS With the above validity rules we can define the precise semantics of a Creation instruction Consider such an instruction with target x and creation type T If T is a reference type the effect of executing the instruction is the following sequence of steps le If there is not enough memory available to create a new direct instance of T trigger an exception in the routine that executed the instruction Steps 2 to 5 do not apply in this case 2 Create a new direct instance of TC 3 Assign a value to every field of the new instance for a field corresponding to a constant attribute the value defined in the class text for a field corre
50. f some of the above examples 25 9 IDENTIFIERS An Identifier is a sequence of one or more characters of which the first is a letter and each of the subsequent ones if any is a letter a decimal digit 0 to 9 or the underscore character _ The definition indicates that the first character of an identifier must be a letter in particular an identifier may not begin with an underscore Also no intervening blank is permitted within an identifier The validity constraint is obvious An identifier is valid if and only if it is not one of the language s reserved words There is no limit to the length of identifiers and all characters are significant in other words to determine whether two identifiers are the same or not you must take all their characters into account Letter case is not significant for letters if you write two identifiers as a and A or lInKeD_liST and LINKED_LIST they are considered the same The recommended style includes some standard conventions class names and other type names in upper case as in LINKED_LIST names of routines variable attributes and local entities in lower case as in item names of constant attributes and predefined entities with an initial upper case ee and the rest in lower case ae in deena or nthe ovangias ws name lower H ge a a aA 25 10 OPERATORS When it comes to defining a function with one or two arguments you may wish to use a prefix or infix name Definition
51. fect is to leave the state of the computation unchanged If the Compound has one or more instructions its effect is that of executing the first instruction of the Compound and then recursively to execute the Compound obtained by removing the first instruction 14 3 NULL INSTRUCTION Specimens of the null instruction are empty The effect of the null instruction is to leave the state of the computation unchanged 14 4 CONDITIONAL Conditional f Then_part_list Else_part end Then_part_list A Then_part elseif Then_part 2 Boolean_expression then Compound Else_part 4 else Compound To define precisely the semantics of this construct a few auxiliary notions are useful As the syntactical specification shows a Conditional begins with if condition then compound where condition is a boolean expression and compound is a Compound Definition secondary part The remaining part may optionally begin with elseif If so replacing the first elseif by if would transform the remaining part into a new syntactically correct Conditional such an instruction is called the secondary part of the enclosing Conditional The final part also optional is of the form else compound 52 13 11 13 11 BIT TYPES The possible direct conformance cases involving a Bit_type are the following for any positive integers N and P le BIT N conforms directly to ANY 2e BIT N conforms directly to BIT P for N
52. g has the same final name 2e The signature of g conforms to the signature of f 3 If gis a routine its Precondition if any begins with require else not just require and its Postcondition if any begins with ensure then not just ensure 4e Ifthe redeclaration is a redefinition rather than an effecting the Redefine subclause of the Parent part for B lists the final name of f in its Feature_list 5 If fis inherited as effective then g is also effective 6 If fis an attribute g is an attribute f and g are both variable and their types are either both expanded or both non expanded 7 If either one of fand g is an External routine so is the other Definition declared type Any feature or entity of a class C has a declared type as follows e For a feature which is immediate in C or redeclared in C dt is the type given by the declaration or redeclaration e For an inherited feature which is not redeclared in C dt is recursively the declared type of its precursors in the corresponding parents e For the predefined entity Current dt is C with its formal generic parameters if any e For the predefined entity Result appearing in a function dt is the return type declared for the function e For any other entity e dt is the type used in the declaration of e Definition type of a feature In this book the type of a feature or entity without further qualification always means its declared type rather
53. has an empty Clients list that is to say begins with feature then the features it introduces are secret If a non redeclared inherited feature f has more than one precursor it is available to all classes to which it would be available as a consequence of applying the preceding rule separately to each of its precursors 7 14 DESCRIBING A CLASS FOR CLIENTS THE SHORT FORM Definition short form abstract form The short form of a class also called its abstract form is a text which has the same structure as the class but does not include non public elements The short form is the one that should be used as interface documentation for the class 7 15 THE FLAT SHORT FORM Definition flat short form The flat short form of a class is similar to the short form but applies to the reconstructed full text of a class you may view it as resulting from a shortening step that has been preceded by a flattening step which expands the class text to unfold all the features obtained from proper ancestors putting them at the same level as the immediate features of the class Clearly flattening must take both renaming and redefinition into account 28 7 12 7 12 THE EXPORT STATUS OF FEATURES Definition exported selectively available The status of a feature of a class is one of the following le The feature may be available to all classes Such a feature is said to be exported of generally available 2e The f
54. he following holds le Vand 7 are identical 2 V conforms directly to T 3 Vis NONE and T is a reference type 4 Vis B Y Y for some generic class B Tis B X X and every one of the Y conforms recursively to the corresponding X 5e T is a reference type and for some type U V conforms to U and U conforms recursively to T 13 5 CONFORMANCE TO A NON GENERIC REFERENCE TYPE Let CT be a Class_type of base class C and BT be a reference type whose base class B is not generic CT conforms directly to BT if and only if the Inheritance clause of C lists B in one or more of its Parent items 13 6 GENERICALLY DERIVED REFERENCE TYPES Let BT be a generically derived reference type of base type B X7 X for some n gt 1 where the formal generic parameters of B are G G Let CT be a Class_type of base class C different from B To determine whether CT conforms directly to BT define the substitution oas follows e If CT is non generic is the identity substitution e If CT is a a generically derived type of the form C Yj Yml and class C is declared with formal generic parameters Hj H then applied to any of the H for 1 lt i lt m is Y and applied to any other element is the element itself Then CT conforms directly to BT if and only if the Inheritance clause of C lists B Zj Z as one of its Parent items and for every j such that lt lt n applying substitution
55. hich do not set satisfy the requirements at the previous levels In almost all cases authors of tools should follow a stricter guideline and make sure that their tools reject any input that does not satisfy the rules applying to the earlier levels Such rejection should include a clear error message For syntax the message should identify the production which is not properly observed for validity it should give the code of the violated validity constraint and the clause number for constraints divided into clauses Two special considerations may justify occasional departures from this general obligation of rejection le A semantic tool may be able to process valid parts of a text even if other parts are invalid For example a compiler may generate code for some valid classes in a system rejecting classes which are not valid 2e A tool author may have a particular reason for providing a tool or tool option which accepts input violating a specific validity constraint A possible application would be for a prototyping mode which attempts to execute incomplete systems or skips certain checks Such tool variants are outside of the semantics of Eiffel proper and should be clearly labeled as such reminding developers that acceptance of an input text provides no guarantee that the text satisfies the full language rules One final note intended for implementors of Eiffel and regarding what they might not find here Although this book goes to gre
56. hose origin is a class C Let a a be the set containing all the attributes of C except for Semantics those listed in the Attribute_list if present of the Strip expression Let CO be the current object for the call CO is an instance not necessarily direct of a type based on C Then the value of the expression is an array whose elements are the fields of CO corresponding 23 11 ENTITIES WALID VWID SYANTAR WALIDITY VWCA SYNTAX An Equality expression 23 11 ENTITIES Identifier rule The value of an entity of each possible form evaluated during a call to the enclosing routine is defined as follows involving two bit sequences yields true if and only if after right padding if necessary they are bit by bit identical An Identifier appearing in an expression as part of the text of a routine r in a class C either by itself or as the target or actual argument of a Call must be the name of a feature of C a local entity of r or a formal argument of r e The value of a Local entity including Result results from the successive instructions that may have been applied to the entity since the default initializations performed anew on each call e The value of a routine s Formal argument is obtained according to the rules of direct reattachment from the value of the corresponding actual argument at the time of the current call This value may not change for the duration of tha
57. ically based on a number of selection rules not unlike those of a little expert systems Anyone familiar with the Eiffel method will have recognized the idea it is the notion of short form To document an Eiffel class you do not as a rule write a separate document you should instead include the relevant information in the class text itself and rely on computer tools to extract views of the class at various levels of abstraction in particular the short or flat short form which only keeps the interface properties of the class signatures preconditions postconditions and header comments of exported features class invariant while discarding the implementation information non exported features routine bodies distinction between functions and attributes Here we are doing the same with respect to the language itself using the power of the computer to remove the non essential information from a complete description The major advantage in the case of classes is that we can keep a single description So when things evolve as they inevitably will be they classes or languages we have only one document to maintain This may be called the principle of single reference and is essential to the smooth evolution of the language and its description Although the work of the NICE library committee may cause changes to be brought to the present document independently of Eiffel The Language every effort will be made to maintain the pri
58. identifier feature A feature declared with an operator is called an operator feature Unary A pot enn Binary Ayn yn mg ng pan ee S Brae Ws NNN lesa and or xor and then or else implies 18 5 9 A routine is either a procedure or a function e A procedure does not return a result it may perform a number of operations some of which may modify the instance to which the procedure is applied e A function returns a result and may also perform operations 5 9 FEATURE DECLARATIONS SYNTAX Feature_declaration N ew_feature_list Declaration_body Declaration_body 2 Formal_arguments Type_mark Constant_or_routine Constant_or_routine is Feature_value Feature_value eS Manifest_constant Unique Routine 5 10 COMPONENTS OF A FEATURE DECLARATION n yE New_feature_list New_feature New_feature frozen Feature_name 5 11 HOW TO RECOGNIZE FEATURES A Feature_declaration is a variable attribute declaration if and only if it satisfies the following conditions e There is no Formal_arguments part e There is a Type_mark part e There is no Constant_or_routine part A Feature_declaration is a constant attribute declaration if and only if it satisfies the following conditions e There is no Formal_arguments part e There is a Type_mark part e There is a Constant_or_routine part which contains either a Manifest_constant or a Unique A Fe
59. ition field By introducing an attribute in a class you specify that at run time every instance of the class will possess a certain value or field corresponding to the attribute An attribute is either variable or constant If an attribute is variable the corresponding fields may be different for various instances of the class and may change at run time As a consequence the actual values must be stored in the representation of each instance If an attribute is constant the corresponding field is the same value for all instances and may not change at run time This value appears in the class as part of the attribute declaration By introducing a routine in a class you specify that a certain computation an algorithm must be applicable to instances of the class 16 4 9 4 9 FORMAL GENERIC PARAMETERS Formal_generics Formal_generic_list Formal_generic_list a Formal_generic Formal_generic A Formal_generic_name Constraint Formal_generic_name 2 Identifier Constraint ea eS Class_type A Formal_generics part of a Class_declaration is valid if and only if every Formal_ generic_name G appearing in it satisfies the following three conditions le Gis different from the name of any class in the surrounding universe 2e Gis different from any other Formal_generic_name appearing in the same Formal_ generics_part 3 If a Constraint is given it does not involve any types other than class nam
60. l have no effect on the semantics of system execution except through possible side effects of the functions called by assertions For an incorrect system the effect depends on compilation or execution options Various options of the environment will make it possible to evaluate assertions If an assertion evaluates to true it has no further effect on the outcome of the computation If it evaluates to false it will trigger an exception disrupting the normal flow of computation as discussed in the chapter on exception handling An assertion violation detected as a result of enabling assertion monitoring at one of the above levels triggers an exception An exception will also result at level loop or higher if a loop iteration fails to decrease the variant or gives it a negative value Here is the rule for determining the recipient of an exception resulting from an assertion violation le For postconditions class invariants loop invariants variants and Check instructions the recipient is the routine whose text contains the violated assertion or variant 2e For a violated precondition the recipient is the calling routine In this case no component of the routine s body is executed the routine fails immediately not performing any of its normal actions and triggering an exception in the caller 9 13 CHECK INSTRUCTIONS 35 VAVE In this rule VV is the invariant of C and for any routine s pre is the precondi
61. list inherited of inherited features of D Feature Name rule It is valid for a class C to introduce a feature with the Feature_name fname or to inherit a feature under the final name fname if and only if no other feature of C has that same name le A class may not introduce two different features both deferred or both effective with the same name 2e Ifaclass introduces a feature with the same name as a feature it inherits in effective form it must rename the inherited version 42 11 Repeated inheritance 11 2 CASES OF REPEATED INHERITANCE Definition repeated inheritance ancestor descendant Repeated inheritance occurs whenever as a result of multiple inheritance two or more of the ancestors of a class D have a common parent A D is then called a repeated descendant of A and A a repeated ancestor of D Definition direct repeated inheritance The simplest case called direct repeated inheritance corresponds to the following scheme where D is a repeated heir of A class D inherit A rename redefine end A rename redefine end Rest of class omitted Definition indirect repeated inheritance The second case indirect repeated inheritance arises when one parent of D is a proper descendant of A and one or more other parents are descendants of A Some of the paths may be direct 11 3 SHARING AND REPLICATION Repeated Inheritance rule Let D be a class and B B n 2
62. m with a they must include at least one B 2 6 VALIDITY The productions and other elements labeled smx as described so far specify the structure of constructs Definition validity constraint valid Supplementary requirements on the syntactically well formed specimens of a construct are called validity constraints on the construct Paragraphs introducing them are labeled by the VALIDITY road sign A specimen which follows the syntactic rules and satisfies the constraints will be accepted by the language processing tools of any Eiffel environment and is said to be valid 2 2 2 Syntax validity and semantics 2 2 SYNTAX COMPONENTS SPECIMENS AND CONSTRUCTS Eiffel s syntax is the set of rules describing the structure of class texts It covers neither limitations on valid texts described by validity constraints nor the execution time meaning or effect of these texts described by semantic rules Definition component Any class text or syntactically meaningful part of that text such as an instruction an expression or an identifier is called a component Definition construct specimen The structure of any components is described by a construct A component conforming to a certain construct is called a specimen of that construct For example the construct Class describes the structure of class texts any particular class text built according to the rules given in this book is a specimen of this con
63. n the absence of further action from C that feature remains deferred Of course C may also provide an effective declaration for the feature killing several abstract birds with one concrete stone by using a single redeclaration to effect several features inherited as deferred 10 19 REDEFINITION AND UNDEFINITION RULES Redefine redefine Feature_list Undefine undefine Feature_list Redefine Subclause rule Consider a class C with a parent B If a Parent part for B in C contains a Redefine subclause that clause is valid if and only if every Feature_identifier fname that it lists in its Feature_list satisfies the following conditions WALICITY le fname is the final name in C of a feature inherited from B VDRS 2 That feature was not frozen and was not a constant attribute 3 fname appears only once in the Feature_list 4e The Features part of C contains oneHeature declaration for fname which is a valid redeclaration but not an effecting of the original feature The effect of redefining a feature in a class is that any use of the feature in the class its clients or barring further redefinitions its proper descendants will refer to the redefined version rather than the original Undefine Subclause rule Consider a class C that inherits from a class B If a Parent part for B in C contains an Undefine subclause that clause is valid if and only if for every Feature_identifier fname S that it lists in its Featu
64. n_list Local Entity rule Let ld be the Local_declarations part of a routine r in a class C Let locals be the concatenation of every Identifier_list of every Entity_declaration_group in d Then ld is valid if and only if every Identifier e in ld satisfies the following two conditions le No feature of C has e as its final name 2e No formal argument of r has e as its Identifier Definition local entity Most of the rules governing the validity and semantics of declared local entities also apply to a special predefined entity Result which may only appear in the Routine_ body or Postcondition of a function and denotes the result to be returned by the function Reflecting this similarity this book uses the term local entity to cover Result as well as declared local entities 30 8 3 Routines 8 3 FORMAL ARGUMENTS Formal_arguments 2 Entity_declaration_list Entity_declaration_list 2 Entity_declaration_group Entity_declaration_group 4 Identifier_list Type_mark Identifier_list Identifier Type_mark aa Type Formal Argument rule Let fa be the Formal_arguments part of a routine r in a class C Let formals be the concatenation of every Identifier_list of every Entity_declaration_group in fa Then fa is valid if and only if no Identifier e appearing in formals is the final name of a feature of C Let el be an Entity_declaration_list Let identifiers be the concatenation of eve
65. name A rename Rename_list Rename_list Rename_pair Rename_pair A Feature_name as Feature _name Rename Clause rule A Rename_pair of the form old_name as new_name appearing in the Rename subclause of the Parent clause for B in a class C is valid if and only if it satisfies the following five conditions l old_name is the final name of a feature f of B 2e old_name does not appear as the first element of any other Rename_pair in the same Rename subclause 3e new_name satisfies the Feature Name rule for C 4e If new_name is of the Prefix form fis an attribute or a function with no argument Se If new_name is of the Infix form fis a function with one argument Renaming does not affect the semantics of an inherited feature but simply gives it a new final name in an heir as defined below 6 3 FORM OF THE INHERITANCE PART 23 STAT AR 6 3 FORM OF THE INHERITANCE PART Inheritance inherit Parent_list xk inherit Parent_list A Parent lt 7 Parent Class_type Feature_adaptation Feature_adaptation Rename New_exports Undefine Redefine Select end Inheritance inherit Parent_list Parent_list A Parent Parent Class_type Feature_adaptation Feature_adaptation Rename New_exports Undefine Redefine Select end Definition parent clause for a class The Parent_list names a number of Parent clauses Each Parent clause is relative t
66. nciple of single reference avoiding the disaster that divergence between the two documents would mean for Eiffel In particular the structure and section numbering will be kept the same for the two books and changes made to the present one as a result of the committee s work will be continuously reflected back into the electronic version of Eiffel The Language LIBRARY ELEMENTS The definition of Eiffel relies on a number of predefined classes in the Kernel covering such fundamental notions as basic types BOOLEAN INTEGER and the like arrays input and output WHAT IS EIFFEL Preface This document serves two purposes e It is submitted to the Language Committee of the Nonprofit International Consortium for Eiffel as the second step towards a standard definition of Eiffel the first step was the book Eiffel The Language e It provides users of Eiffel with a short language definition For the first of these goals note that the present Preface is not part of the intended Standard WHAT IS EIFFEL The name Eiffel covers a method and a language for the systematic development of quality software based on the full application of object oriented principles Only the language aspect is covered in this book Other documents are available on the method and on the various compilers tools and environments that make it possible to develop software with Eiffel A complete description of the Eiffel language has been pre
67. nd operand of value v2 is defined as follows e For and true if and only if both v and v2 are false e For or false if and only if either v or v2 is false e For xor true if and only if v and v2 have different values In other words a xor b has the same value as a or b and not a and b 23 8 SEMI STRICT BOOLEAN OPERATORS For operands of values v and v2 these operators yield the following results e and then false if v is false otherwise the value of v2 e or else true if v is true otherwise the value of v2 e implies true if v is false otherwise the value of v2 In other words a implies b has the same value as not a or else b 23 9 ARITHMETIC OPERATORS AND THE BALANCING RULE Arithmetic Expression Balancing rule In determining the equivalent dot form of a Binary expression involving operands of arithmetic types one or more of DOUBLE REAL and INTEGER first convert all operands to the heaviest operand type occurring in the expression 23 10 OPERATIONS ON BIT SEQUENCES If b is a bit sequence not b is a bit sequence of the same length with a one at every position where b has a zero and conversely If sa and sb are bit sequences each of the binary operators pads the shorter operand with zeros on the right if necessary to reach the number of bits N of the larger For any position 7 1 lt i lt N let 4 c be the i th bits of s sb sc Then c is a one if and only if e For and a and b are ones e Fo
68. ned as the semantics of the corresponding sequence of declarations naming only one feature each and with identical declaration bodies as in f some_declaration_body J some_declaration_body J some_declaration_body 5 12 THE SIGNATURE OF A FEATURE 19 5 12 THE SIGNATURE OF A FEATURE Definition signature The signature of a feature f is a pair lt argument_types result_type gt where both argument_types and result_type are sequences of types defined as follows e If fis a routine argument_types is the possibly empty sequence of the types of its arguments If fis an attribute argument_types is an empty sequence e If fis an attribute or a function result_type is a one element sequence whose single element is the type of f If fis a procedure fis an empty sequence Definition argument signature The first component of a feature s signature written argument_types in this definition is called the argument signature of the feature The argument signature gives the types of the feature s arguments it is an empty sequence for attributes and for routines without arguments 5 13 FEATURE NAMES emmy Feature_name A Identifier Prefix Infix Prefix prefix Prefix_operator Infix infix Infix_operator Prefix_operator Unary Free_operator Infix_operator 4 Binary Free_operator Definition identifier feature operator feature A feature declared with an identifier is called an
69. no yes no yes all or a Name representing a Debug_key Value yes means the same as all no yes all or a Name representing a specific optimization level offered by the compiler no yes or all Value yes means the same as all Default all Default Monitoring at each level in this list also applies to the subsequent levels ensure implies precondition checking etc Value invariant means class invariant loop means monitoring of loop invariants and of loop variant decrease check adds execution of check instructions all means the same as check In the Defaults or Options clause for a given cluster yes governs class level optimization and all means the same as yes In the Ace level Defaults clause yes governs systemwide optimization and all means the same as yes plus class level optimization 100 D 11 D 11 SPECIFYING OPTIONS Defaults Options Option_clause Target_list Option_tag System_tag Class_tag Free_tag Option_mark Option_value Standard_value Class_value Free_value I gt We We We We We I gt l gt We We We I gt I gt default Option_clause SYNTAX option Option_clause tl Option_tag Option_mark Target_list Class_name Class_tag System_tag collect Free_tag assertion debug optimize trace Free_tag Name Option_value Standard_value Class_value yes no
70. nt 3 Clone Expanded 2 Copy will fail if source 4 Copy is void For the exception raised in case 2 if the value of y is void the Kernel Library class EXCEPTIONS introduces the integer code Void_assigned_to_expanded 20 9 POLYMORPHISM Definition dynamic type The dynamic type of an entity or expression x at some instant of execution when x is not void is the type of the object to which x is attached This should not be confused with the type of x called its static type if there is any ambiguity which for an entity is the type with which it is declared and for an expression is the type deduced from the types of its constituents Definition polymorphism The ability to have more than one dynamic type is called polymorphism an entity or expression which has two or more possible dynamic types that is to say which may become attached at run time to objects of two or more types is itself a polymorphic entity Only entities or expressions of reference types may be polymorphic 19 8 DEEP EQUALITY 67 36 4e If TY does not conform to TX the result is false Cases 4 to 6 assume that TY conforms to TX If TX is BOOLEAN CHARACTER INTEGER REAL DOUBLE or POINTER the result is true if and only if OX and OY are the same value after possible conversion to the heavier type if OX and OY are different arithmetic types If OX and OY are special objects strings or arrays that is to sa
71. o a Class_type that is to say a class name B possibly followed by actual generic parameters as in B T U B must be the name of a class in the universe to which the current class belongs The clause is said to be a Parent clause for B 6 4 RELATIONS INDUCED BY INHERITANCE Definition heir parent If class C has a Parent clause for B then C is said to inherit from B B is said to be a parent of C and C is said to be an heir of B 22 6 2 6 The inheritance relation 6 2 AN INHERITANCE PART Definition multiple single inheritance Multiple inheritance occurs as soon as there is more than one Parent clause even if they all refer to the same parent class a case called repeated inheritance and studied in chapter 11 5 17 OBSOLETE FEATURES 21 The special case is that of a multiple declaration introducing Unique constant attributes which is covered by the Unique Declaration rule 5 17 OBSOLETE FEATURES Declaring a routine as Obsolete does not affect its semantics But language processing tools or at least some of them should produce a warning when they process a client or descendant class that uses the routine The warning should include the Message 20 5 14 Definition same feature name By convention two feature names are the same if and only if either of the following conditions holds e They are both identifier features with identical lower name e They are bo
72. ok references to the name of a feature if not further qualified always denote the final name 6 11 INHERITANCE AND EXPANSION The only consequence of the expansion status of a class is the semantics of entities of the corresponding types such as x above An expanded class may inherit from an non expanded one and conversely The expansion status is not transmitted under inheritance it is entirely determined by the presence or absence of the expanded mark in the class s own Class_header not by any property of its parents 24 6 6 Definition ancestor descendant Class A is an ancestor of class B if and only if A is B itself or recursively an ancestor of one of B s parents Class B is a descendant of class A if and only if A is an ancestor of B in other words if B is A or recursively a descendant of one of its heirs Definition proper ancestor descendant The proper ancestors of a class C are its ancestors other than C itself The proper descendants of a class B are its descendants other than B itself 6 6 THE INHERITANCE STRUCTURE Parent rule The Inheritance clause of a class D is valid if and only if it meets the following two conditions le In every Parent clause for a class B B is not a descendant of D 2 If two or more Parent clauses are for classes which have a common ancestor A D meets the conditions of the Repeated Inheritance Consistency constraint for A 6 9 RENAMING Re
73. or a standard non free operator such as and star but not including the alphabetic operators such as or else and not e A word is any token which is not a symbol It is permitted to write two adjacent tokens without an intervening break if and only if one is a word and the other is a symbol 25 6 TOKEN CATEGORIES There are two categories of tokens fixed and variable e Fixed tokens have a single frozen form They include reserved words such as class or Current containing letters only and special symbols such as or containing non alphabetic characters e Variable tokens are specimens of terminal constructs such as Integer Identifier or Free_operator 25 7 RESERVED WORDS Reserved words include keywords and predefined names e Keywords such as class or feature serve to introduce and delimit the various components of constructs e Predefined names come at positions where variable tokens would also be permissible for example the predefined entity Result denoting the result of a 84 25 2 25 Lexical components 25 2 CHARACTER CATEGORIES The discussion will rely on a classification of characters into letters digits and other categories To start with it is useful to have a precise although obvious definition Definition letter digit alphabetic Printable A letter is one of the twenty six elements of the Roman alphabet lower case or upper case abcdefghijklmnopaqrstuvwxyz ABCDEFGHIJKLMNOPQ
74. ormal description of the structure of its specimens Such a description is called the production for the construct A production has the form Construct right hand side Every non terminal construct appears on the left hand side of exactly one such production The symbolS means is defined as The right hand side of the production describes the structure of specimens of the left hand side construct Three forms of right hand side are available e Aggregate describing a construct made of a fixed number of parts some of which may be optional to be concatenated in a given order e Choice describing a construct having a number of given variants e Repetition describing a construct made of a variable number of parts which are all specimens of a given construct An aggregate right hand side is a non empty sequence of constructs some of which may be in square brackets to indicate optional parts A choice right hand side is a non empty sequence of constructs separated by vertical bars A repetition right hand side is of one of the two forms Construct Construct where the separator is some construct usually but not necessarily terminal Appearing in a production for a left hand side construct R this states that a specimen of R consists of zero or more specimens of B separated if more than one by the separator In the first form without a specimens of R may be empty in the second for
75. otherwise Clearly attempting to describe the effect of an invalid component would be useless 2 9 CORRECTNESS Validity is only a structural property valid Eiffel software is not guaranteed to perform according to any expected behavior In fact execution of valid software may lead to non termination or to exceptions and eventual failure For a valid component then we need a more advanced criterion its ability to operate properly at run time This is called correctness and is a more elusive aim than validity since it involves semantic properties 2 10 THE CONTEXT OF EXECUTING SYSTEMS Definitions run time machine platform language processing tool The following terminology will serve to discuss the context of system execution e Run time is the period during which a system is executed e The machine is the combination of hardware one or more computers and operating system which makes it possible to execute systems e The machine type that is to say a certain kind of computer and a certain operating system is called a platform e To make the text of an Eiffel system executable by a machine you will need software tools such as compilers and interpreters for which this book will use the term language processing tool general enough to cover various implementation techniques 3 VBGV 2 4 PRODUCTIONS VVVV 2 4 PRODUCTIONS Definition production To understand a non terminal you need a f
76. pearing after the then in the When_part of the matching inspect constant If there is no such constant le If the Else_part is present the effect of the Compound is that of the Compound appearing in the Else_part 2e Otherwise an exception is triggered and the current routine execution fails 14 7 LOOP s ntax Initialization 2 from Compound Loop_body Exit loop Compound Exit until Boolean_expression The effect of a Loop is the effect of executing its Initialization followed by the effect of executing its Loop_body The effect of executing an Initialization clause is the effect of executing its Compound The effect of executing a Loop_body is to leave the state of the 54 14 5 Definition prevail immediately If the value of condition is true when the instruction is executed then the Conditional is said to prevail immediately Finally we may consider that every Conditional has an Else_part if we understand an empty Else_part to stand for one with an empty Compound With these conventions the effect of a Conditional may be defined as follows If the Conditional prevails immediately then its effect is that of its compound part as defined above Otherwise e If it has a secondary part the effect of the entire Conditional is recursively the effect of the secondary part e If it has no secondary part its effect is that of the possibly empty Compound in its Else part 14 5 MULTI BRAN
77. plies that f will be replicated in D and f is potentially ambiguous then the Select subclause of exactly one of the Parent parts of D lists the corresponding version of f under its final D name Select Subclause rule A Select subclause appearing in the Parent part for a class B in a class D is valid if and only if for every Feature_name fname in its Feature_list fname is the final name in D of a feature that has two or more potential versions in D and fname appears only once in the Feature_list VMSS 44 11 12 3 Jfa class inherits two features as effective from different parents and they have the same name the class must also except under sharing for repeated inheritance remove the name clash through renaming 11 12 VERSIONS OF A FEATURE Definition potential version Let f be a feature of a class A and D a descendant of A A potential version of fin D is any inherited feature of D which is either f itself e A feature resulting recursively from a redeclaration of a potential version of f e Recursively a potential version of a feature of which fis a redeclaration e A feature resulting recursively from a generic derivation of A Definition version Let fbe a feature of a class A and D a descendant of A The version of fin D is the feature df defined as follows le If Dhas only one potential version of f then df is that feature 2e If D has two or more potential versions of f the Repeated Inh
78. r a call xe copy y As with any call the target x must be non void the first precondition clause of copy states that y must also be non void Let OX and OY be the attached objects at the time of the call le If OX and OY are bit sequences the conformance rule on Bit_type requires OX to be at least as long as OY The call then copies onto OX the contents of OY extended with zeros on the left if OY is shorter than OX Cases 2 to 4 assume that OX and OY are not bit sequences 2e Ifthe types of OX and OY are basic types BOOLEAN CHARACTER INTEGER REAL DOUBLE or POINTER then OX is identical to OY or is a heavier type Then the call copies the value of OY onto OX after conversion to the heavier type if necessary 3 If OX and OY are special objects that is to say sequences of values strings or arrays the call copies the value of OY onto OX The implementation must ensure that whenever this occurs the size of OX is at least as large as the size of OY 4e In the remaining cases OX and OY are standard objects made of zero or more fields and the second precondition clause implies that the type of OY is a descendant of the type of OX as a result for every field of OX there is a corresponding field in OY Then the call copies onto every field of OX the corresponding field of OY 19 3 CLONING AN OBJECT Here is the definition of the semantics of a call clone y le Ifthe value of y is void the call returns a void value 2e
79. r or a or b or both are ones e For implies a is a zero or b is a one or both of these conditions e For xor one among a and b is a one and the other is a zero 23 2 GENERAL FORM OF EXPRESSIONS 75 SYANTAR SYNTAX ALIOM VWOE 23 Expressions and constants 23 2 GENERAL FORM OF EXPRESSIONS I gt Expression Call Operator_expression Equality Manifest_constant Manifest_array Old I Strip Boolean_expression Expression A Boolean_expression is valid if and only if it is an Expression of type BOOLEAN 23 3 EQUALITY EXPRESSIONS Equality Expression Comparison Expression We Ne Comparison An Equality expression is valid if and only if either of its operands conforms to the other The expression e fhas value true if and only if e fhas value false 23 5 OPERATOR EXPRESSION SYNTAX AND PRECEDENCE RULES Operator_expression Parenthesized Unary_expression Binary_expression Parenthesized Expression Unary_expression Prefix_operator Expression Binary_expression Expression Infix_operator Expression 23 6 VALIDITY AND SEMANTICS OF OPERATOR EXPRESSIONS An Operator_expression is valid if and only if its equivalent dot form is a valid Call 22 9 If a routine of C contains an Assignment of target x and source e the dynamic type set of x for T includes recursively every member of the
80. re_list VALIDITY le fname is the final name in C of a feature inherited from B VDUS 2 That feature was not frozen and was not an attribute 3 That feature was effective in B 4e fname appears only once in the Feature_list 10 20 DEFERRED AND EFFECTIVE FEATURES AND CLASSES Definition effective feature deferred feature effecting A feature f of a class C is said to be an effective feature of C if and only if any of the following conditions holds 10 2 REDECLARING INHERITED FEATURES WHY AND HOW 37 10 Feature adaptation 10 2 REDECLARING INHERITED FEATURES WHY AND HOW Definition redeclaration A class that contains a redefinition or effecting of an inherited feature will be said to redeclare that feature 10 6 THE REDEFINITION CLAUSE Definition precursor If a class inherits a feature from a parent either keeping the feature unchanged or redefining it the parent s version of the feature is called the precursor of the feature 10 15 REDECLARATION AND ASSERTIONS Consider a routine redeclaration and let pre pre be the precursors preconditions and post post be the precursors postconditions Assume that new assertion clauses are present of the form require else alternative_precondition ensure then extra_postcondition Then the redeclared routine will be considered to have the precondition and postcondition alternative_precondition or else pre or else or else pre ex
81. recursors for any set of two or more elements representing features that are repeatedly inherited in D under the same name so that the Repeated Inheritance rule yields sharing keep only one of these elements The Repeated Inheritance Consistency constraint sharing case indicates that these elements must all represent the same feature so that it does not matter which one is kept 2 For every feature f in the resulting list if D undefines f replace f by a deferred feature with the same signature and specification 3 In the resulting list for any set of deferred features with the same final name in D keep only one of these features with assertions joined as per the Join Semantics rule Keep the signature which the Join rule requires to be the same for all the features involved 4e In the resulting list remove any deferred feature such that there is an effective feature with the same final name in the list This is the case in which a feature f inherited as effective effects one or more deferred features of the whole group only jf remains 5e Let merged_features be the resulting list All its elements have different feature names they are the inherited features of D in their parent form From this list produce a new one as follows for any feature which D redeclares by redefinition or effecting replace the feature by the result of the redeclaration keep any other feature as it is in merged_features 6 The result is the
82. routine r of C exported generally or selectively pre INVc do post A INVc 9 5 FORM OF ASSERTIONS 33 STANT AR ALIGI VAPE 9 Correctness 9 5 FORM OF ASSERTIONS Precondition A require else Assertion Postcondition A ensure then Assertion Invariant invariant Assertion Assertion Assertion_clause Assertion_clause 2 Tag_mark Unlabeled_assertion_clause Unlabeled_assertion_ A Boolean_expression Comment clause Tag_mark 2 Tag g Tag 2 Identifier In an Assertion the semicolon separating each Assertion from the next has the same semantics as the and then infix boolean operator This means that the order of the clauses may be meaningful le The value of an Assertion is true if and only if every Assertion_clause in the Assertion has value true 2 Ifan Assertion_clause has value false the whole Assertion in which it appears has value false even if the value of a subsequent clause is not defined 9 7 THE SPECIFICATION OF A ROUTINE Definition specification subspecification Let pre and post be the precondition and postcondition of a routine rout The specification of rout is the pair of assertions lt pre post gt A specification lt pre post gt is said to be a subspecification of lt pre post gt if and only if pre implies pre and post implies post Here implies is boolean implication 9 8 CONSTRAINTS ON ROUTINE ASSERTIONS A Precondition of
83. run time values which are instances of the corresponding base type 12 14 BIT TYPES A Bit_type declaration is valid if and only if its Constant is of type INTEGER and has a positive value The possible values of an entity declared as BIT N for some N are bit sequences of exactly N bits 12 15 ANCHORED TYPES Definition anchored type anchor An Anchored type is of the form like anchor where anchor is called the anchor of the type An anchored type of the form like anchor appearing in a class C is valid if and only if one of the following holds le anchor is the final name of an attribute or function of C whose declared type is a non Anchored reference type 2e The type appears in the text of a routine r of C and anchor is a formal argument of r whose declared type is a non Anchored reference type 3 anchor is the reserved word Current WALIDITY VTBT VTAT 12 8 CONSTRAINED GENERICITY 47 AALIDE VTUG VTCG Definition generic class generic derivation non generic Any class declared with a non empty Formal_generics part constrained or not is said to be a generic class A generic class does not describe a type but a template for a set of possible types To derive an actual type from this template you must provide an Actual_generics list whose elements are themselves types The result is called a generic derivation Unconstrained Genericity rule Let CT be a Class_type having
84. ry Identifier_list of every Entity_declaration_group in fa Then el is valid if and only if no Identifier appears more than once in the list identifiers 8 5 ROUTINE STRUCTURE We Routine Obsolete Header_comment Precondition Local_declarations Routine_body Postcondition Rescue end Feature_name Routine rule A Routine part of a routine declaration is valid if and only if one of the following conditions holds le Its Routine_body is an Internal body beginning with do or once SYNTAX VRFA VREG SYNTAX WALIGITY VRRR 7 14 DESCRIBING A CLASS FOR CLIENTS THE SHORT FORM 29 1 At most one of the feature_list is the keyword all 2 All the other feature_list are lists of final names of features of C obtained from B 3 No final feature name appears twice in any such list or appears in more than one list An immediate feature of a class has the following export status e If the Feature_clause which introduces it has no Clients part that is to say begins with the keyword feature with no further qualification the feature is exported generally available e If the Feature_clause which introduces it has a Clients part that is to say begins with feature A B C the feature is selectively available to the descendants of the classes listed in that Clients part and to these descendants only Definition secret If a Feature_clause
85. s an instance of any type T based on C and similarly for direct instances 17 5 INSTANCES AND DIRECT INSTANCES OF A TYPE Definition instances of a type The instances of a type 7X are the direct instances of any type conforming to TX Definition diirect instances of non basic expanded types Let TX be an expanded type which is neither one of the basic types BOOLEAN CHARACTER INTEGER REAL DOUBLE POINTER nor a Bit_type The direct instances of TX are the the direct instances of the base type of TX 17 6 DIRECT INSTANCES OF BASIC TYPES Direct instances of basic types The direct instances of the basic types are the following e For BOOLEAN the boolean values true and false e For CHARACTER any character e For INTEGER all the integer values which may be represented on Jnteger_bits bits e For REAL all floating point values which may be represented on Real_bits bits e For DOUBLE all the floating point values which may be represented on Double_ bits bits e For POINTER all possible feature addresses for transmission to non Eiffel routines 16 5 CONSTANT ATTRIBUTES WITH MANIFEST VALUES 59 VQMC VALIDITY VQUI 16 Attributes 16 5 CONSTANT ATTRIBUTES WITH MANIFEST VALUES A declaration of a feature f introducing a manifest constant is valid if and only if the Manifest_constant m used in the declaration matches the type T declared for f in one of the following ways e mis a Boolean_constan
86. set of x The set of base classes of these types is called the dynamic class set of x 20 14 RULES ON ASSIGNMENT ATTEMPT Assignment_attempt A Writable Expression Assignment Attempt rule An Assignment_attempt is valid if and only if the type of the target entity is a reference type The effect of an Assignment_attempt of source y and target x of type TX is the following le Ify is attached to an object whose type conforms to TX then the effect is that of a direct reattachment of y to x as given earlier in this chapter 2e Ify is void or attached to an object whose type does not conform to TX the effect is to make the value of x void 20 17 SEMANTICS OF EQUALITY There are in fact two operators to consider here equality and inequality However only one semantic definition is needed since the effect of x y is defined in all cases to be that of not x y TYPE OF FIRST Reference TYPE OF SECOND Reference 1 Reference equality 3 standard_equal 4 standard_equal 2 standard_equal 68 20 4 20 Reattaching entities 20 4 SYNTAX AND VALIDITY OF DIRECT REATTACHMENT Assignment Writable Expression Assignment rule An Assignment is valid if and only if its source expression conforms to its target entity 20 5 SEMANTICS OF DIRECT REATTACHMENT SOURCE TYPE Reference Expanded TARGET TYPE Reference 1 Reference reattachme
87. sponding to a variable attribute the default value of the attribute s type according to the rules given below 4e Ifthe Creation instruction includes a Creation_call that is to say an Unqualified_ call execute that call on the resulting object Se Attach x to the object 18 13 DEFAULT INITIALIZATION VALUES Consider a field of a newly created object corresponding to an attribute of type FT in the base class of the object s type The default initialization value init for the field is determined as follows according to the nature of FT e For a reference type a void reference 18 8 CREATION SYNTAX 63 VGCP WALIDITY VGCC SYNTAX j 18 Creating objects 18 8 CREATION SYNTAX Creators A creation Creation_clause creation Creation_clause Clients Header_comment Feature_list Creation_clause rule A Creation_clause appearing in the Creators part of a class C is valid if and only if it satisfies the following five conditions the last four for every Feature_identifier fname in the clause s Feature_list le Cis effective 2e fname appears only once in the Feature_list 3 fname is the final name of a procedure of C let cp be that procedure 4e cp is nota once routine 5e If Cis expanded cp has no arguments and no other Feature_identifier appears in the Feature_list Definition creation procedure We Creation I Type Writable Creation_call Cre
88. stem_name system System_name Name root Class_name Cluster _mark Creation_procedure Name Cluster_name Name Name D 7 BASICS OF CLUSTER CLAUSES Clusters Cluster_clause Cluster_tag Directory _name Cluster_properties l gt I gt We We cluster Cluster_clause Cluster_tag Directory_name Cluster_properties Cluster_name Name Use Include Exclude Name_adaptation STATA STAT AR D OVERVIEW 97 ALIGI VDCN D Specifying systems in Lace D 1 OVERVIEW Language processing tools need a specification of where to find the classes and what to do with them Such a specification is called an Assembly of Classes in Eiffel or Ace for short This appendix presents a notation the Language for Assembling Classes in Eiffel or Lace for writing Aces This appendix describes Lace D 5 Basic conventions Here is the list of Lace keywords adapt all check cluster creation default end ensure exclude export external generate ignore include invarian keep loop t option require rename system visible yes Name Identifier Manifest_string A consistency condition applies to names used in an Ace the Cluster_name must be different for each cluster It is valid however to use the same identifier in two or more of the roles of Cluster_name System_name Class_name 96 25 15 25 15 CHARACTERS 9
89. struct All constructs have names beginning with a capital letter and written in the default roman font Each appears in the index with a reference to the page of its syntactical definition An important convention will simplify the discussions the phrase an X where X is the name of a construct serves as a shorthand for a specimen of construct X For example a Class means a specimen of construct Class in other words a text built according to the syntactical specification of construct Class 2 3 TERMINALS NON TERMINALS AND TOKENS Every construct is either a terminal or a non terminal as defined next y Definitions Terminal Non terminal Token Specimens of a terminal construct have no further syntactical structure Examples include reserved words such as class Result etc constants such as integers and identifiers used to denote classes features and entities In contrast the specimens of a non terminal construct are defined in terms of other constructs Definition token lexical component The specimens of terminal constructs are called tokens or lexical components They form the basic vocabulary out of which you may build more complex texts specimens of non terminals CHANGES A preliminary standard document for the Kernel Library known as the PELKS Propesed Eiffel Library Kernel Standard s currenth under study by NICE To avoid any ambiguity or contradic
90. t and T is BOOLEAN e mis aCharacter_constant and Tis CHARACTER e mis an Integer_constant and T is INTEGER e mis aReal_constant and T is REAL or DOUBLE e mis a Manifest_string and Tis STRING e mis a Bit_constant consisting of exactly M bits for some positive integer M and T is BIT M for the same M 16 6 UNIQUE ATTRIBUTES Unique unique A declaration of a feature f introducing a Unique constant is valid if and only if the type T declared for f is INTEGER Unique Declaration rule The value of an attribute declared as unique is a positive integer If two unique attributes are introduced in the same class their values are different Furthermore unique attributes declared as part of the same Feature_declaration are guaranteed to have consecutive values in the order given 58 15 9 A Retry instruction is valid if and only if it appears in a Rescue clause 15 9 SEMANTICS OF EXCEPTION HANDLING Definition rescue block Any Internal routine r of a class C has a rescue block rb which is a Compound defined as follows le Ifrhas a Rescue clause rb is the Compound contained in that clause 2e Ifrhas no Rescue clause rb is a Compound made of a single instruction a call to the version of default_rescue in C An exception triggered during an execution of a routine r leads if it is neither ignored nor continued to the following sequence of events le Some or all of the remaining instructions are not e
91. t call although fields of the attached object if any may change 23 12 CONSTANTS e The value of Current is the current object Constant Constant_attribute Manifest_constant Sign Integer_constant Character_constant Boolean_constant Real_constant Manifest_string A We H gt We We We Wo Manifest_constant Constant_attribute Entity A Constant_attribute appearing in a class C is valid if and only if its Entity is the final name of a constant attribute of C Boolean_constant Character_constant Integer_constant Real_constant Manifest_string Bit_constant mpm pn Sign Integer e Character true false Sign Real Tn gt Simple_string 76 23 7 Definition multiary operator Some binary operators are actually multiary that is to say may take three or more operands whose types all conform to the type of the first while other operators are limited to two arguments The semantics of an Operator_expression is simply the semantics of calls the value of an operator expression is the value that would be returned by the equivalent dot form This is complemented by a special rule seen below semi strict evaluation for boolean operators 23 7 ORDINARY BOOLEAN OPERATORS The value of not a is true if and only if a has value false The others are binary operators the value they yield when applied to a first operand of value v and a seco
92. tes the earlier CO and cr resume their roles as current object and current routine 4e When cr executes an unqualified call the current object remains the same and there is a new current routine for the duration of the call as in case 3 21 14 SEMANTICS OF CALLS Consider the execution at a certain run time instant of a call targete fname yj Yn To define its effect call target_value the value of target at that instant The first possibility is for target_value to be void Then the call cannot be executed correctly it will fail triggering an exception For the exception raised in this case the Kernel Library class EXCEPTIONS introduces the integer code Void_call_target The rest of this section assumes that target_value is not void Then target_value is attached to some object OD which must be a direct instance of some type DT for dynamic type based on some class D Let ST be the type of expression target and S the base class of ST The rules of reattachment indicate that DT conforms to ST and that D is a descendant of S Also because DT has a direct instance OD D must be an effective class If the call is valid the constraint on calls implies that fname is the final name of a feature sf of class S available to the class which contains the call Let df be the version of sf in D dynamic binding means that the effect of the call is determined by df not sf If df is an external routine the effect of the call
93. th operator features both Prefix or both Infix with identical lower names 5 14 VALIDITY OF FEATURE DECLARATIONS Feature Declaration rule A Feature_declaration appearing in a class C and whose New_feature_list contains one or more feature names f fa is valid if and only if it satisfies all of the following conditions VALIDITY e Its Declaration_body describes a feature which according to the rules of 5 11 is VFFD one of variable attribute constant attribute procedure function e None of the f has the same name as another feature introduced in C in particular fiis not the same name as f for different 7 and j Ifthe name of any of the f is the same as the final name of any inherited feature the Declaration_body satisfies the Redeclaration rule e If the Declaration_body describes a deferred feature then none of the f is preceded by the keyword frozen e If any of the f is a Prefix name the Declaration_body describes an attribute or a function with no argument e If any of the f is an Infix name the Declaration_body describes a function with exactly one argument e If the Declaration_body describes a once function the result type involves neither a Formal_generic_name nor an Anchored type 5 16 SYNONYMS Multiple Declaration rule The semantics of a feature declaration applying to more than one feature name as in fi f2 fa Some_declaration_body is except in one special case defi
94. than its base type 10 23 RULES ON JOINING FEATURES Definition precursor joined features A precursor of an inherited feature is a version of the feature in the parent from which it is inherited Without the join mechanism there was just one precursor but a feature which results from the join of two or more deferred features will have all of them as precursors Join rule It is valid for a class C to inherit two different features as deferred under the same final name if and only if after possible redeclaration in C they have identical signatures VDIR 10 21 ORIGIN AND SEED 39 le fis introduced in C as an attribute or a routine whose Routine_body is of the Effective form that is to say not the keyword deferred but beginning with do once or external 2e fis an inherited feature coming from a parent B of C where it is recursively effective and C does not undefine it 3 Another feature of C with the same final name is recursively effective That feature is then said to effect f in C A feature of C is a deferred feature of C if and only if it is not an effective feature of C Condition 3 defines the effecting case an effective feature which has the same final name as one or more deferred features serves as effecting for all of them Definition deferred effective class A class is deferred if it has at least one deferred feature It is effective otherwise 10 21 ORIGIN AND SEED
95. tion all discussions of Kernel Library classes in this book have been replaced by references to the PELKS CHANGES This book cerrespads to the second printing of Eiffel The Language It incorporates a few corrections corresponding to mistakes that have been detected since the publication of the book The principal among these changes are listed in chapter 14 of ISE Eiffel The Environment Technical Report TR 39 IE A complete list of the changes will be made available separately Time prevented inclusion of change bars for the first release of this document but they will be part of future editions all changes have been carefully logged In case of discrepancy between Eiffel The Language and Eiffel The Reference follow the document that has the latest printing date unless the problem appears to be due to a text processing mistake Remember that at the basis of Eiffel there is only one document the only differences result from how one selects and prints portions of that document This is the required condition for the stability maturity and credibility of the Eiffel language in other words for its success The present book uses the same conventions as Eiffel The Language To avoid any confusion the original chapter and section numbers have been retained So if you have leafed through this book before reading the present preface you may have wondered why the first chapter is number 2 and its first section 2 2 but the purpose
96. tion of s post its postcondition and do its body 9 13 CHECK INSTRUCTIONS Check A check Assertion end Definition check correct An effective routine r is check correct if for every Check instruction c in r any execution of c as part of an execution of r satisfies all its assertions 9 14 LOOP INVARIANTS AND VARIANTS Variant 4 variant Tag_mark Expression A Variant is valid if and only if its Expression is of type INTEGER when 2 7 at Exit condition preserves the truth of INV Gn other words leaves IVY true if it finds INV oricinal Definition loop correct A routine is loop correct if every loop it contains satisfies the following four conditions le true NIT INV 2 true INIT VAR gt 0 3 INV and then not EXIT BODY INV 4e INV and then not EXIT and then VAR v BODY 0 lt VAR lt v where INV is the loop s invariant VAR its variant NIT its Initialization EXIT its Exit condition and BODY its Loop_body 34 9 9 Definition availability of an assertion clause An Assertion_clause a of a routine Precondition or Postcondition is available to a class B if and only if all the entities involved in a are available to B with the convention that formal arguments and Result are available to all classes 9 9 OLD EXPRESSION Old old Expression An Old expression of the form old e where e is an expression of type TE is valid if and only if it satisfi
97. tra_postcondition and then post and then and then post If in a routine redeclaration the Precondition part is absent the redeclared routine is considered to have false as its alternative_precondition if the Postcondition part is absent the redeclared routine is considered to have true as its extra_postcondition Because of the rules of boolean algebra the absence of one of these assertions means that the corresponding precursor assertion is kept as it was Or ing a boolean value with false or and ing it with true does not change the condition For a declaration of an immediate feature of a class the require else form of Precondition clause has the same meaning as if it were introduced by just require and the ensure then form of Postcondition clause has the same meaning as if it were introduced by just ensure 10 16 UNDEFINING A FEATURE Definition inherited as effective as deferred In the rest of this discussion an inherited feature is said to be inherited as effective if it has at least one effective precursor and the corresponding Parent part does not undefine it Otherwise the feature is inherited as deferred 36 9 16 9 16 CORRECTNES S OF A CLASS Definition correctness class A class is correct if and only if it is consistent and every routine of the class is check correct loop correct and exception correct 9 17 SEMANTICS OF ASSERTIONS For a correct system assertions in all cases wil
98. uence 23 17 CHARACTER CONSTANTS The value of a Character_constant is its middle Character 23 18 MANIFEST STRINGS A Manifest_string is valid if and only if it satisfies the following two conditions le None of the characters of its associated Simple_string is a double quote 2 Inthe extended form no characters other than blanks or tabs may appear before the initial sign on the second and subsequent lines 23 20 MANIFEST ARRAYS Manifest_array lt lt Expression_list gt gt l gt We Expression_list Expression Manifest Array rule A Manifest_array lt lt e e2 e gt gt is a valid expression of type ARRAY T if and only if the type of every e conforms to T VALIDITY The value of a Manifest_array made of N expressions is an array of bounds 1 and N VWMA whose elements are the values of the successive expressions in the Manifest_array In this definition an array is an instance of the Kernel Library class ARRAY 23 21 STRIP EXPRESSIONS Strip strip Attribute_list Identifier I gt Attribute_list A Strip expression appearing in a class C is valid if and only if its Attribute_list satisfies the following two conditions le Every Identifier appearing in the list is the final name of an attribute of C 2 No Identifier appears twice in the list eT Consider the evaluation of a Strip expression as part of a call to a routine r w
99. unction on appropriate arguments There is a major difference between the Eiffel call and its C emulation Cecil does not apply dynamic binding What you get from eif_proc or one of its sisters is a pointer to a function representing the exact Eiffel routine of the given name in the given class In the presence of polymorphism and redeclaration the Eiffel call may trigger a different version of go depending on the type of the object attached to your_list at the time the call is executed The C form will always call the same version regardless of the object s type 80 24 3 24 Interfaces with other languages 24 3 EXTERNAL ROUTINES We external Language_name External_ name External Language_name Manifest_string We We External_name alias Manifest_string 24 5 ARGUMENT AND RESULT TRANSMISSION For external routines follow the the semantics of direct reattachment interpreted as if each formal argument were declared with exactly the same type as the corresponding actual 24 6 PASSING THE ADDRESS OF AN EIFFEL FEATURE An argument of the Address form is of type POINTER The validity constraint on actual arguments of the Address form is clause 4 of the argument validity rule which makes f valid as actual argument to a call if and only if f is the final name of a feature of the enclosing class and that feature is not a constant attribute which has no address If the rule is satisfied the
100. universes they are not language constructs but mechanisms for grouping and storing classes using the facilities provided by the underlying operating system such as files and directories 3 3 CLASS TEXTS AND CLASS NAMES Every class has a name such as DOCUMENT or PARAGRAPH and a class text which describes the features of the class and its other properties For class names as for all uses of identifiers letter case is not significant identifiers such as DOCUMENT document and even dOcUmEnT have the same semantics when viewed as class names Definition U N The standard recommended style in Eiffel texts is to write all class names using exclusively the upper name of the classthatiste saythe name_all in upper case suchas Definition Class Name The term class name as used in this book denotes the upper name of a class In particular two classes are said to have the same class name if they have the same 2 11 REQUIREMENTS ON LANGUAGE PROCESSING TOOLS 11 2 11 REQUIREMENTS ON LANGUAGE PROCESSING TOOLS The definition of Eiffel syntax validity and semantics contained in this book is also a specification of certain aspects of the corresponding language processing tools Not all aspects apply to all language processing tools A language processing tool that processes software components at a certain level syntax validity semantics is not required to perform the tasks associated with that level on components w
101. viously published the book Eiffel The Language Prentice Hall second printing with corrections 1992 ISBN Q 13 24795 7 The intent of that document is to provide under a single cover a precise reference a tutorial a Guide for the Perplexed and a detailed user s manual for the language As a result much of the space in it is occupied by examples explanations justifications discussions previews reminders and comments Eiffel The Language does not shun repetition occasionally for example some part of the discussion needs to refer to the syntax of a construct seen in a distant chapter and simply reproduces the syntax specification for the convenience of the reader In some cases however a shorter reference may be necessary A typical example is that of someone implementing an Eiffel compiler who may be presumed to be familiar with the rationale behind the various components of the language but will need a document were he can quickly find precise answers to specific questions often on fine points Can the target of an anchored declaration be anchored too Another example is that of a user who is familiar with the language but wants to keep a concise reference on his desk Providing such a no frills description of Eiffel is the purpose of Eiffel The Reference This book is not meant as a first introduction to Eiffel If you do not know the language or know it only superficially you should read Eiffel The Language
102. xecuted 2 The rescue block of the routine is executed 3 Ifthe rescue block executes a Retry the body of the routine is executed again This terminates processing of the current exception Any new triggering of an exception is a new occurrence which will recursively be handled according to the present semantics 4e Ifthe rescue block is executed to the end without executing a Retry this terminates the processing of the current exception and the current execution of r causing a failure of that execution If there is a calling routine this failure triggers an exception in the calling routine which will be handled recursively according to the same semantics If there is no calling routine r is the root s creation procedure its execution will terminate The definition mentions that it applies only to a eutine which is neither ignored nor continued This corresponds to two facilities provided through features of the Kernel Library class EXCEPTIONS implementing the false alarm response e You may specify that a certain type exception must be altogether ignored e You may specify that a certain type of exception must cause execution of a designated procedure and then continuation 15 10 EXCEPTION CORRECTNESS Definition exception correct A routine r of a class C is exception correct if and only if for every branch b of its rescue block le If bends with a Retry true b INVc pre 2 If b does not end with a Retr
103. y true b INV In this rule INV is the invariant of class C and pre is the precondition of r VALIDITY VXRT 15 2 WHAT IS AN EXCEPTION 57 15 Exceptions 15 2 WHAT IS AN EXCEPTION Definitions failure exception Under certain circumstances the execution of a construct such as an instruction may be unable to terminate as you normally expect it to The execution is then said to result in a failure If a routine executes a component and that component fails this will prevent the routine s execution from proceeding as planned such an event is called an exception 15 3 EXCEPTION HANDLING POLICY Only three possibilities make sense for handling an exception e A favorable albeit unlikely case is one in which the exception was in fact not justified This is called the false alarm e When writing the component you may have anticipated the possibility of an exception and provided for an alternative way to fulfil the contract Then the execution will try that alternative This case is called resumption e If you have no way of fulfilling the contract then you should try to return the objects involved into an acceptable state and signal your failure to the client This is called organized panic Definition recipient Any execution of a software component is part of the execution of a call to a certain routine known as the current routine If the component s execution fails this will trigger an
104. y sequences of values the result is true if and only if the sequences have the same length and every field in one is recursively equal to the field at the same position in the other Otherwise OX and OY are standard complex objects and conformance of TY to TX implies that for every field of OX there is a corresponding field in OY Then the result is true if and only if every reference field of OX is attached to the same object as the corresponding field in OY and every object field of OX coming from an expanded attribute in TX s base class is recursively equal to the corresponding field in OY 19 8 DEEP EQUALITY Two references x and y are deep equal if and only if they are either both void or attached to deep equal objects Two objects OX and OY are deep equal and only if they satisfy the following four conditions le OX and OY have the same exact type 2e The objects obtained by setting all the reference fields of OX and OY if any to void references are equal 3 For every void reference field of OX the corresponding field of OY is void 4e For every non void reference field of OX attached to an object PX the corresponding field of OY is attached to an object PY and it is possible recursively to show under the assumption that OX is deep equal to OY that PX is deep equal to PY 66 19 2 19 Duplicating and comparing objects 19 2 COPYING AN OBJECT Assume copy has not been redefined and conside

35) p. 66. This is embarassing but I`m not sure I`ve ever understood

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