85:. In these cases identifiers must be defined (variables initialized, functions defined) before they can be employed during runtime without the need for pre-definition in the source code for either compilation or interpretation: identifiers do not need to be immediately resolved to an existing entity.
73:
in such languages, as it is impossible to define such functions (or data structures) without a forward reference in one definition: one of the functions (respectively, data structures) must be defined first. It is also useful to allow flexible code organization, for example if one wishes to place the
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programming languages, it is a general rule that all entities must be declared before use, and thus forward declaration is necessary for mutual recursion, for instance. In C, the same general rule applies, but with an exception for undeclared functions and incomplete types. Thus, in C it is possible
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in the rest of the program. The definition for a function must be provided somewhere (same file or other, where it would be the responsibility of the linker to correctly match references to a particular function in one or several object files with the definition, which must be unique, in another):
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Variables may have only forward declaration and lack definition. During compilation time these are initialized by language specific rules (to undefined values, 0, NULL pointers, ...). Variables that are defined in other source/object files must have a forward declaration specified with a keyword
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In C++, classes can be forward-declared if you only need to use the pointer-to-that-class type (since all object pointers are the same size, and this is what the compiler cares about). This is especially useful inside class definitions, e.g. if a class contains a member that is a pointer (or a
514:. This is especially useful inside class definitions, e.g. if a class contains a member that is a pointer to another class; to avoid circular references (i.e. that class might also contain a member that is a pointer to this class), we simply forward-declare the classes instead.
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Forward declaration of a class is not sufficient if you need to use the actual class type, for example, if you have a member whose type is that class directly (not a pointer), or if you need to use it as a base class, or if you need to use the methods of the class in a method.
435:, it is sometimes necessary to forward-declare classes. This is done in situations when it is necessary to know that the name of the class is a type, but where it is unnecessary to know the structure.
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of functions), but not other details, like the particular value it holds (in case of variables or constants) or definition (in the case of functions). This is particularly useful for
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Forward-declaration is used to avoid unnecessary coupling which help reducing compilation time by reducing the number of header inclusion. This has a triple advantage:
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Permitting forward references can greatly increase the complexity and memory requirements of a compiler, and generally prevents the compiler from being implemented in
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before it is declared. C++ generally prohibits forward references, but they are allowed in the special case of class members. Since the member function
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In
Objective-C, classes and protocols can be forward-declared if you only need to use them as part of an object pointer type, e.g.
35:(denoting an entity such as a type, a variable, a constant, or a function) for which the programmer has not yet given a complete
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in the code above is a forward reference. Thus, we may say that because forward declarations are mandatory in Pascal, forward
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Forward declaration of a class or protocol is not sufficient if you need to subclass that class or implement that protocol.
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Forward declaration is used in languages that require declaration before use; it is necessary for
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In other languages forward declarations are not necessary, which generally requires instead a
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reduce the number of files opened by #include (hence the number of operating system calls)
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reducing the volume of the pre-processed files (as the header is not included)
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reducing recompilation impact when the forward declared class is modified.
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In
Objective-C, classes and protocols can be forward-declared like this:
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http://pages.cs.wisc.edu/~fischer/cs536.s07/lectures/Lecture25.4up.pdf
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of an entity before any declaration; that is, the first reference to
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In Pascal, the same implementation requires a forward declaration of
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In C++, classes and structs can be forward-declared like this:
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cannot be compiled until the compiler knows the type of the
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to know certain properties of an identifier (size for
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main body at the top, and called functions below it.
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would allow the program code to refer to the entity
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A computer programming declaration of an identifier
701:MSDN: Converting to a Forward-Reference Class Type
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655:In this example, there are two references to
214://foo might be defined somewhere in this file
723:Thinking in C++: Inlines & the compiler
548:An example of (valid) forward reference in
81:and for some compilation to be deferred to
243:(although unwise) to implement a pair of
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229://bar must be defined in some other file
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419:has been used without being declared.
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396:// backward reference to first
321:// forward reference to second
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93:A basic example in C is:
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512:id<MyProtocol>
129:function's prototype
64:separate compilation
21:computer programming
531:forward declaration
79:multi-pass compiler
25:forward declaration
245:mutually recursive
60:one-pass compilers
527:forward reference
521:Forward reference
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433:Objective-C
127:and is the
29:declaration
689:References
543:references
500:MyProtocol
414:identifier
238:and other
37:definition
33:identifier
525:The term
508:MyClass *
497:@protocol
83:link time
52:data type
734:Category
683:one pass
672:accessor
661:accessor
611:accessor
455:MyStruct
172:"%d
133:compiler
125:function
89:Examples
44:compiler
676:myValue
668:myValue
657:myValue
644:myValue
635:private
626:myValue
593:myValue
575:mutator
493:MyClass
446:MyClass
423:Classes
623:return
566:public
539:second
490:@class
452:struct
417:second
406:second
378:return
366:return
330:second
306:second
303:return
291:return
236:Pascal
217:extern
198:extern
178:"
166:printf
31:of an
617:const
557:class
443:class
410:first
381:first
255:first
240:Wirth
27:is a
572:void
431:and
375:else
300:else
145:void
119:and
98:void
62:and
23:, a
641:int
608:int
581:int
550:C++
535:use
510:or
429:C++
336:int
327:int
261:int
252:int
234:In
223:bar
220:int
208:foo
205:int
154:int
121:C++
115:In
107:int
19:In
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650:};
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390:-1
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315:-1
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