Incomputer programming, aforward declaration is adeclaration of anidentifier (denoting an entity such as a type, a variable, a constant, or a function) for which the programmer has not yet given a completedefinition.
It is required for acompiler to know certain properties of an identifier (size formemory allocation,data type fortype checking, such astype signature of functions), but it isn't required to know some 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 forone-pass compilers andseparate compilation.
Forward declaration is used in languages that require declaration before use; it is necessary formutual recursion 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 declared first. It is also useful to allow flexible code organization, for example if one wishes to place the main body at the top, and called functions below it.
In other languages forward declarations are not necessary, which generally requires instead amulti-pass compiler and for some compilation to be deferred tolink time. 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.
A basic example in C is:
voidprintThisInteger(int);
InC andC++, the line above represents a forward declaration of afunction and is thefunction's prototype. After processing this declaration, thecompiler would allow the program code to refer to the entityprintThisInteger 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):
voidprintThisInteger(intx){printf("%d\n",x);}
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 keywordextern:
intfoo;//foo might be defined somewhere in this fileexternintbar;//bar must be defined in some other file
InPascal and otherWirth 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 (although unwise) to implement a pair ofmutually recursive functions thus:
intfirst(intx){if(x==0)return1;elsereturnsecond(x-1);// forward reference to second}intsecond(intx){if(x==0)return0;elsereturnfirst(x-1);// backward reference to first}
In Pascal, the same implementation requires a forward declaration ofsecond to precede its use infirst. Without the forward declaration, the compiler will produce anerror message indicating that theidentifiersecond has been used without being declared.
In some object-oriented languages likeC++ andObjective-C, 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.
In C++, classes and structs can be forward-declared like this:
classMyClass;structMyStruct;
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 reference) to another class.
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:
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.
In Objective-C, classes and protocols can be forward-declared like this:
@classMyClass;@protocolMyProtocol;
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.MyClass * orid<MyProtocol>. This is especially useful inside class definitions, e.g. if a class contains a member that is a pointer to another class; to avoidcircular references (i.e. that class might also contain a member that is a pointer to this class), we simply forward-declare the classes instead.
Forward declaration of a class or protocol is not sufficient if you need to subclass that class or implement that protocol.
The termforward reference is sometimes used as a synonym offorward declaration.[1] However, more often it is taken to refer to the actualuse of an entity before any declaration; that is, the first reference tosecond in the code above is a forward reference.[2][3] Thus, we may say that because forward declarations are mandatory in Pascal, forwardreferences are prohibited.
An example of (valid) forward reference inC++:
classC{public:voidmutator(intx){myValue=x;}intaccessor()const{returnmyValue;}private:intmyValue;};
In this example, there are two references tomyValue before it is declared. C++ generally prohibits forward references, but they are allowed in the special case of class members. Since the member functionaccessor cannot be compiled until the compiler knows the type of themember variablemyValue, it is the compiler's responsibility to remember the definition ofaccessor until it seesmyValue's declaration.
Permitting forward references can greatly increase the complexity and memory requirements of a compiler, and generally prevents the compiler from being implemented inone pass.