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In computer programming, a declaration is a language construct specifying identifier properties: it declares a word's (identifier's) meaning. [1] Declarations are most commonly used for functions, variables, constants, and classes, but can also be used for other entities such as enumerations and type definitions. [1]
C functions are akin to the subroutines of Fortran or the procedures of Pascal. A definition is a special type of declaration. A variable definition sets aside storage and possibly initializes it, a function definition provides its body. An implementation of C providing all of the standard library functions is called a hosted implementation.
An external variable can be accessed by all the functions in all the modules of a program. It is a global variable.For a function to be able to use the variable, a declaration or the definition of the external variable must lie before the function definition in the source code.
The latter list is sometimes called the "initializer list" or "initialization list" (although the term "initializer list" is formally reserved for initialization of class/struct members in C++; see below). A declaration which creates a data object, instead of merely describing its existence, is commonly called a definition.
A definition or declaration specifies the data on which a program is to operate, while a statement specifies the actions to be taken with that data. Statements which cannot contain other statements are simple ; those which can contain other statements are compound .
C++ generally prohibits forward references, but they are allowed in the special case of class members. Since the member function accessor cannot be compiled until the compiler knows the type of the member variable myValue, it is the compiler's responsibility to remember the definition of accessor until it sees myValue's declaration.
In C and C++, the type signature is declared by what is commonly known as a function prototype. In C/C++, a function declaration reflects its use; for example, a function pointer with the signature (int)(char, double) would be called as:
This is an accepted version of this page This is the latest accepted revision, reviewed on 6 January 2025. General-purpose programming language "C programming language" redirects here. For the book, see The C Programming Language. Not to be confused with C++ or C#. C Logotype used on the cover of the first edition of The C Programming Language Paradigm Multi-paradigm: imperative (procedural ...