Declaration (computer programming)

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] Beyond the name (the identifier itself) and the kind of entity (function, variable, etc.), declarations typically specify the data type (for variables and constants), or the type signature (for functions); types may also include dimensions, such as for arrays. A declaration is used to announce the existence of the entity to the compiler; this is important in those strongly typed languages that require functions, variables, and constants, and their types to be specified with a declaration before use, and is used in forward declaration.^[2] The term "declaration" is frequently contrasted with the term "definition",^[1] but meaning and usage varies significantly between languages; see below.

Declarations are particularly prominent in languages in the ALGOL tradition, including the BCPL family, most prominently C and C++, and also Pascal. Java uses the term "declaration", though Java does not require separate declarations and definitions.

^ ^a ^b ^c
"A declaration specifies the interpretation and attributes of a set of identifiers. A definition of an identifier is a declaration for that identifier that:
- for an object [variable or constant], causes storage to be reserved for that object;
- for a function, includes the function body;
- for an enumeration constant, is the (only) declaration of the identifier;
- for a typedef name, is the first (or only) declaration of the identifier."
C11 specification, 6.7: Declarations, paragraph 5.
^ Mike Banahan. "2.5. Declaration of variables". GBdirect. Retrieved 2011-06-08. [A] declaration [...] introduces just the name and type of something but allocates no storage[...].

[c11-1] "A declaration specifies the interpretation and attributes of a set of identifiers. A definition of an identifier is a declaration for that identifier that:
for an object [variable or constant], causes storage to be reserved for that object;

for a function, includes the function body;

for an enumeration constant, is the (only) declaration of the identifier;

for a typedef name, is the first (or only) declaration of the identifier."
C11 specification, 6.7: Declarations, paragraph 5.

[2] r an object [variable or constant], causes storage to be reserved for that object;

[3] r a function, includes the function body;

[4] r an enumeration constant, is the (only) declaration of the identifier;

[5] r a typedef name, is the first (or only) declaration of the identifier."

[2] Mike Banahan. "2.5. Declaration of variables". GBdirect. Retrieved 2011-06-08. [A] declaration [...] introduces just the name and type of something but allocates no storage[...].

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