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Concept
Declaration (computer programming)
Summary
In computer programming, a declaration is a language construct specifying identifier properties: it declares a word's (identifier's) meaning. 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. 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. The term "declaration" is frequently contrasted with the term "definition", 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.
One basic dichotomy is whether or not a declaration contains a definition: for example, whether a variable or constant declaration specifies its value, or only its type; and similarly whether a declaration of a function specifies the body (implementation) of the function, or only its type signature. Not all languages make this distinction: in many languages, declarations always include a definition, and may be referred to as either "declarations" or "definitions", depending on the language. However, these concepts are distinguished in languages that require declaration before use (for which forward declarations are used), and in languages where interface and implementation are separated: the interface contains declarations, the implementation contains definitions.
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