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In computer programming, a declaration specifies properties of an identifier: it declares what a word (identifier) ''means.''〔"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 (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.〕 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 have separate declarations and definitions. ==Declaration vs. definition== A basic dichotomy is whether a declaration contains a ''definition'' or not: for example, whether a declaration of a constant or variable specifies the value of the constant (respectively, initial value of a variable), 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. In informal usage, a "declaration" refers only to a pure declaration (types only, no value or body), while a "definition" refers to a declaration that includes a value or body. However, in formal usage (in language specifications), "declaration" includes ''both'' of these senses, with finer distinctions by language: in C and C++, a declaration of a function that does not include a body is called a function prototype, while a declaration of a function that does include a body is called a "function definition". By contrast in Java declarations always include the body, and the word "definition" has no technical meaning in Java. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Declaration (computer programming)」の詳細全文を読む スポンサード リンク
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