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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.
A declaration which creates a data object, instead of merely describing its existence, is commonly called a definition. Many find it convenient to draw a distinction between the terms "declaration" and "definition", as in the commonly seen phrase "the distinction between a declaration and definition ...", implying that a declaration merely ...
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 .
In C and C++, the line above represents a forward declaration of a function and is the function's prototype. After processing this declaration, the compiler would allow the program code to refer to the entity printThisInteger in the rest of the program. The definition for a function must be provided somewhere (same file or other, where it would ...
In the C programming language, struct is the keyword used to define a composite, a.k.a. record, data type – a named set of values that occupy a block of memory. It allows for the different values to be accessed via a single identifier, often a pointer. A struct can contain other data types so is used for mixed-data-type records.
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A function prototype is always a declaration and never a definition. Also, in standard C, a function is always external, but some compiler extensions allow a function to be defined inside a function. An external variable must be defined, exactly once, outside of any function; this sets aside storage for it.