Declaration (computer science)

In programming languages, a declaration specifies the identifier, type, and other aspects of language elements such as variables and functions. It is used to announce the existence of the element to the compiler; this is important in many languages (such as C) which require variables to be declared before use.

In the BCPL family languages such as C++ and Java, it can also specify the variable's dimensions to declare a scalar, array or matrix. In this family, declarations (announcing the existence and properties of the element) and "definitions" (providing the actual implementation) can be made independent of each other. Declarations are generally made in header files, which are meant to be included in other files that reference and use these declarations but don't have access to the definition. If a definition type doesn't match the previous declaration for the same element, the situation generates a compiler error.

For variables, definitions assign values to an area of memory that was reserved during the declaration phase. For functions, definitions supply the function body. While a variable or function may be declared many times, it is typically defined once. Dynamic languages such as Javascript or Python allow the redefinition of functions, though.

A declaration is often used in order to be able to access functions or variables defined in different source files, or in a library.

Here are some examples of declarations which are not definitions, in the language C:extern int example1;void example2(void);

Here are some examples of definitions, again in C:int example1 = 5;

void example2(void){ int x = 7;}

ee also

* Function prototype
* Scope