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The declaration informs the compiler that a variable by that name and type exists, but the compiler does not need to allocate memory for it since it is allocated elsewhere. The extern keyword means "declare without defining". In other words, it is a way to explicitly declare a variable, or to force a declaration without a definition.
Likewise, for C code to call a C++ function bar(), the C++ code for bar() must be declared with extern "C". A common practice for header files to maintain both C and C++ compatibility is to make its declaration be extern "C" for the scope of the header: [21]
The CPP is run during the compilation of each translation unit before the C compiler. The build process (e.g. make) can be implemented in a conditional way such that just different versions of a symbol are created or different (specialized) libraries are used and linked depending on the target.
In programming languages, particularly the compiled ones like C, C++, and D, linkage describes how names can or can not refer to the same entity throughout the whole program or one single translation unit. The static keyword is used in C to restrict the visibility of a function or variable to its translation unit. This is also valid in C++.
Encapsulation is also possible in non-object-oriented languages. In C, for example, a structure can be declared in the public API via the header file for a set of functions that operate on an item of data containing data members that are not accessible to clients of the API with the extern keyword. [13]
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]
static is a reserved word in many programming languages to modify a declaration. The effect of the keyword varies depending on the details of the specific programming language, most commonly used to modify the lifetime (as a static variable) and visibility (depending on linkage), or to specify a class member instead of an instance member in classes.
struct birthday {char name [20]; int day; int month; int year;}; struct birthday John; The memory layout of a structure is a language implementation issue for each platform, with a few restrictions. The memory address of the first member must be the same as the address of structure itself.