Functions and Linkage

Like variables, functions have linkage properties, although the selection is more limited than for variables. C++, like C, does not allow you to define one function inside another, so all functions automatically have static storage duration, meaning they are all present as long as the program is running. By default, functions have external linkage, meaning they can be shared across files. You can, in fact, use the keyword extern in a function prototype to indicate that the function is defined in another file, but that is optional. (For the program to find the function in another file, that file must be one of the files being compiled as part of the program or a library file searched by the linker.) You can also use the keyword static to give a function internal linkage, confining its use to a single file. You would apply this keyword to the prototype and to the function definition:

static int private(double x);
...
static int private(double x)
{
    ...
}

This means the function is known only in that file. It also means you can use the same name for another function in a different file. As with variables, a static function overrides an external definition for the file containing the static declaration, so a file containing a static function definition will use that version of the function even if there is an external definition of a function that has the same name.

The one definition rule extends to non-inline functions, too. Therefore, every program shall contain exactly one definition of every non-inline function. For functions with external linkage, this means that only one file of a multifile program can contain the function definition. (This file could be a library file rather than one you supply.) However, each file that uses the function should have the function prototype.

Inline functions are excepted from this rule to allow you to place inline function definitions in a header file. Thus, each file that includes the header file ends up having the inline function definition. However, C++ does require that all the inline definitions for a particular function be identical.