C++ Primer Plus

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 ...