C++ Primer Plus

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.

Where C++ Finds Functions

Suppose you call a function in a particular file in a program. Where does C++ look for the function definition? If the function prototype in that file indicates that the function is static, the compiler looks only in that file for the function definition. Otherwise, the compiler (and the linker, too) looks in all the program files. If it finds two definitions, the compiler sends you an error message because you can have only one definition for an external function. If it fails to find any definition in the files, the function then searches the libraries. This implies that if you define a function that has the same name as a library function, the compiler uses your version rather than the library version. (However, C++ reserves the names of the standard library functions, so you shouldn’t reuse them.) Some compiler-linkers need explicit instructions to identify which libraries to search.

Language Linking

Another form of linking, called language linking, affects functions. First, a little background. A linker needs a different symbolic name for each distinct function. In C, this is simple to implement because there can be only one C function with a given name. So for internal purposes, a C compiler might translate a C function name such as spiff to _spiff. The C approach is termed C language linkage. However, C++ can have several functions with the same C++ name that have to be translated to separate symbolic names. Thus, the C++ compiler indulges in the process of name mangling or name decoration (as discussed in Chapter 8) to generate different symbolic names for overloaded functions. For example, it could convert spiff(int) to, say, _spiff_i, and spiff(double, double) to _spiff_d_d. The C++ approach is C++ language linkage.

When the linker looks for a function to match a C++ function call, it uses a different look-up method than it does to match a C function call. But suppose you want to use a precompiled function from a C library in a C++ program? For example, suppose you have this code:

spiff(22); // want spiff(int) from a C library

Its hypothetical symbolic name in the C library file is _spiff, but for our hypothetical linker, the C++ look-up convention is to look for the symbolic name _spiff_i. To get around this problem, you can use the function prototype to indicate which protocol to use:

extern "C" void spiff(int);   // use C protocol for name look-up

extern void spoff(int);       // use C++ protocol for name look-up

extern "C++" void spaff(int); // use C++ protocol for name look-up

The first example here uses C language linkage. The second and third examples use C++ language linkage; the second does so by default, and the third does so explicitly.