6.1.2. Function Declarations

Like any other name, the name of a function must be declared before we can use it. As with variables (§ 2.2.2, p. 45), a function may be defined only once but may be declared multiple times. With one exception that we’ll cover in § 15.3 (p. 603), we can declare a function that is not defined so long as we never use that function.

A function declaration is just like a function definition except that a declaration has no function body. In a declaration, a semicolon replaces the function body.

Because a function declaration has no body, there is no need for parameter names. Hence, parameter names are often omitted in a declaration. Although parameter names are not required, they can be used to help users of the function understand what the function does:

// parameter names chosen to indicate that the iterators denote a range of values to print
void print(vector<int>::const_iterator beg,
           vector<int>::const_iterator end);

These three elements—the return type, function name, and parameter types—describe the function’s interface. They specify all the information we need to call the function. Function declarations are also known as the function prototype.

Function Declarations Go in Header Files

Recall that variables are declared in header files (§ 2.6.3, p. 76) and defined in source files. For the same reasons, functions should be declared in header files and defined in source files.

It may be tempting—and would be legal—to put a function declaration directly in each source file that uses the function. However, doing so is tedious and error-prone. When we use header files for our function declarations, we can ensure that all the declarations for a given function agree. Moreover, if the interface to the function changes, only one declaration has to be changed.

The source file that defines a function should include the header that contains that function’s declaration. That way the compiler will verify that the definition and declaration are consistent.