Software Engineering Observation 6.3

Function prototypes are required unless the function is defined before it is used. Use #include preprocessing directives to obtain function prototypes for the C++ Standard Library functions from the headers of the appropriate libraries (e.g., the prototype for sqrt is in header <cmath>; a partial list of C++ Standard Library headers appears in Section 6.5). Also use #include to obtain headers containing function prototypes written by you or other programmers.

Common Programming Error 6.3

If a function is defined before it’s invoked, then its definition also serves as the function’s prototype, so a separate prototype is unnecessary. If a function is invoked before it’s defined, and that function does not have a function prototype, a compilation error occurs.

Software Engineering Observation 6.4

Always provide function prototypes, even though it’s possible to omit them when functions are defined before they’re used. Providing the prototypes avoids tying the code to the order in which functions are defined (which can easily change as a program evolves).