Define Function? There are two types of function built-in functions and user-defined functions.   Built-in functions are the function within C++ like main, setw, sqrt,  etc.  User-defined functions are the functions produced by the user or the programmer for a specific job.  The advantage of producing function is it reduces program size, easy to maintain and modify.   Only the statements called within the main function will be executed.

The use-defined  function must have  a prototype  (declaration) before it can be called

except for inline function. There are other functions like friendly function and virtual function which will be discussed later.