A function must be defined exactly once, but through overloading, you can have many functions with the same name that differ by their parameters. Code that uses a function has to have access to the name of the function, and so it needs to have access to either the function definition (for example, the function is defined earlier in the source file) or the declaration of the function (also called the function prototype). The compiler uses the prototype to type-check that the calling code is calling the function, using the right types.
Typically, libraries are implemented as separate compiled library files and prototypes of the library functions are provided in header files so that many source files can use ...
