At the most basic level, a function has parameters, has code to manipulate the parameters, and returns a value. C++ gives you several ways to determine these three aspects. In the following section, we will cover those parts of a C++ function from the left to the right of the declaration. Functions can also be templated, but this will be left to a later section.
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...