C++ Predefined, User-Defined, and Void Functions

This video is a tutorial on writing predefined, user-defined, and void functions in C++. Predefined functions are those found within C++'s builtin libraries such as the cmath library. The cmath library offers functions such as a square root function, power function, absolute value function, etc.
User-defined functions are for those functions that a user wants to code from scratch. They are defined outside of main and return some specific value type such as a char or int.
A void function is also defined outside of main and uses the keyword void. It does not return any value type. They are typically used to write a message.