Functions

Functions in C++ are really procedures. They are not related to mathematical notion of a function in that they must have an input and an output, and be pure (for a given input, the function must always return the same output).

Simple functions

// Function prototype to let whoever uses the function know about it.
// A function prototype is only needed if code above your function uses it.
int isEven(int)

// some code in between...

int isEven(int aNumber) {
    return aNumber % 2 == 0;
}