Functions

A function declares its return type, name, and parameter types up front: int add(int a, int b) { ... }.

Example: a function used before main()

#include <iostream>
using namespace std;

int square(int n) {
    return n * n;
}

int main() {
    cout << "Square of 6 is " << square(6) << endl;
    return 0;
}

Square of 6 is 36

Unlike some languages, C++ generally requires a function to be declared (or at least its signature known) before the point where it's called, here, square is defined entirely above main, so by the time main calls it, the compiler already knows exactly what square looks like. Use void as the return type for a function that doesn't return a value at all.