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.