#include <simplecpp>

int &f(int &x, int &y) {
    if (x > y) return x;
    else return y;
}

main_program{
    int p=5, q=4;
    f(p,q) = 2;
    cout << p << ' ' << q << endl;
    cout << f(p, q) << endl;
}

(It’s a little simple version of C++ used to teach students). What I am not able to understand is the & before the function declaration. What exactly is it doing?