读程序题目，并给出解释

#include <iostream>
using namespace std;
int &f1() {
    int i = 1;
    return i;
}
int g1() {
    int j = 3;
    return j;
}
int f(int i) {
    return ++i;
}
int g(int &i) {
    return ++i;
}

int main () {
    int &ri = f1();
    int rj = g1();
    cout << ri << endl;
    cout << rj << endl;

    int a = 0;
    a += f(g(a));
    cout << a << endl;
    return 0;
}