reference class member in C++

struct A {
    int &m;
    A(int &x) : m(x) {}
};

int main() {
    int x = 5;
    A a(x);
    cout << a.m << endl; // 5
    x = 6;
    cout << a.m << endl; // 6
    a.m = 7;
    cout << x << endl; // 7
    return 0;
}