#include <iostream>
using namespace std;

struct X {
    X() : d{100} {}

    double d;
};

void f1(X& t)       { t.d *= 2;  cout << t.d << endl; }

void f2(const X& t) { cout << "can't change t" << endl; }

void f3(X&& t)      { t.d *= 3;  cout << t.d << endl; }

int main() {
    X x;           // x is an lvalue
                  
    f1(x);         // passing an lvalue to X&       --> ok
    f2(x);         // passing an lvalue to const X& --> ok
    // f3(x);      // passing an lvalue to X&&      --> not ok

    // f1( X{} );  // passing an rvalue to X&       --> not ok
    f2( X{} );     // passing an rvalue to const X& --> ok
    f3( X{} );     // passing an rvalue to X&&      --> ok
}