Question

Can someone explain to me why output for R-value is different from L-value?

#include <iostream>
#include <vector>
using namespace std;

template<typename Ct>
struct  ct_wrapper {
    Ct&& ct; // R or L ref
    explicit ct_wrapper(Ct&& ct) 
      : ct(std::forward<Ct>(ct)) { std::cout <<  this->ct[1];};
};

int main() {

    // L-val
    vector<int> v{1,2,3};
    ct_wrapper<vector<int>&> lv(v);
    cout << endl << lv.ct[0] << lv.ct[1] << lv.ct[2] << endl;

    // R-val
    ct_wrapper<vector<int>&&> rv(vector<int>{1,2,3});
    cout << endl << rv.ct[0] << rv.ct[1] << rv.ct[2] << endl;
}

Output (same for gcc48 and clang32):

2
123
2
003

Answer

It was buried somewhat in my chat with Johannes Schaub, so I’ve put it here.

When temporary vector initializes r-value-ref member-variable rv.ct, temporary lifetime is not extended because there’s a special exception: [class.temporary]p5: “A temporary bound to a reference member in a constructor’s ctor-initializer (12.6.2) persists until the constructor exits.”

Answer 1

Because your member is just a reference. In the second case the object it references is already dead after the definition of the local rv variable has finished. So the later access in the cout is undefined behavior.