Printing a vector of reference_wrapper<int>

Question

I have the following test code that's runnable under clang.

#include <algorithm>
#include <vector>
#include <iostream>

int main() {

    std::vector<int> vs{1, 2, 4, 5};

    std::vector<std::reference_wrapper<int>> vs1;

    for (int i : vs) {
        std::cout << "loop: " << i << std::endl;
        vs1.emplace_back(i);
    }

    for (auto p : vs1) {
        std::cout << p << std::endl;
    }
    return 0;
}

You can plug that into https://rextester.com/l/cpp_online_compiler_clang (or locally). The result is:

loop: 1
loop: 2
loop: 4
loop: 5
5
5
5
5

1. I'd expect 1,2,4,5, not 5 all the way.
2. The code won't work on non-clang. Where's the problem?

Answer 1

i is a local variable inside its declaring for loop. It is a copy of each int in the vs vector. You are thus (via the emplace_back() call) creating reference_wrapper objects that refer to a local variable, keeping the references alive after the lifetime of the referred-to variable (i) has ended. This is undefined behavior.

The fix is to make i be a reference to each int, not a copy, that way the reference_wrappers refer to the ints in vs as expected:

for (int& i : vs)

Answer 2

First, you forgot <functional> header. Second, reference_wrapper<int> stores reference to an int. Not its value. So in this loop:

for (int i : vs) {
    std::cout << "loop: " << i << std::endl;
    vs1.emplace_back(i);
}

You are changing value of i but not its place in memory. It is always the same variable. That's why it prints the last value stored in that variable, which is 5.