What will be the output of the following C++ code?

#include <iostream>
#include <vector>
using namespace std;
int main ()
{
    vector<bool> mask;
    mask.push_back(true);
    mask.flip();
    cout << boolalpha;
    for (unsigned i = 0; i < mask.size(); i++)
    cout << mask.at(i);
    return 0;
}