std::forward_as_tuple

#include <iostream>
#include <map>
#include <tuple>
#include <string>

int main()
{
    std::map<int, std::string> m;

    m.emplace(std::piecewise_construct,
              std::forward_as_tuple(10),
              std::forward_as_tuple(20, 'a'));
    std::cout << "m[10] = " << m[10] << '\n';

    //   : 
    // std::tuple<int&&, char&&>    .
    //
    // auto t = std::forward_as_tuple(20, 'a');
    // m.emplace(std::piecewise_construct, std::forward_as_tuple(10), t);
}