C++11: std::forward_as_tuple()

C++11 added the template function std::forward_as_tuple(), which creates a tuple that can be forwarded. If the arguments are rvalues, then the tuple members are rvalue references, otherwise the tuple members are lvalue members. Here is an example:

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

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

  myMap.emplace(
    std::piecewise_construct       ,  // pair CTOR selector
    std::forward_as_tuple("four_c"),  // pair first  CTOR args
                                      //  {string(string&& other)}
    std::forward_as_tuple(4, 'c'  )); // pair second CTOR args
                                      //  {string(size_type count,
                                      //   CharT ch)}
  std::cout << myMap["four_c"] << " ";

  // The '5' and 'c' are dangling references after the first semicolon.
  auto myTupleWithRvalueReferences = std::forward_as_tuple(5, 'c');

#if 0 // The following line does not compile, 
      //   because of the '5' and 'c' dangling references above.
  myMap.emplace(
    std::piecewise_construct       ,
    std::forward_as_tuple("five_c"),
    myTupleWithRvalueReferences    );
#endif

  std::cout << myMap["five_c"] << " ";

  int  count                       = 6  ;
  char ch                          = 'c';
  auto myTupleWithLvalueReferences = std::forward_as_tuple(count, ch);

  myMap.emplace(
    std::piecewise_construct      ,
    std::forward_as_tuple("six_c"),
    myTupleWithLvalueReferences   );

  std::cout << myMap["six_c"] << " ";

  std::cout << std::endl;
  return 0;
}
// Output: cccc  cccccc

Reference: http://en.cppreference.com/w/cpp/utility/tuple/forward_as_tuple