std::forward

// forward example
#include <utility>      // std::forward
#include <iostream>     // std::cout
// function with lvalue and rvalue reference overloads:
void overloaded (const int& x) {std::cout << "[lvalue]";}
void overloaded (int&& x) {std::cout << "[rvalue]";}
// function template taking rvalue reference to deduced type:
template <class T> void fn (T&& x) {
  overloaded (x);                   // always an lvalue
  overloaded (std::forward<T>(x));  // rvalue if argument is rvalue
}
int main () {
  int a;
  std::cout << "calling fn with lvalue: ";
  fn (a);
  std::cout << '\n';
  std::cout << "calling fn with rvalue: ";
  fn (0);
  std::cout << '\n';
  return 0;
}