std::forward_as_tuple (3) - Linux Manuals

std::forward_as_tuple: std::forward_as_tuple

NAME

std::forward_as_tuple - std::forward_as_tuple

Synopsis

Defined in header <tuple>
template< class... Types > (since C++11)
tuple<Types&&...> forward_as_tuple( Types&&... args ) noexcept; (constexpr since C++14)

Constructs a tuple of references to the arguments in args suitable for forwarding as an argument to a function. The tuple has rvalue reference data members when rvalues are used as arguments, and otherwise has lvalue reference data members.

Parameters

args - zero or more arguments to construct the tuple from

Return value

A std::tuple object created as if by std::tuple<Types&&...>(std::forward<Types>(args)...)

Notes

If the arguments are temporaries, forward_as_tuple does not extend their lifetime; they have to be used before the end of the full expression.

Example

// Run this code

  #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';

      // The following is an error: it produces a
      // std::tuple<int&&, char&&> holding two dangling references.
      //
      // auto t = std::forward_as_tuple(20, 'a');
      // m.emplace(std::piecewise_construct, std::forward_as_tuple(10), t);
  }

Output:

  m[10] = aaaaaaaaaaaaaaaaaaaa