std::forward_as_tuple (3) - Linux Man Pages

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

See also


           creates a tuple object of the type defined by the argument types
make_tuple (function template)
           creates a tuple of lvalue references or unpacks a tuple into individual objects
tie (function template)
           creates a tuple by concatenating any number of tuples
tuple_cat (function template)


apply calls a function with a tuple of arguments
           (function template)
(C++17)