std::forward_as_tuple

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

make_tuple	creates a tuple object of the type defined by the argument types (function template)
tie	creates a tuple of lvalue references or unpacks a tuple into individual objects (function template)
tuple_cat	creates a tuple by concatenating any number of tuples (function template)
apply (C++17)	calls a function with a tuple of arguments (function template)