std::forward_as_tuple

 

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std::tuple

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tuple::tuple

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make_tuple

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forward_as_tuple

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operator==operator!=operator<operator<=operator>operator>=operator<=> (until C++20)(until C++20)(until C++20)(until C++20)(until C++20)(C++20)

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Deduction guides(C++17)

 

Defined in header <tuple>
template< class... Types > std::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 <string>
#include <tuple>
 
int main()
{
    std::map<int, std::string> m;
 
    m.emplace(std::piecewise_construct,
              std::forward_as_tuple(6),
              std::forward_as_tuple(9, 'g');
    std::cout << "m[6] = " << m[6] << '\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[6] = ggggggggg

See also

make_tuple (C++11)	creates a tuple object of the type defined by the argument types (function template)
tie (C++11)	creates a tuple of lvalue references or unpacks a tuple into individual objects (function template)
tuple_cat (C++11)	creates a tuple by concatenating any number of tuples (function template)
apply (C++17)	calls a function with a tuple of arguments (function template)

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