std::forward_as_tuple

 

C++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Diagnostics library

Memory management library

Metaprogramming library (C++11)

General utilities library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Strings library

Text processing library

Numerics library

Date and time library

Input/output library

Filesystem library (C++17)

Concurrency support library (C++11)

Execution control library (C++26)

Technical specifications

Symbols index

External libraries

 

Utilities library

Language support Type support (basic types, RTTI) Library feature-test macros (C++20) Program utilities Variadic functions initializer_list (C++11) is_constant_evaluated (C++20) is_within_lifetime (C++26) source_location (C++20) Coroutine support (C++20) Contract support (C++26) Three-way comparison three_way_comparablethree_way_comparable_with (C++20)(C++20) strong_ordering (C++20) weak_ordering (C++20) partial_ordering (C++20) common_comparison_category (C++20) compare_three_way_result (C++20) compare_three_way (C++20) strong_order (C++20) weak_order (C++20) partial_order (C++20) compare_strong_order_fallback (C++20) compare_weak_order_fallback (C++20) compare_partial_order_fallback (C++20)   is_eqis_ltis_lteq (C++20)(C++20)(C++20)   is_neqis_gtis_gteq (C++20)(C++20)(C++20)

General utilities Function objects Bit manipulation (C++20) bitset hash (C++11) Relational operators (deprecated in C++20) rel_ops::operator!=rel_ops::operator>    rel_ops::operator<=rel_ops::operator>= Integer comparison functions cmp_equalcmp_lesscmp_less_than (C++20)(C++20)(C++20)   cmp_not_equalcmp_greatercmp_greater_than (C++20)(C++20)(C++20) in_range (C++20) Swap and type operations swap ranges::swap (C++20) exchange (C++14) declval (C++11) to_underlying (C++23) forward (C++11) forward_like (C++23) move (C++11) move_if_noexcept (C++11) as_const (C++17) Common vocabulary types pair tuple (C++11) optional (C++17) any (C++17) variant (C++17) tuple_size (C++11) tuple_element (C++11) apply (C++17) make_from_tuple (C++17) expected (C++23)

 

std::tuple

Member functions

tuple::tuple

tuple::operator=

tuple::swap

Non-member functions

make_tuple

tie

forward_as_tuple

tuple_cat

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

swap(std::tuple)

get(std::tuple)

Helper concepts

tuple-like (C++23)

Helper classes

tuple_size<std::tuple>

tuple_element<std::tuple>

uses_allocator<std::tuple>

basic_common_reference<std::tuple> (C++23)

common_type<std::tuple> (C++23)

formatter<std::tuple> (C++23)

ignore

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)

See More

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 std::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)