std::make_from_tuple

 

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Defined in header <tuple>
template<class T, class Tuple > constexpr T make_from_tuple( Tuple&& t );		(since C++17) (until C++23)
template<class T, tuple-like Tuple > constexpr T make_from_tuple( Tuple&& t );		(since C++23)

Construct an object of type T, using the elements of the tuple t as the arguments to the constructor.

Given the exposition-only function /*make-from-tuple-impl*/ defined as follows:
template<class T, tuple-like Tuple, std::size_t... I>// no constraint on Tuple before C++23
constexpr T /*make-from-tuple-impl*/(Tuple&& t, std::index_sequence<I...>)
{
    return T(std::get<I>(std::forward<Tuple>(t))...);
}

The effect is equivalent to:
return/*make-from-tuple-impl*/<T>(
    std::forward<Tuple>(t),
    std::make_index_sequence<std::tuple_size_v<std::remove_reference_t<Tuple>>>{}
);.

If

std::tuple_size_v<std::remove_reference_t<Tuple>> is 1 and std::reference_constructs_from_temporary_v<     T, decltype(std::get<0>(std::declval<Tuple>()))> is true, or

(since C++23)

• std::is_constructible_v<T, decltype(std::get<I>(std::declval<Tuple>()))...> is false,

the program is ill-formed.

Parameters

t

-

tuple whose elements to be used as arguments to the constructor of T

Return value

The constructed T object or reference.

Notes

Tuple need not be std::tuple, and instead may be anything that supports std::get and std::tuple_size; in particular, std::array and std::pair may be used.	(until C++23)
Tuple is constrained to be tuple-like, i.e. each type therein is required to be a specialization of std::tuple or another type (such as std::array and std::pair) that models tuple-like.	(since C++23)

Due to guaranteed copy elision, T need not be movable.

Feature-test macro	Value	Std	Feature
__cpp_lib_make_from_tuple	201606L	(C++17)	std::make_from_tuple

Example

Run this code

#include <iostream>#include <tuple>   struct Foo { Foo(int first, float second, int third){std::cout<< first <<", "<< second <<", "<< third <<'\n';}};   int main(){auto tuple =std::make_tuple(42, 3.14f, 0); std::make_from_tuple<Foo>(std::move(tuple));}

Output:

42, 3.14, 0

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 3528	C++17	cast containing reinterpret_cast etc. was allowed in the case of 1-tuple	prohibited

See also

make_tuple (C++11)	creates a tuple object of the type defined by the argument types (function template)
forward_as_tuple (C++11)	creates a tuple of forwarding references (function template)
apply (C++17)	calls a function with a tuple of arguments (function template)