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 typeT, using the elements of the tuplet 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>>>{}
);.

std::tuple_size_v<std::remove_reference_t<Tuple>> is1 andstd::reference_constructs_from_temporary_v<
T, decltype(std::get<0>(std::declval<Tuple>()))> istrue, or

(since C++23)

std::is_constructible_v<T, decltype(std::get<I>(std::declval<Tuple>()))...> isfalse,

the program is ill-formed.

[edit]Parameters

t - tuple whose elements to be used as arguments to the constructor ofT

[edit]Return value

The constructedT object or reference.

[edit]Notes

`Tuple` need not bestd::tuple, and instead may be anything that supports`std::get` and`std::tuple_size`; in particular,std::array andstd::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 ofstd::tuple or another type (such asstd::array andstd::pair) that models`tuple-like`.	(since C++23)

Due toguaranteed 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`

[edit]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

[edit]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 containingreinterpret_cast etc. was allowed in the case of 1-tuple	prohibited

[edit]See also

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