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std::make_from_tuple

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...>) // exposition only { 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	Comment
`__cpp_lib_make_from_tuple`	201606L	(C++17)	`std::make_from_tuple`

Example

#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