std::make_from_tuple - cppreference.com (original) (raw)

| Defined in header | | | | ----------------------------------------------------------------------------------- | | --------------------------- | | 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>>>{}
);.

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.

[edit] Parameters

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

[edit] Return value

The constructed T object or reference.

[edit] Notes

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

[edit] Example

#include #include 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(std::move(tuple)); }

Output:

[edit] Defect reports

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