NAME

std::make_from_tuple - std::make_from_tuple

Synopsis

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

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

The program is ill-formed if the hypothetical variable definition T
var(std::get<Is>(std::forward<Tuple>(t))...); is ill-formed
, or T is a reference type and var is bound to a temporary object
(since C++23), where Is... denotes the parameter pack 0, 1, ...,
std::tuple_size_v<std::remove_reference_t<Tuple>> - 1.

Parameters

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

Return value

The constructed T object or reference.

Notes

The 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.

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

Feature-test macro: __cpp_lib_make_from_tuple

Possible implementation

namespace detail {
template<class T, class Tuple, std::size_t... I>
constexpr T make_from_tuple_impl(Tuple&& t, std::index_sequence<I...>)
{
static_assert(std::is_constructible_v<T,
decltype(std::get<I>(std::declval<Tuple>()))...>);
#if __cpp_lib_reference_from_temporary >= 202202L
if constexpr (std::tuple_size_v<std::remove_reference_t<Tuple>> == 1) {
using tuple_first_t = decltype(std::get<0>(std::declval<Tuple>()));
static_assert(!std::reference_constructs_from_temporary_v<T, tuple_first_t>);
}
#endif
return T(std::get<I>(std::forward<Tuple>(t))...);
}
} // namespace detail

template<class T, class Tuple>
constexpr T make_from_tuple(Tuple&& t)
{
return detail::make_from_tuple_impl<T>(std::forward<Tuple>(t),
std::make_index_sequence<std::tuple_size_v<std::remove_reference_t<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 disallowed
allowed in the case of 1-tuple