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 ); (until C++23)
template< class T, tuple-like Tuple > (since C++23)
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.

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 (since
std::reference_constructs_from_temporary_v< C++23)
T, decltype(std::get<0>(std::declval<Tuple>()))> is true, or

* 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 (until C++23)
std::pair may be used.
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 (since C++23)
std::array and std::pair) that models tuple-like.

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 prohibited
allowed in the case of 1-tuple

See also¶

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