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)

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 allowed in the case of 1-tuple	disallowed

See also

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