std:: make_from_tuple

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:: is_constructible_v < T, decltype ( std :: get < I > ( std:: declval < Tuple > ( ) ) ) ... > is false ,

the program is ill-formed.

Parameters

Return value

The constructed T object or reference.

Notes

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

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));
}

42, 3.14, 0

Defect reports

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

See also