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std::make_from_tuple(3) | C++ Standard Libary | std::make_from_tuple(3) |
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
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)
2022.07.31 | http://cppreference.com |