< cpp‎ | types
Metaprogramming library
Type traits
Type categories
Type properties
(C++11)(until C++20*)
(C++11)(deprecated in C++20)
Type trait constants
Supported operations
Relationships and property queries
Type modifications
Type transformations
(C++11)(deprecated in C++23)
(C++11)(deprecated in C++23)

(C++11)(until C++20*)(C++17)
Compile-time rational arithmetic
Compile-time integer sequences
Defined in header <type_traits>
template< class T, class U >
struct reference_constructs_from_temporary;
(since C++23)

Let V be std::remove_cv_t<U> if U is a scalar type or cv void, or U otherwise. If T is a reference type, and given a hypothetic expression e such that decltype(e) is V, the variable definition T ref(e); is well-formed and binds a temporary object to ref, then provides the member constant value equal to true. Otherwise, value is false.

If T is an lvalue reference type to a const- but not volatile-qualified object type or an rvalue reference type, both std::remove_reference_t<T> and std::remove_reference_t<U> shall be complete types, cv void, or an arrays of unknown bound; otherwise the behavior is undefined.

If an instantiation of a template above depends, directly or indirectly, on an incomplete type, and that instantiation could yield a different result if that type were hypothetically completed, the behavior is undefined.

If the program adds specializations for std::reference_constructs_from_temporary or std::reference_constructs_from_temporary_v, the behavior is undefined.


[edit] Helper variable template

template< class T, class U >

inline constexpr bool reference_constructs_from_temporary_v =

    std::reference_constructs_from_temporary<T, U>::value;
(since C++23)

Inherited from std::integral_constant

Member constants

true if T is a reference type, a U value can be bound to T in direct-initialization, and a temporary object would be bound to the reference, false otherwise
(public static member constant)

Member functions

operator bool
converts the object to bool, returns value
(public member function)
returns value
(public member function)

Member types

Type Definition
value_type bool
type std::integral_constant<bool, value>

[edit] Notes

std::reference_constructs_from_temporary can be used for rejecting some cases that always produce dangling references.

It is also possible to use member initializer list to reject binding a temporary object to a reference if the compiler has implemented CWG1696.

[edit] Example

#include <type_traits>
static_assert(std::reference_constructs_from_temporary_v<int&&, int> == true);
static_assert(std::reference_constructs_from_temporary_v<const int&, int> == true);
static_assert(std::reference_constructs_from_temporary_v<int&&, int&&> == false);
static_assert(std::reference_constructs_from_temporary_v<const int&, int&&> == false);
static_assert(std::reference_constructs_from_temporary_v<int&&, long&&> == true);
static_assert(std::reference_constructs_from_temporary_v<int&&, long> == true);
int main() {}

[edit] See also

checks if a type has a constructor for specific arguments
(class template) [edit]
constructs a new tuple
(public member function of std::tuple<Types...>) [edit]
constructs new pair
(public member function of std::pair<T1,T2>) [edit]
construct an object with a tuple of arguments
(function template) [edit]