std::reference_constructs_from_temporary - cppreference.com (original) (raw)

| | | | | ---------------------------------------------------------------------------- | | ------------- | | 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.

Contents

• 1 Helper variable template
• 2 Inherited from std::integral_constant
  • 2.1 Member constants
  • 2.2 Member functions
  • 2.3 Member types
  • 2.4 Notes
  • 2.5 Example
  • 2.6 See also

[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

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

Member types

[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   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