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.
The behavior of a program that adds specializations for reference_constructs_from_temporary or reference_constructs_from_temporary_v is undefined.
template< class T, class U >
inline constexpr bool reference_constructs_from_temporary_v =
std::reference_constructs_from_temporary<T, U>::value;
| (since C++23) |
| value
[static] | 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) |
| operator bool | converts the object to bool, returns value (public member function) |
| operator()
(C++14) | returns value (public member function) |
| Type | Definition |
|---|---|
value_type | bool |
type | std::integral_constant<bool, value> |
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.
#include <iostream>
#include <type_traits>
int main()
{
std::cout << std::boolalpha
<< std::reference_constructs_from_temporary_v<int&&, int> << '\n'
<< std::reference_constructs_from_temporary_v<const int&, int> << '\n'
<< std::reference_constructs_from_temporary_v<int&&, int&&> << '\n'
<< std::reference_constructs_from_temporary_v<const int&, int&&> << '\n'
<< std::reference_constructs_from_temporary_v<int&&, long&&> << '\n';
<< std::reference_constructs_from_temporary_v<int&&, long> << '\n';
}Output:
true true false false true true
|
(C++11)(C++11)(C++11) | checks if a type has a constructor for specific arguments (class template) |
|
(C++11) | constructs a new tuple (public member function of std::tuple<Types...>) |
| constructs new pair (public member function of std::pair<T1,T2>) |
|
|
(C++17) | construct an object with a tuple of arguments (function template) |
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