Namespaces
Variants

    std::reference_constructs_from_temporary

    From cppreference.com
     
    C++
     
    Metaprogramming library
    Type traits
    Type categories
    (C++11)
    (C++11)(DR*)
    (C++11)
    (C++11)
    (C++11)
    (C++11)
    (C++11)
    (C++11)
    (C++11) 
    (C++11)
    (C++11)
    (C++11)
    (C++11)
    (C++11)  
    (C++11)
    (C++11)
    (C++11)
    (C++11)
    (C++11)
    (C++11)
    (C++11)
    Type properties
    (C++11)
    (C++11)
    (C++11)
    (C++11)
    (C++14)
    (C++11)
    (C++17)
    (C++23)
    (C++11)(deprecated in C++26)
    (C++11)
    (C++11)
    (C++11)(until C++20*)
    (C++11)(deprecated in C++20)
    (C++11)
    (C++11)
    (C++20)
    (C++20)
    (C++23)
    (C++17)
    Type trait constants
    (C++11)(C++17)(C++11)(C++11)
    Metafunctions
    (C++17)
    (C++17)
    (C++17)
    Supported operations
    Relationships and property queries
    Type modifications
    (C++11)(C++11)(C++11)
    (C++11)(C++11)(C++11)
    (C++11)
    (C++11)
    Type transformations
    (C++11)(deprecated in C++23)
    (C++11)(deprecated in C++23)
    (C++11)
    (C++20)
    (C++11)(until C++20*)(C++17)

    (C++11)
    (C++11)
    (C++20)
    (C++11)
    (C++20)
    (C++11)
    (C++17)
    Compile-time rational arithmetic
    Compile-time integer sequences
    (C++14)
     
    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.

    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)
    [edit]

    Inherited from std::integral_constant

    Member constants

    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)

    Member functions

    operator bool
    		 converts the object to bool, returns value
    (public member function)
    operator()
    (C++14)
    		 returns value
    (public member function)

    Member types

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

    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.

    Example

    Run this code
    #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() {}

    See also

    (C++11)(C++11)(C++11)
    		 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]
    (C++17)
    		 construct an object with a tuple of arguments
    (function template) [edit]