std::reference_converts_from_temporary

 

C++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Diagnostics library

Memory management library

Metaprogramming library (C++11)

General utilities library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Strings library

Text processing library

Numerics library

Date and time library

Input/output library

Filesystem library (C++17)

Concurrency support library (C++11)

Execution control library (C++26)

Technical specifications

Symbols index

External libraries

 

Metaprogramming library

Type traits (C++11)

Compile-time rational arithmetic (C++11)

Compile-time integer sequences (C++14)

integer_sequence (C++14)

Compile-time reflection (C++26)

 

Type traits

Type traits (since C++11)

Type categories is_void is_null_pointer (DR*) is_reflection (C++26) is_array is_pointer is_enum is_union is_class is_function is_reference      is_object is_scalar is_compound is_integral is_floating_point      is_fundamental is_arithmetic is_lvalue_reference is_rvalue_reference is_member_pointer is_member_object_pointer is_member_function_pointer Type properties is_const is_volatile is_empty is_polymorphic is_final (C++14) is_abstract is_aggregate (C++17) is_structural (C++26) is_implicit_lifetime (C++23) is_trivial (deprecated in C++26) is_trivially_copyable is_standard_layout is_literal_type (until C++20*) is_pod (deprecated in C++20) is_signed is_unsigned is_bounded_array (C++20) is_unbounded_array (C++20) is_scoped_enum (C++23) has_unique_object_   representations (C++17)     Type trait constants integral_constantbool_constanttrue_typefalse_type (C++17) Metafunctions conjunction (C++17) disjunction (C++17) negation (C++17)

Supported operations is_constructibleis_trivially_constructibleis_nothrow_constructible is_default_constructibleis_trivially_default_constructibleis_nothrow_default_constructible is_copy_constructibleis_trivially_copy_constructibleis_nothrow_copy_constructible is_move_constructibleis_trivially_move_constructibleis_nothrow_move_constructible is_assignableis_trivially_assignableis_nothrow_assignable is_copy_assignableis_trivially_copy_assignableis_nothrow_copy_assignable is_move_assignableis_trivially_move_assignableis_nothrow_move_assignable is_destructibleis_trivially_destructibleis_nothrow_destructible has_virtual_destructor is_swappable_withis_swappableis_nothrow_swappable_withis_nothrow_swappable (C++17)(C++17)(C++17)(C++17) Relationships and property queries is_same is_convertibleis_nothrow_convertible (C++20) is_layout_compatible (C++20) is_pointer_interconvertible_base_of (C++20) is_pointer_interconvertible_with_class (C++20)     is_corresponding_member (C++20) reference_constructs_from_temporary (C++23) reference_converts_from_temporary (C++23) alignment_of is_base_of is_virtual_base_of (C++26) rank extent is_invocableis_invocable_ris_nothrow_invocableis_nothrow_invocable_r (C++17)(C++17)(C++17)(C++17) is_applicableis_nothrow_applicable (C++26)(C++26) Type modifications remove_cvremove_constremove_volatile add_cvadd_constadd_volatile make_signed make_unsigned remove_reference add_lvalue_referenceadd_rvalue_reference remove_pointer add_pointer remove_extent remove_all_extents Type transformations aligned_storage (deprecated in C++23) aligned_union (deprecated in C++23) decay remove_cvref (C++20) result_ofinvoke_result (until C++20*)(C++17) apply_result (C++26) conditional common_type common_reference (C++20) underlying_type type_identity (C++20) enable_if void_t (C++17)

 

Defined in header <type_traits>
template< class To, class From > struct reference_converts_from_temporary;		(since C++23)

Let V be std::remove_cv_t<From> if From is a scalar type or cv void, or From otherwise. If To is a reference type, and given a hypothetical expression e such that decltype(e) is V, the variable definition To 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 To is an lvalue reference type to a const- but not volatile-qualified object type or an rvalue reference type, both std::remove_reference_t<To> and std::remove_reference_t<From> must 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_converts_from_temporary or std::reference_converts_from_temporary_v, the behavior is undefined.

Helper variable template

template< class To, class From > constexpr bool reference_converts_from_temporary_v = std::reference_converts_from_temporary<To, From>::value;		(since C++23)

Inherited from std::integral_constant

Member constants

value [static]

true if To is a reference type, a From value can be bound to To in copy-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_converts_from_temporary can be used for rejecting some cases that always produce dangling references.

Example

Run this code

#include <type_traits>

static_assert
(""
    && std::reference_converts_from_temporary_v<int&&, int> == true
    && std::reference_converts_from_temporary_v<const int&, int> == true
    && std::reference_converts_from_temporary_v<int&&, int&&> == false
    && std::reference_converts_from_temporary_v<const int&, int&&> == false
    && std::reference_converts_from_temporary_v<int&&, long&&> == true
    && std::reference_converts_from_temporary_v<int&&, long> == true
);

int main() {}

See also

is_convertibleis_nothrow_convertible (C++11)(C++20)	checks if a type can be converted to the other type (class template)
invokeinvoke_r (C++17)(C++23)	invokes any Callable object with given arguments and possibility to specify return type(since C++23) (function template)
bind (C++11)	binds one or more arguments to a function object (function template)
visit (C++17)	calls the provided functor with the arguments held by one or more variants (function template)
(constructor)	constructs a new std::function instance (public member function of std::function<R(Args...)>)
(constructor)	constructs a new std::move_only_function object (public member function of std::move_only_function)
(constructor)	constructs the task object (public member function of std::packaged_task<R(Args...)>)
reference_constructs_from_temporary (C++23)	checks if a reference is bound to a temporary in direct-initialization (class template)
reference_converts_from_temporary (C++26)	checks if a reference is bound to a temporary in copy-initialization (function)