std::reference_constructs_from_temporary

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Standard library
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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
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Date and time library
Input/output library
Filesystem library(C++17)
Concurrency support library(C++11)
Execution control library(C++26)
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External libraries

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Metaprogramming library

Type traits

Type categories

is_void

(C++11)

is_null_pointer

(C++11)(DR*)

is_array (C++11)
is_pointer (C++11)
is_enum (C++11)
is_union (C++11)
is_class (C++11)
is_function (C++11)
is_reference (C++11)

is_object (C++11)
is_scalar (C++11)
is_compound (C++11)
is_integral (C++11)
is_floating_point (C++11)
is_fundamental (C++11)
is_arithmetic (C++11)

is_lvalue_reference

(C++11)

is_rvalue_reference

(C++11)

is_member_pointer

(C++11)

is_member_object_pointer

(C++11)

is_member_function_pointer

(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)

is_trivially_copyable

(C++11)

is_standard_layout

(C++11)

is_literal_type

(C++11)(until C++20*)

is_pod

(C++11)(deprecated in C++20)

(C++11)

(C++11)

(C++20)

(C++20)

(C++23)

has_unique_object_representations

(C++17)

Type trait constants

integral_constantbool_constanttrue_typefalse_type

(C++11)(C++17)(C++11)(C++11)

Metafunctions

conjunction

(C++17)

disjunction

(C++17)

negation

(C++17)

Supported operations

is_constructibleis_trivially_constructibleis_nothrow_constructible (C++11)(C++11)(C++11)
is_default_constructibleis_trivially_default_constructibleis_nothrow_default_constructible (C++11)(C++11)(C++11)
is_copy_constructibleis_trivially_copy_constructibleis_nothrow_copy_constructible (C++11)(C++11)(C++11)
is_move_constructibleis_trivially_move_constructibleis_nothrow_move_constructible (C++11)(C++11)(C++11)
is_assignableis_trivially_assignableis_nothrow_assignable (C++11)(C++11)(C++11)

is_copy_assignableis_trivially_copy_assignableis_nothrow_copy_assignable (C++11)(C++11)(C++11)
is_move_assignableis_trivially_move_assignableis_nothrow_move_assignable (C++11)(C++11)(C++11)
is_destructibleis_trivially_destructibleis_nothrow_destructible (C++11)(C++11)(C++11)
has_virtual_destructor (C++11)
is_swappable_withis_swappableis_nothrow_swappable_withis_nothrow_swappable (C++17)(C++17)(C++17)(C++17)

Relationships and property queries

is_same (C++11)
is_convertibleis_nothrow_convertible (C++11)(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)

is_base_of (C++11)
is_virtual_base_of (C++26)
alignment_of (C++11)
rank (C++11)
extent (C++11)
is_invocableis_invocable_ris_nothrow_invocableis_nothrow_invocable_r (C++17)(C++17)(C++17)(C++17)

Type modifications

remove_cvremove_constremove_volatile (C++11)(C++11)(C++11)
add_cvadd_constadd_volatile (C++11)(C++11)(C++11)
make_signed (C++11)
make_unsigned (C++11)

remove_reference (C++11)
add_lvalue_referenceadd_rvalue_reference (C++11)(C++11)
remove_pointer (C++11)
add_pointer (C++11)
remove_extent (C++11)
remove_all_extents (C++11)

Type transformations

aligned_storage (C++11)(deprecated in C++23)
aligned_union (C++11)(deprecated in C++23)
decay (C++11)
remove_cvref (C++20)
result_ofinvoke_result (C++11)(until C++20*)(C++17)

conditional (C++11)
common_type (C++11)
common_reference (C++20)
underlying_type (C++11)
type_identity (C++20)
enable_if (C++11)
void_t (C++17)

Compile-time rational arithmetic

Compile-time integer sequences

integer_sequence

(C++14)

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Defined in header`<type_traits>`
template<class T,class U> struct reference_constructs_from_temporary;		(since C++23)

LetV bestd::remove_cv_t<U> ifU is a scalar type orcvvoid, orU otherwise. IfT is a reference type, and given a hypothetic expressione such thatdecltype(e) isV, the variable definitionT ref(e); is well-formed andbinds a temporary object toref, then provides the member constantvalue equal totrue. Otherwise,value isfalse.

IfT is an lvalue reference type to a const- but not volatile-qualified object type or an rvalue reference type, bothstd::remove_reference_t<T> andstd::remove_reference_t<U> shall becomplete types,cvvoid, or anarrays 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 forstd::reference_constructs_from_temporary orstd::reference_constructs_from_temporary_v, the behavior is undefined.

[edit]Helper variable template

template<class T,class U> inlineconstexprbool reference_constructs_from_temporary_v= std::reference_constructs_from_temporary<T, U>::value;		(since C++23)

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Inherited fromstd::integral_constant

Member constants

value

[static]

true ifT is a reference type, aU value can be bound toT 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 tobool, returnsvalue (public member function)
operator() (C++14)	returnsvalue (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 implementedCWG1696.

[edit]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<constint&,int>==true);static_assert(std::reference_constructs_from_temporary_v<int&&,int&&>==false);static_assert(std::reference_constructs_from_temporary_v<constint&,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

is_constructibleis_trivially_constructibleis_nothrow_constructible (C++11)(C++11)(C++11)	checks if a type has a constructor for specific arguments (class template)[edit]
(constructor)	constructs a new`tuple` (public member function of`std::tuple<Types...>`)[edit]
(constructor)	constructs new`pair` (public member function of`std::pair<T1,T2>`)[edit]
make_from_tuple (C++17)	construct an object with a tuple of arguments (function template)[edit]