std::is_copy_assignable,std::is_trivially_copy_assignable,std::is_nothrow_copy_assignable

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Feature test macros(C++20)
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Concepts library(C++20)
Diagnostics library
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Metaprogramming library(C++11)
General utilities library
Containers library
Iterators library
Ranges library(C++20)
Algorithms library
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Date and time library
Input/output library
Filesystem library(C++17)
Concurrency support library(C++11)
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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> struct is_copy_assignable;	(1)	(since C++11)
template<class T> struct is_trivially_copy_assignable;	(2)	(since C++11)
template<class T> struct is_nothrow_copy_assignable;	(3)	(since C++11)

Type trait	The value of the member constant`value`
Type trait	`T` is areferenceable type	`T` is not a referenceable type
(1)	std::is_assignable<T&,const T&>::value	false
(2)	std::is_trivially_assignable<T&,const T&>::value
(3)	std::is_nothrow_assignable<T&,const T&>::value

IfT is not a complete type, (possibly cv-qualified)void, or an array of unknown bound, 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 any of the templates described on this page, the behavior is undefined.

[edit]Helper variable templates

template<class T> inlineconstexprbool is_copy_assignable_v= is_copy_assignable<T>::value;		(since C++17)
template<class T> inlineconstexprbool is_trivially_copy_assignable_v= is_trivially_copy_assignable<T>::value;		(since C++17)
template<class T> inlineconstexprbool is_nothrow_copy_assignable_v= is_nothrow_copy_assignable<T>::value;		(since C++17)

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Inherited fromstd::integral_constant

Member constants

value

[static]

true ifT is copy-assignable,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]Possible implementation

template<class T>struct is_copy_assignable:std::is_assignable<typenamestd::add_lvalue_reference<T>::type,typenamestd::add_lvalue_reference<const T>::type>{}; template<class T>struct is_trivially_copy_assignable:std::is_trivially_assignable<typenamestd::add_lvalue_reference<T>::type,typenamestd::add_lvalue_reference<const T>::type>{}; template<class T>struct is_nothrow_copy_assignable:std::is_nothrow_assignable<typenamestd::add_lvalue_reference<T>::type,typenamestd::add_lvalue_reference<const T>::type>{};

[edit]Notes

The traitstd::is_copy_assignable is less strict thanCopyAssignable because it does not check the type of the result of the assignment (which, for aCopyAssignable type, must be an lvalue of typeT) and does not check the semantic requirement that the argument expression remains unchanged. It also does not check thatT satisfiesMoveAssignable, which is required of allCopyAssignable types.

[edit]Example

Run this code

#include <iostream>#include <type_traits>#include <utility> struct Foo{int n;}; int main(){std::cout<<std::boolalpha<<"Foo is trivially copy-assignable? "<< std::is_trivially_copy_assignable<Foo>::value<<'\n'<<"int[2] is copy-assignable? "<< std::is_copy_assignable<int[2]>::value<<'\n'<<"int is nothrow copy-assignable? "<< std::is_nothrow_copy_assignable<int>::value<<'\n';}

Output:

Foo is trivially copy-assignable? trueint[2] is copy-assignable? falseint is nothrow copy-assignable? true

[edit]Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 2196	C++11	the behavior was unclear ifconst T& cannot be formed	the value produced isfalse in this case

[edit]See also

is_assignableis_trivially_assignableis_nothrow_assignable (C++11)(C++11)(C++11)	checks if a type has an assignment operator for a specific argument (class template)[edit]
is_move_assignableis_trivially_move_assignableis_nothrow_move_assignable (C++11)(C++11)(C++11)	checks if a type has a move assignment operator (class template)[edit]