std::is_assignable,std::is_trivially_assignable,std::is_nothrow_assignable

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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

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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 is_assignable;	(1)	(since C++11)
template<class T,class U> struct is_trivially_assignable;	(2)	(since C++11)
template<class T,class U> struct is_nothrow_assignable;	(3)	(since C++11)

1) If the expressionstd::declval<T>()=std::declval<U>() is well-formed in unevaluated context, provides the member constantvalue equal totrue. Otherwise,value isfalse.Access checks are performed as if from a context unrelated to either type.

2) Same as(1), but the evaluation of the assignment expression will not call any operation that is not trivial. For the purposes of this check, a call tostd::declval is considered trivial and not considered anodr-use ofstd::declval.

3) Same as(1), but the evaluation of the assignment expression will not call any operation that is not noexcept.

IfT orU 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,class U> constexprbool is_assignable_v= is_assignable<T, U>::value;		(since C++17)
template<class T,class U> constexprbool is_trivially_assignable_v= is_trivially_assignable<T, U>::value;		(since C++17)
template<class T,class U> constexprbool is_nothrow_assignable_v= is_nothrow_assignable<T, U>::value;		(since C++17)

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Inherited fromstd::integral_constant

Member constants

value

[static]

true ifT is assignable fromU,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

This trait does not check anything outside the immediate context of the assignment expression: if the use ofT orU would trigger template specializations, generation of implicitly-defined special member functions etc, and those have errors, the actual assignment may not compile even ifstd::is_assignable<T,U>::value compiles and evaluates totrue.

[edit]Example

Run this code

#include <iostream>#include <string>#include <type_traits> struct Ex1{int n;}; int main(){std::cout<<std::boolalpha<<"int is assignable from int? "<< std::is_assignable<int,int>::value<<'\n'// 1 = 1; wouldn't compile<<"int& is assignable from int? "<< std::is_assignable<int&,int>::value<<'\n'// int a; a = 1; works<<"int is assignable from double? "<< std::is_assignable<int,double>::value<<'\n'<<"int& is nothrow assignable from double? "<< std::is_nothrow_assignable<int&,double>::value<<'\n'<<"string is assignable from double? "<< std::is_assignable<std::string,double>::value<<'\n'<<"Ex1& is trivially assignable from const Ex1&? "<< std::is_trivially_assignable<Ex1&,const Ex1&>::value<<'\n';}

Output:

int is assignable from int? falseint& is assignable from int? trueint is assignable from double? falseint& is nothrow assignable from double? truestring is assignable from double? trueEx1& is trivially assignable from const Ex1&? true

[edit]See also

is_copy_assignableis_trivially_copy_assignableis_nothrow_copy_assignable (C++11)(C++11)(C++11)	checks if a type has a copy assignment operator (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]
assignable_from (C++20)	specifies that a type is assignable from another type (concept)[edit]