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

 

C++

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Technical Specifications

 

Utilities library

Type support (basic types, RTTI, type traits)

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bitset

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rel_ops::operator!=rel_ops::operator>rel_ops::operator<=rel_ops::operator>=

optional, any and variant (C++17)

optional

any

variant

in_placein_place_typein_place_indexin_place_tin_place_type_tin_place_index_t

Pairs and tuples

pair

tuple (C++11)

apply (C++17)

make_from_tuple (C++17)

piecewise_construct_t (C++11)

piecewise_construct (C++11)

integer_sequence (C++14)

Swap, forward and move

swap

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Elementary string conversions

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Type operations

declval (C++11)

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Type support

Basic types
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ptrdiff_t size_t max_align_t (C++11) byte (C++17)	nullptr_t (C++11) offsetof NULL
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numeric_limits
C numeric limits interface
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type_info type_index (C++11)	bad_typeid bad_cast
Type traits
Type categories
is_void (C++11) is_null_pointer (C++14) 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_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_reference (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
is_const (C++11) is_volatile (C++11) is_pod (C++11) is_empty (C++11) is_polymorphic (C++11) is_final (C++14) is_abstract (C++11)	is_trivial (C++11) is_trivially_copyable (C++11) is_standard_layout (C++11) is_literal_type (C++11)(deprecated in C++17) is_aggregate (C++17) is_signed (C++11) is_unsigned (C++11)
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_base_of (C++11) is_convertible (C++11)	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) aligned_union (C++11) decay (C++11) enable_if (C++11) void_t (C++17)	conditional (C++11) common_type (C++11) underlying_type (C++11) result_ofinvoke_result (C++11)(deprecated in C++17)(C++17)

 

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 expression std::declval<T>() = std::declval<U>() is well-formed in unevaluated context, provides the member constant value equal true. Otherwise, value is false. 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 to std::declval is considered trivial and not considered an odr-use of std::declval.

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

T and U shall each be a complete type, (possibly cv-qualified) void, or an array of unknown bound. Otherwise, the behavior is undefined.

Helper variable templates

template< class T, class U > inline constexpr bool is_assignable_v = is_assignable<T, U>::value;		(since C++17)
template< class T, class U > inline constexpr bool is_trivially_assignable_v = is_trivially_assignable<T, U>::value;		(since C++17)
template< class T, class U > inline constexpr bool is_nothrow_assignable_v = is_nothrow_assignable<T, U>::value;		(since C++17)

Inherited from std::integral_constant

Member constants

value [static]

true if T is assignable from U , 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

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

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? false
int& is assignable from int? true
int is assignable from double? false
int& is nothrow assignable from double? true
string is assignable from double? true
Ex1& is trivially assignable from const Ex1&? true

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)
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)