Namespaces
Variants
Views
Actions

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

From cppreference.com
< cpp‎ | types
 
 
 
Type support
Basic types
Fundamental types
Fixed width integer types (C++11)
Numeric limits
C numeric limits interface
Runtime type information
Type traits
Type categories
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
Type properties
(C++11)
(C++11)
(C++11)
(C++14)
(C++11)
(C++11)(deprecated in C++17)
(C++11)
Type trait constants
Metafunctions
(C++17)
Supported operations
is_copy_assignableis_trivially_copy_assignableis_nothrow_copy_assignable
(C++11)(C++11)(C++11)
Relationships and property queries
(C++11)
(C++11)
(C++11)
(C++11)
Type modifications
(C++11)(C++11)(C++11)
Type transformations
(C++11)
(C++11)
(C++17)
(C++11)(deprecated in C++17)(C++17)
 
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)
1) If T is not a referenceable type (i.e., possibly cv-qualified void or a function type with a cv-qualifier-seq or a ref-qualifier), provides a member constant value equal to false. Otherwise, provides a member constant value equal to std::is_assignable<T&, const T&>::value.
2) Same as 1), but uses std::is_trivially_assignable<T&, const T&>
2) Same as 1), but uses std::is_nothrow_assignable<T&, const T&>

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

Contents

[edit] Helper variable templates

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

Inherited from std::integral_constant

Member constants

value
[static]
true if T is copy-assignable, 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>

[edit] Possible implementation

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

[edit] Notes

The trait std::is_copy_assignable is less strict than CopyAssignable because it does not check the type of the result of the assignment (which, for a CopyAssignable type, must be an lvalue of type T) and does not check the semantic requirement that the argument expression remains unchanged. It also does not check that T satisfies MoveAssignable, which is required of all CopyAssignable types.

[edit] Example

#include <iostream>
#include <utility>
#include <type_traits>
struct Foo { int n; };
int main() {
    std::cout << std::boolalpha
              << "Foo is trivally 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 trivally copy-assignable? true
int[2] is copy-assignable? false
int is nothrow copy-assignable? true

[edit] See also

checks if a type has a assignment operator for a specific argument
(class template) [edit]
checks if a type has a move assignment operator
(class template) [edit]