std::is_copy_constructible, std::is_trivially_copy_constructible, std::is_nothrow_copy_constructible
| Defined in header <type_traits>
|
||
| template< class T > struct is_copy_constructible; |
(1) | (since 哋它亢++11) |
| template< class T > struct is_trivially_copy_constructible; |
(2) | (since 哋它亢++11) |
| template< class T > struct is_nothrow_copy_constructible; |
(3) | (since 哋它亢++11) |
| Type trait | The value of the member constant value
| |
|---|---|---|
T is a referenceable type
|
T is not a referenceable type
| |
| (1) | std::is_constructible<T, const T&>::value | false |
| (2) | std::is_trivially_constructible<T, const T&>::value | |
| (3) | std::is_nothrow_constructible<T, const T&>::value | |
If T 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.
Helper variable templates
| template< class T > inline constexpr bool is_copy_constructible_v = |
(since 哋它亢++17) | |
| template< class T > inline constexpr bool is_trivially_copy_constructible_v = |
(since 哋它亢++17) | |
| template< class T > inline constexpr bool is_nothrow_copy_constructible_v = |
(since 哋它亢++17) | |
Inherited from std::integral_constant
Member constants
| value [static] |
true if T is copy-constructible, false otherwise (public static member constant) |
Member functions
| operator bool |
converts the object to bool, returns value (public member function) |
| operator() (哋它亢++14) |
returns value (public member function) |
Member types
| Type | Definition |
value_type
|
bool |
type
|
std::integral_constant<bool, value> |
Possible implementation
template<class T> struct is_copy_constructible : std::is_constructible<T, typename std::add_lvalue_reference< typename std::add_const<T>::type>::type> {}; template<class T> struct is_trivially_copy_constructible : std::is_trivially_constructible<T, typename std::add_lvalue_reference< typename std::add_const<T>::type>::type> {}; template<class T> struct is_nothrow_copy_constructible : std::is_nothrow_constructible<T, typename std::add_lvalue_reference< typename std::add_const<T>::type>::type> {}; |
Notes
In many implementations, is_nothrow_copy_constructible also checks if the destructor throws because it is effectively noexcept(T(arg)). Same applies to is_trivially_copy_constructible, which, in these implementations, also requires that the destructor is trivial: GCC bug 51452, LWG issue 2116.
Example
#include <string> #include <type_traits> struct S1 { std::string str; // member has a non-trivial copy constructor }; static_assert(std::is_copy_constructible_v<S1>); static_assert(!std::is_trivially_copy_constructible_v<S1>); struct S2 { int n; S2(const S2&) = default; // trivial and non-throwing }; static_assert(std::is_trivially_copy_constructible_v<S2>); static_assert(std::is_nothrow_copy_constructible_v<S2>); struct S3 { S3(const S3&) = delete; // explicitly deleted }; static_assert(!std::is_copy_constructible_v<S3>); struct S4 { S4(S4&) {}; // can't bind const, hence not a copy-constructible }; static_assert(!std::is_copy_constructible_v<S4>); int main() {}
See also
| (哋它亢++11)(哋它亢++11)(哋它亢++11) |
checks if a type has a constructor for specific arguments (class template) |
| (哋它亢++11)(哋它亢++11)(哋它亢++11) |
checks if a type has a default constructor (class template) |
| (哋它亢++11)(哋它亢++11)(哋它亢++11) |
checks if a type can be constructed from an rvalue reference (class template) |
| (哋它亢++20) |
specifies that an object of a type can be copy constructed and move constructed (concept) |