std::is_move_constructible, std::is_trivially_move_constructible, std::is_nothrow_move_constructible
| Defined in header <type_traits>
|
||
| template< class T > struct is_move_constructible; |
(1) | (since 哋它亢++11) |
| template< class T > struct is_trivially_move_constructible; |
(2) | (since 哋它亢++11) |
| template< class T > struct is_nothrow_move_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, T&&>::value | false |
| (2) | std::is_trivially_constructible<T, T&&>::value | |
| (3) | std::is_nothrow_constructible<T, 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_move_constructible_v = |
(since 哋它亢++17) | |
| template< class T > inline constexpr bool is_trivially_move_constructible_v = |
(since 哋它亢++17) | |
| template< class T > inline constexpr bool is_nothrow_move_constructible_v = |
(since 哋它亢++17) | |
Inherited from std::integral_constant
Member constants
| value [static] |
true if T is move-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_move_constructible : std::is_constructible<T, typename std::add_rvalue_reference<T>::type> {}; template<class T> struct is_trivially_move_constructible : std::is_trivially_constructible<T, typename std::add_rvalue_reference<T>::type> {}; template<class T> struct is_nothrow_move_constructible : std::is_nothrow_constructible<T, typename std::add_rvalue_reference<T>::type> {}; |
Notes
Types without a move constructor, but with a copy constructor that accepts const T& arguments, satisfy std::is_move_constructible.
Move constructors are usually noexcept, since otherwise they are unusable in any code that provides strong exception guarantee.
In many implementations, std::is_nothrow_move_constructible also checks if the destructor throws because it is effectively noexcept(T(arg)). Same applies to std::is_trivially_move_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 Ex1 { std::string str; // member has a non-trivial but non-throwing move constructor }; static_assert(std::is_move_constructible_v<Ex1>); static_assert(!std::is_trivially_move_constructible_v<Ex1>); static_assert(std::is_nothrow_move_constructible_v<Ex1>); struct Ex2 { int n; Ex2(Ex2&&) = default; // trivial and non-throwing }; static_assert(std::is_move_constructible_v<Ex2>); static_assert(std::is_trivially_move_constructible_v<Ex2>); static_assert(std::is_nothrow_move_constructible_v<Ex2>); struct NoMove1 { // prevents implicit declaration of default move constructor; // however, the class is still move-constructible because its // copy constructor can bind to an rvalue argument NoMove1(const NoMove1&) {} }; static_assert(std::is_move_constructible_v<NoMove1>); static_assert(!std::is_trivially_move_constructible_v<NoMove1>); static_assert(!std::is_nothrow_move_constructible_v<NoMove1>); struct NoMove2 { // Not move-constructible since the lvalue reference // can't bind to the rvalue argument NoMove2(NoMove2&) {} }; static_assert(!std::is_move_constructible_v<NoMove2>); static_assert(!std::is_trivially_move_constructible_v<NoMove2>); static_assert(!std::is_nothrow_move_constructible_v<NoMove2>); 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 has a copy constructor (class template) |
| (哋它亢++20) |
specifies that an object of a type can be move constructed (concept) |
| (哋它亢++11) |
obtains an rvalue reference (function template) |
| (哋它亢++11) |
obtains an rvalue reference if the move constructor does not throw (function template) |