| Defined in header <type_traits> | ||
|---|---|---|
template< class T > struct is_move_constructible; | (1) | (since C++11) |
template< class T > struct is_trivially_move_constructible; | (2) | (since C++11) |
template< class T > struct is_nothrow_move_constructible; | (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_constructible<T, T&&>::value.
2) Same as (1), but uses std::is_trivially_constructible<T, T&&>.
3) Same as (1), but uses std::is_nothrow_constructible<T, T&&>.
T shall 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 > constexpr bool is_move_constructible_v = is_move_constructible<T>::value; | (since C++17) | |
template< class T > constexpr bool is_trivially_move_constructible_v = is_trivially_move_constructible<T>::value; | (since C++17) | |
template< class T > constexpr bool is_nothrow_move_constructible_v = is_nothrow_move_constructible<T>::value; | (since C++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() (C++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, is_nothrow_move_constructible also checks if the destructor throws because it is effectively noexept(T(arg)): GCC bug 51452 LWG issue 2116.
Example
#include <iostream>
#include <type_traits>
struct Ex1 {
std::string str; // member has a non-trivial but non-throwing move ctor
};
struct Ex2 {
int n;
Ex2(Ex2&&) = default; // trivial and non-throwing
};
struct NoMove {
// prevents implicit declaration of default move constructor
// however, the class is still move-constructible because its
// copy constructor can bind to an rvalue argument
NoMove(const NoMove&) {}
};
int main() {
std::cout << std::boolalpha << "Ex1 is move-constructible? "
<< std::is_move_constructible<Ex1>::value << '\n'
<< "Ex1 is trivially move-constructible? "
<< std::is_trivially_move_constructible<Ex1>::value << '\n'
<< "Ex1 is nothrow move-constructible? "
<< std::is_nothrow_move_constructible<Ex1>::value << '\n'
<< "Ex2 is trivially move-constructible? "
<< std::is_trivially_move_constructible<Ex2>::value << '\n'
<< "Ex2 is nothrow move-constructible? "
<< std::is_nothrow_move_constructible<Ex2>::value << '\n';
std::cout << std::boolalpha
<< "NoMove is move-constructible? "
<< std::is_move_constructible<NoMove>::value << '\n'
<< "NoMove is nothrow move-constructible? "
<< std::is_nothrow_move_constructible<NoMove>::value << '\n';
}Output:
Ex1 is move-constructible? true Ex1 is trivially move-constructible? false Ex1 is nothrow move-constructible? true Ex2 is trivially move-constructible? true Ex2 is nothrow move-constructible? true NoMove is move-constructible? true NoMove is nothrow move-constructible? false
See also
| (C++11)(C++11)(C++11) | checks if a type has a constructor for specific arguments (class template) |
| checks if a type has a default constructor (class template) | |
| (C++11)(C++11)(C++11) | checks if a type has a copy constructor (class template) |
| (C++11) | obtains an rvalue reference if the move constructor does not throw (function template) |
| std::experimental::is_move_constructible_v
(library fundamentals TS) | variable template alias of std::is_move_constructible::value (variable template) |
| std::experimental::is_trivially_move_constructible_v
(library fundamentals TS) | variable template alias of std::is_trivially_move_constructible::value (variable template) |
| std::experimental::is_nothrow_move_constructible_v
(library fundamentals TS) | variable template alias of std::is_nothrow_move_constructible::value (variable template) |
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