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std::is_move_constructible, std::is_trivially_move_constructible, std::is_nothrow_move_constructible

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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.

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.

The behavior of a program that adds specializations for any of the templates described on this page is undefined.

Contents

[edit] Helper variable templates

template< class T >
inline constexpr bool is_move_constructible_v = is_move_constructible<T>::value;
(since C++17)
template< class T >
inline constexpr bool is_trivially_move_constructible_v = is_trivially_move_constructible<T>::value;
(since C++17)
template< class T >
inline 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>

[edit] 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> {};

[edit] 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 noexcept(T(arg)). Same applies to is_trivially_move_constructible, which, in these implementations, also requires that the destructor is trivial: GCC bug 51452, LWG issue 2116.

[edit] 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&) {}
};
 
#define OUT(...) std::cout << #__VA_ARGS__ << " : " << __VA_ARGS__ << '\n'
 
int main() {
    std::cout << std::boolalpha;
    OUT( std::is_move_constructible_v<Ex1> );
    OUT( std::is_trivially_move_constructible_v<Ex1> );
    OUT( std::is_nothrow_move_constructible_v<Ex1> );
    OUT( std::is_trivially_move_constructible_v<Ex2> );
    OUT( std::is_nothrow_move_constructible_v<Ex2> );
    OUT( std::is_move_constructible_v<NoMove> );
    OUT( std::is_nothrow_move_constructible_v<NoMove> );
}

Output:

std::is_move_constructible_v<Ex1> : true
std::is_trivially_move_constructible_v<Ex1> : false
std::is_nothrow_move_constructible_v<Ex1> : true
std::is_trivially_move_constructible_v<Ex2> : true
std::is_nothrow_move_constructible_v<Ex2> : true
std::is_move_constructible_v<NoMove> : true
std::is_nothrow_move_constructible_v<NoMove> : false

[edit] See also

checks if a type has a constructor for specific arguments
(class template) [edit]
checks if a type has a default constructor
(class template) [edit]
checks if a type has a copy constructor
(class template) [edit]
specifies that an object of a type can be move constructed
(concept) [edit]
(C++11)
obtains an rvalue reference
(function template) [edit]
obtains an rvalue reference if the move constructor does not throw
(function template) [edit]