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

template< class T >
struct is_move_constructible;

template< class T >
struct is_trivially_move_constructible;

template< class T >
struct is_nothrow_move_constructible;

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.

Helper variable templates

template< class T >
inline constexpr bool is_move_constructible_v =
    is_move_constructible<T>::value;

template< class T >
inline constexpr bool is_trivially_move_constructible_v =
    is_trivially_move_constructible<T>::value;

template< class T >
inline constexpr bool is_nothrow_move_constructible_v =
    is_nothrow_move_constructible<T>::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 <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>);
}

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

See also