NAME¶

std::is_move_constructible,std::is_trivially_move_constructible, - std::is_move_constructible,std::is_trivially_move_constructible,

Synopsis¶

Defined in header <type_traits>
template< class T > (1) (since C++11)
struct is_move_constructible;
template< class T > (2) (since C++11)
struct is_trivially_move_constructible;
template< class T > (3) (since C++11)
struct is_nothrow_move_constructible;

The value of the member constant value
Type trait T is a referenceable type T is not a
referenceable type
(1) std::is_constructible<T, T&&>::value
(2) std::is_trivially_constructible<T, T&&>::value false
(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.

Member constants¶

value true if T is move-constructible, false otherwise
[static] (public static member constant)

Member functions¶

operator bool converts the object to bool, returns value
(public member function)
operator() returns value
(C++14) (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¶

// Run this code

#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¶

is_constructible
is_trivially_constructible checks if a type has a constructor for specific
is_nothrow_constructible arguments
(C++11) (class template)
(C++11)
(C++11)
is_default_constructible
is_trivially_default_constructible
is_nothrow_default_constructible checks if a type has a default constructor
(C++11) (class template)
(C++11)
(C++11)
is_copy_constructible
is_trivially_copy_constructible
is_nothrow_copy_constructible checks if a type has a copy constructor
(C++11) (class template)
(C++11)
(C++11)
move_constructible specifies that an object of a type can be move
(C++20) constructed
(concept)
move obtains an rvalue reference
(C++11) (function template)
move_if_noexcept obtains an rvalue reference if the move
(C++11) constructor does not throw
(function template)