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;

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.

Helper variable templates

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

Inherited from std::integral_constant

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, 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.

Example¶

// Run this code

#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

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)