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

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.

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>

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&) {}
};
 
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

Language
Standard Library Headers
Freestanding and hosted implementations
Named requirements
Language support library
Concepts library (C++20)
Diagnostics library
Utilities library
Strings library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Numerics library
Input/output library
Localizations library
Regular expressions library (C++11)
Atomic operations library (C++11)
Thread support library (C++11)
Filesystem library (C++17)
Technical Specifications

is_constructibleis_trivially_constructibleis_nothrow_constructible (C++11)(C++11)(C++11)	checks if a type has a constructor for specific arguments (class template)
is_default_constructibleis_trivially_default_constructibleis_nothrow_default_constructible (C++11)(C++11)(C++11)	checks if a type has a default constructor (class template)
is_copy_constructibleis_trivially_copy_constructibleis_nothrow_copy_constructible (C++11)(C++11)(C++11)	checks if a type has a copy constructor (class template)
move (C++11)	obtains an rvalue reference (function template)
move_if_noexcept (C++11)	obtains an rvalue reference if the move constructor does not throw (function template)