std::is_move_constructible,std::is_trivially_move_constructible,std::is_nothrow_move_constructible (3) - Linux Manuals

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

NAME

std::is_move_constructible,std::is_trivially_move_constructible,std::is_nothrow_move_constructible - std::is_move_constructible,std::is_trivially_move_constructible,std::is_nothrow_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.

Helper variable templates

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

Inherited from std::integral_constant

Member constants

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

Member functions

              converts the object to bool, returns value
operator bool (public member function)

operator() returns value
              (public member function)
(C++14)

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