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

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

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.

  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&) {}
 };

 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