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) |
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.std::is_trivially_constructible<T, T&&>.std::is_nothrow_constructible<T, T&&>.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.
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, 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
#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() {}
Defect reports
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
| DR | Applied to | Behavior as published | Correct behavior |
|---|---|---|---|
| LWG 2196 | C++11 | the behavior was unclear if T&& cannot be formed
|
the value produced is false in this case
|