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
Defined in header <type_traits>
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||
---|---|---|
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(1) | (since C++11) |
|
(2) | (since C++11) |
|
(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
|
(since C++17) | |
|
(since C++17) | |
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(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
|
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
#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
See also
(C++11)(C++11)(C++11) |
checks if a type has a constructor for specific arguments (class template) |
(C++11)(C++11)(C++11) |
checks if a type has a default constructor (class template) |
(C++11)(C++11)(C++11) |
checks if a type has a copy constructor (class template) |
(C++11) |
obtains an rvalue reference (function template) |
(C++11) |
obtains an rvalue reference if the move constructor does not throw (function template) |
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