std::is_empty(3) | C++ Standard Libary | std::is_empty(3) |
NAME¶
std::is_empty - std::is_empty
Synopsis¶
Defined in header <type_traits>
template< class T > (since C++11)
struct is_empty;
If T is an empty type (that is, a non-union class type with no non-static
data
members other than bit-fields of size 0, no virtual functions, no virtual
base
classes, and no non-empty base classes), provides the member constant value
equal to
true. For any other type, value is false.
If T is a non-union class type, T shall be a complete type; otherwise, the
behavior
is undefined.
The behavior of a program that adds specializations for is_empty
or is_empty_v
(since C++17) is undefined.
Template parameters¶
T - a type to check
Helper variable template
template< class T > (since C++17)
inline constexpr bool is_empty_v = is_empty<T>::value;
Inherited from std::integral_constant
Member constants¶
value true if T is an empty class type , 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>
Notes¶
Inheriting from empty base classes usually does not increase the
size of a class due
to empty base optimization.
std::is_empty<T> and all other type traits are empty classes.
Example¶
// Run this code
#include <iostream>
#include <type_traits>
struct A {};
struct B {
int m;
};
struct C {
static int m;
};
struct D {
virtual ~D();
};
union E {};
struct F {
[[no_unique_address]] E e;
};
struct G {
int:0;
// C++ standard allow "as a special case, an unnamed bit-field with a
width of zero
// specifies alignment of the next bit-field at an allocation unit boundary.
// Only when declaring an unnamed bit-field may the width be zero."
};
int main()
{
std::cout << std::boolalpha;
std::cout << "A " << std::is_empty<A>::value
<< '\n';
std::cout << "B " << std::is_empty<B>::value
<< '\n';
std::cout << "C " << std::is_empty<C>::value
<< '\n';
std::cout << "D " << std::is_empty<D>::value
<< '\n';
std::cout << "E " << std::is_empty<E>::value
<< '\n';
std::cout << "F " << std::is_empty<F>::value
<< '\n'; // the result is ABI-dependent
std::cout << "G " << std::is_empty<G>::value
<< '\n'; // unnamed bit-fields of width of 0
}
Possible output:¶
A true
B false
C true
D false
E false
F true
G true
See also¶
is_class checks if a type is a non-union class type
(C++11) (class template)
2022.07.31 | http://cppreference.com |