std::is_empty
Defined in header <type_traits> |
||
|---|---|---|
template< class T > struct is_empty; |
(since C++11) |
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 > inline constexpr bool is_empty_v = is_empty<T>::value; |
(since C++17) |
Inherited from std::integral_constant
Member constants
|
value
[static]
|
true if T is an empty class type , 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> |
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
#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
|
(C++11)
|
checks if a type is a non-union class type (class template) |
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https://en.cppreference.com/w/cpp/types/is_empty
