std::is_empty
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                    | 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 true. For any other type, value is false.
| Contents | 
Inherited from std::integral_constant
Member constants
| value [static] | true if Tis an empty class type , false otherwise(public static member constant) | 
Member functions
| operator bool | converts the object to bool, returns value(public member function) | 
Member types
| Type | Definition | 
| value_type | bool | 
| type | std::integral_constant<bool, value> | 
[edit] Notes
sizeof(T) always returns 1 if T is empty, but 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.
[edit] Example
Run this code
#include <iostream> #include <type_traits> struct A {}; struct B { int m; }; struct C { virtual ~C(); }; int main() { std::cout << std::boolalpha; std::cout << std::is_empty<A>::value << '\n'; std::cout << std::is_empty<B>::value << '\n'; std::cout << std::is_empty<C>::value << '\n'; }
Output:
true false false
[edit] See also
| (C++11) | checks if a type is a class type (but not union type) (class template) | 


