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

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

Compiler support
Freestanding and hosted
Language
Standard library headers
Named requirements
Feature test macros (C++20)
Language support library
Concepts library (C++20)
Metaprogramming library (C++11)
Diagnostics library
General utilities library
Strings library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Numerics library
Localizations library
Input/output library
Filesystem library (C++17)
Regular expressions library (C++11)
Concurrency support library (C++11)
Technical specifications
Symbols index
External libraries