Why is the size of an empty class not zero in C++?

Difficulty Level : Medium
Last Updated : 29 Oct, 2020

Predict the output of following program?

CPP

#include<iostream>
using namespace std;
 
class Empty {};
 
int main()
{
  cout << sizeof(Empty);
  return 0;
}

Output

Size of an empty class is not zero. It is 1 byte generally. It is nonzero to ensure that the two different objects will have different addresses. See the following example.

CPP

#include<iostream>
using namespace std;
 
class Empty { };
 
int main()
{
    Empty a, b;
 
    if (&a == &b)
      cout << "impossible " << endl;
    else
      cout << "Fine " << endl;
 
   return 0;
}

Output

Fine

For the same reason (different objects should have different addresses), “new” always returns pointers to distinct objects. See the following example.

CPP

#include<iostream>
using namespace std;
 
class Empty { };
 
int main()
{
    Empty* p1 = new Empty;
    Empty* p2 = new Empty;
 
    if (p1 == p2)
        cout << "impossible " << endl;
    else
        cout << "Fine " << endl;
 
    return 0;
}

Output

Fine

Now guess the output of following program (This is tricky)

CPP

#include<iostream>
using namespace std;
 
class Empty { };
 
class Derived: Empty { int a; };
 
int main()
{
    cout << sizeof(Derived);
    return 0;
}

Output

Note that the output is not greater than 4. There is an interesting rule that says that an empty base class need not be represented by a separate byte. So compilers are free to make optimization in case of empty base classes. As an exercise, try the following program on your compiler.

CPP

// Thanks to Venki for suggesting this code.
#include <iostream>
using namespace std;
 
class Empty {
};
 
class Derived1 : public Empty {
};
 
class Derived2 : virtual public Empty {
};
 
class Derived3 : public Empty {
    char c;
};
 
class Derived4 : virtual public Empty {
    char c;
};
 
class Dummy {
    char c;
};
 
int main()
{
    cout << "sizeof(Empty) " << sizeof(Empty) << endl;
    cout << "sizeof(Derived1) " << sizeof(Derived1) << endl;
    cout << "sizeof(Derived2) " << sizeof(Derived2) << endl;
    cout << "sizeof(Derived3) " << sizeof(Derived3) << endl;
    cout << "sizeof(Derived4) " << sizeof(Derived4) << endl;
    cout << "sizeof(Dummy) " << sizeof(Dummy) << endl;
 
    return 0;
}

Output

sizeof(Empty) 1
sizeof(Derived1) 1
sizeof(Derived2) 8
sizeof(Derived3) 1
sizeof(Derived4) 16
sizeof(Dummy) 1

Source:
http://www2.research.att.com/~bs/bs_faq2.html
Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.

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