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

Predict the output of following program?

#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.

#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.

#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)

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

Output (with GCC compiler. See this):

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 excercise, try the following program on your compiler.

// 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; 
} 

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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