Virtual inheritance using empty classes

Question

Can anyone tell the exact reason for the output of the following code in C++ ?The output that I received for the code is included in header comments. What does it have to do with virtual table and v pointer.

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

#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;

}

Answer 1

Firstly, even a class with no members must have a non-zero size. The standard insists on that. Otherwise pointer arithmetic and arrays would not work as an array of a zero-sized class would have all its elements in the same place!

The fact that the other sizes differ may well be due to a v-table. But that is not mandated explicitly in the standard, so is a manifestation of the way your compiler is dealing with things.

Note also that polymorphism requires at least one virtual method to be defined in a base class. This accounts for sizeof(Derived1) being the same size as the base class.