Size of class derived from multiple inherited class with virtual function

Question

Consider the diamond scenario below:

class Base {
    int x;
public:
    virtual ~Base(){}
};

class Derived1 : virtual public Base {
    int y;
};

class Derived2 : virtual public Base {
    int z;
};

class Derived3 : public Derived1, public Derived2 {
    int t;
};

Size of Base is 8 [x(4) + vptr(4) (from virtual destructor)]
Size of Derived1 is 16 [x(4) + y(4) + vptr(4) (from virtual destructor) + vptr(4) (from virtual inheritance)]
Size of Derived2 is 16 [x(4) + z(4) + vptr(4) (from virtual destructor) + vptr(4) (from virtual inheritance)]
Size of Derived3 is 28 [x(4) + y(4) + z(4) + t(4) + vptr(4) (from virtual destructor) + 2 times vptr(4) (from virtual inheritance of Derived1 and Derived2)]

Now, if I add another class Derived4 which derives from Derived 3,

class Derived4 : public Derived3 {
    int s;
};

it's size comes out to be 32 (which I'm assuming to be size of Derived3 + s).

I want to know if there is no virtual pointer inside Derived4?

If I take a normal class hierarchy (without diamond structure), then if a base class has a virtual function, then all derived classes have vptrs. So, why not in this case?

I'm compiling the code in codeBlocks 12.11 with GNU GCC compiler.

Scott Jones Scott Jones · Accepted Answer · 2013-04-28T21:51:52

Since Derived4 has not added any virtual functions or virtual base classes, beyond what it has inherited from Derived3, the only increase in size is for the member s.

Size of class derived from multiple inherited class with virtual function

1 Answers