C & C++ Programming - Size Of & VPTR

    class Base
    {
    public:
	    virtual void function1()
	    {}
    };


    class Derive1: public Base
    {
    public:
	    virtual void function1()
	    {}
    };

    class Derive2: public Base
    {
    public:
	    virtual void function1()
	    {}
    };

    class Derive3
    {
    public:
	    virtual void function1()
	    {}
    };

    class DeriveDerive1: public Derive1, Derive2, Derive3
    {
    public:
	    virtual void function1()
	    {
		    cout << __FUNCTION__ << endl;
	    }
    };

    int main()
    {
        Base objB;
	    Derive1 objD1;
	    Derive2 objD2;
	    DeriveDerive1 objDD;

	    cout << "SIZE OF BASE CLASS = " << sizeof(Base) << ", " << sizeof(objB) << endl;
	    cout << "SIZE OF Derived CLASS 1 = " << sizeof(Derive1) << ", " << sizeof(objD1) << endl;
	    cout << "SIZE OF Derived CLASS 2 = " << sizeof(Derive2) << ", " << sizeof(objD2) << endl;
	    cout << "SIZE OF Derive Derive CLASS = " << sizeof(DeriveDerive1) << ", " << sizeof(objDD) << endl;
    }
            

The class DeriveDerive is derived from 3 classes, each having virtual functions thus having individual VTABLE for each.
It could be observed the sizeof objDD is 12. For the fact 3 VTABLE pointers are associated with object objDD to access each base class VTABLE.

    class DeriveDerive1: virtual public Derive1, Derive2, Derive3
    {
    public:
	    virtual void function1()
	    {
		    cout << __FUNCTION__ << endl;
	    }
    };
            

Modifying the code as above will result in sizeof objDD = 16. 4 more is added to accomodate BPTR (Base Table pointer).

    class Base
    {
    public:
	    virtual void function1()
	    {}
    }; 

    class Base2
    {
    public:
	    virtual void function1()
	    {}
    };

    class Derive1: virtual public Base
    {
    public:
	    virtual void function1()
	    {}
    }; // Size of Derive1 = 4 + 4; VPTR, BPTR

    class Derive2: virtual public Base
    {
    public:
	    virtual void function1()
	    {}
    };

    class Derive3 : virtual public Base2
    {
    public:
	    virtual void function1()
	    {}
    };

    class DeriveDerive1:  public Derive1, Derive2, Derive3
    {
    public:
	    virtual void function1()
	    {
		    cout << __FUNCTION__ << endl;
	    }
    };  // Size of Derive1 = 4 + 4 + 4, 4 + 4; 2VPTR, BPTR; VPTR + BPTR;
        // This is derived from 3 different class, of which 2 virually devired from same class and one from diffrent.

    ////////////////////////////////////////////////////////////////////////////////////////////////////////////////
    // WATCH
    
    -		objDD	{...}	DeriveDerive1
    -		Derive1	{...}	Derive1
    -		    Base	{...}	Base
    -		        __vfptr	0x0033991c const DeriveDerive1::`vftable'{for `Derive1'}	*
		                [0]	0x003310af DeriveDerive1::function1(void)	*

    -		Derive2	{...}	Derive2
    -		    Base	{...}	Base
    -		        __vfptr	0x0033991c const DeriveDerive1::`vftable'{for `Derive1'}	*
		                [0]	0x003310af DeriveDerive1::function1(void)	*

    -		Derive3	{...}	Derive3
    -		    Base2	{...}	Base2
    -		        __vfptr	0x003398f4 const DeriveDerive1::`vftable'{for `Derive3'}	*
		                [0]	0x003313a2 [thunk]:DeriveDerive1::function1`adjustor{4}' (void)	*