Go4Expert - Detail about How VPTR and Virtual table works

Originally written by Zeeshan Amjad

Assumption: 32-bit Machine.
Here I am going to explain How Virtual table, Virtual pointer for Virtual functions are internally working.

First we have understand memory layout.

Example 1: How the class's memory layout

Code: cpp

class Test 
{
public:
    int data1;
    int data2;
    int fun1();
};

int main() 
{
    Test obj;
    cout << "obj's Size = " << sizeof(obj) << endl;
    cout << "obj 's Address = " << &obj << endl;
    return 0;
}

OUTPUT:

Sobj's Size = 8
obj 's Address = 0012FF7C

Note: Any Plane member function does not take any memory.

Example 2: Memory Layout of Derived class

Code: cpp

class Test 
{
public:
    int a;
    int b;
};

class dTest : public Test
{
public:
    int c;
};

int main() 
{
    Test obj1;
    cout << "obj1's Size = " << sizeof(obj1) << endl;
    cout << "obj1's Address = " << &obj1 << endl;
    dTest obj2;
    cout << "obj2's Size = "<< sizeof(obj2) << endl;
    cout << "obj2's Address = "<< &obj2 << endl;
    return 0;
}

OUTPUT:
obj1's Size = 8
obj1's Address = 0012FF78
obj2's Size = 12
obj2's Address = 0012FF6C

Example 3: Memory layout If we have one virtual function.

Code: cpp

class Test 
{
public:
    int data;
    virtual void fun1() 
    { 
        cout << "Test::fun1" << endl; 
    }
};

int main() 
{
    Test obj;
    cout << "obj's Size = " << sizeof(obj) << endl;
    cout << "obj's Address = " << &obj << endl;
    return 0;
}

OUTPUT:

obj's Size = 8
obj's Address = 0012FF7C

Note: Adding one virtual function in a class takes 4 Byte extra.

Example 4: More than one Virtual function

Code: cpp

class Test 
{
public:
    int data;
    virtual void fun1() { cout << "Test::fun1" << endl; }
    virtual void fun2() { cout << "Test::fun2" << endl; }
    virtual void fun3() { cout << "Test::fun3" << endl; }
    virtual void fun4() { cout << "Test::fun4" << endl; }
};

int main()
{
    Test obj;
    cout << "obj's Size = " << sizeof(obj) << endl;
    cout << "obj's Address = " << &obj << endl;
    return 0;
}

OUTPUT:

obj's Size = 8
obj's Address = 0012FF7C

Note: Adding more virtual functions in a class, no extra size taking i.e. Only one machine size taking(i.e. 4 byte)

Example 5:

Code: cpp

class Test
{
public:
    int a;
    int b;
    Test(int temp1 = 0, int temp2 = 0)
    {
        a=temp1 ;
        b=temp2 ; 
    }
    int getA()  
    {
        return a;
    }
    int getB()  
    {
        return b;
    }
    virtual ~Test();
};

int main() 
{
    Test obj(5, 10);

    // Changing a and b
    int* pInt = (int*)&obj;
    *(pInt+0) = 100;   
    *(pInt+1) = 200;   

    cout << "a = " << obj.getA() << endl;
    cout << "b = " << obj.getB() << endl;
    return 0;
}

OUTPUT:
a = 200
b = 10

If we Change the code as then

Code: Cpp

// Changing a and b
  int* pInt = (int*)&obj;
  *(pInt+1) = 100;   // In place of 0
  *(pInt+2) = 200;   // In place of 1

OUTPUT:
a = 100
b = 200

Note: Who sits 1st place of Class : Answer is VPTR
VPTR - 1st placed in class and rest sits after it.

Example 6:

Code: cpp

class Test 
{
    virtual void fun1() 
    {
        cout << "Test::fun1" << endl;
    }
};

int main() 
{
    Test obj;
    cout << "VPTR's Address " << (int*)(&obj+0) << endl;
    cout << "VPTR's Value " << (int*)*(int*)(&obj+0) << endl;
    return 0;
}

OUTPUT:

VPTR's Address 0012FF7C
VPTR's Value 0046C060

NOTE: This VPTR's value is a address of Virtual table. Lets see in next Example.

Example 7:

Code: cpp

class Test
{
    virtual void fun1() 
    { 
        cout << "Test::fun1" << endl; 
    }
};
typedef void (*Fun)(void);

int main() 
{
    Test obj;
    cout << "VPTR's Address " << (int*)(&obj+0) << endl;
    cout << " VIRTUAL TABLE 's Address " << (int*)*(int*)(&obj+0) << endl; // Value of VPTR
    cout << "Value at first entry of VIRTUAL TABLE " << (int*)*(int*)*(int*)(&obj+0) << endl;

    Fun pFun = (Fun)*(int*)*(int*)(&obj+0);   // calling Virtual function
    pFun();
    return 0;
}

OUTPUT:
VPTR's Address 0012FF7C
VIRTUAL TABLE 's Address 0046C0EC
Value at first entry of VIRTUAL TABLE 0040100A
Test: fun1

Example 8:

Code: cpp

class Test
{
    virtual void fun1() { cout << "Test::fun1" << endl; }
    virtual void func1() { cout << "Test::func1" << endl; }
};

int main()
{
    Test obj;

    cout << "VPTR's Address " << (int*)(&obj+0) << endl;
    cout << "VIRTUAL TABLE 's Address"<< (int*)*(int*)(&obj+0) << endl;

    // Calling Virtual table functions
    cout << "Value at 1st entry of VTable " << (int*)*((int*)*(int*)(&obj+0)+0) << endl;
    cout << "Value at 2nd entry of VTable " << (int*)*((int*)*(int*)(&obj+0)+1) << endl;

    return 0;
}

OUTPUT:

VPTR's Address 0012FF7C
VIRTUAL TABLE 's Address 0046C0EC
Value at first entry of VIRTUAL TABLE 0040100A
Value at 2nd entry of VIRTUAL TABLE 004012

Example :9

Code: cpp

class Test
{
    virtual void fun1() { cout << "Test::fun1" << endl; }
    virtual void func1() { cout << "Test::func1" << endl; }
};

typedef void(*Fun)(void);

int main() 
{
    Test obj;
    Fun pFun = NULL;

    // calling 1st virtual function
    pFun = (Fun)*((int*)*(int*)(&obj+0)+0);
    pFun();

    // calling 2nd virtual function
    pFun = (Fun)*((int*)*(int*)(&obj+0)+1);
    pFun();

    return 0;
}

OUTPUT:

Test::fun1
Test::func1

Example 10: multiple Inheritance

Code: cpp

class Base1 
{
public:
    virtual void fun();
};

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

class Base3 
{
public:
    virtual void fun();
};

class Derive : public Base1, public Base2, public Base3 
{
};

int main() 
{
    Derive obj;
    cout << "Derive's Size = " << sizeof(obj) << endl;
    return 0;
}

OUTPUT:

Derive's Size = 12

Example 11: Calling Virtual Functions in case of Multiple Inheritance

Code: cpp

class Base1 
{
    virtual void fun1() { cout << "Base1::fun1()" << endl; }
    virtual void func1() { cout << "Base1::func1()" << endl; }
};

class Base2 {
    virtual void fun1() { cout << "Base2::fun1()" << endl; }
    virtual void func1() { cout << "Base2::func1()" << endl; }
};

class Base3 {
    virtual void fun1() { cout << "Base3::fun1()" << endl; }
    virtual void func1() { cout << "Base3::func1()" << endl; }
};

class Derive : public Base1, public Base2, public Base3 
{
public:
    virtual void Fn() 
    { 
        cout << "Derive::Fn" << endl; 
    }
    virtual void Fnc() 
    { 
        cout << "Derive::Fnc" << endl; 
    }
};

typedef void(*Fun)(void);

int main()
{
    Derive obj;
    Fun pFun = NULL;

    // calling 1st virtual function of Base1
    pFun = (Fun)*((int*)*(int*)((int*)&obj+0)+0);
    pFun();

    // calling 2nd virtual function of Base1
    pFun = (Fun)*((int*)*(int*)((int*)&obj+0)+1);
    pFun();

    // calling 1st virtual function of Base2
    pFun = (Fun)*((int*)*(int*)((int*)&obj+1)+0);
    pFun();

    // calling 2nd virtual function of Base2
    pFun = (Fun)*((int*)*(int*)((int*)&obj+1)+1);
    pFun();

    // calling 1st virtual function of Base3
    pFun = (Fun)*((int*)*(int*)((int*)&obj+2)+0);
    pFun();

    // calling 2nd virtual function of Base3
    pFun = (Fun)*((int*)*(int*)((int*)&obj+2)+1);
    pFun();

    // calling 1st virtual function of Drive
    pFun = (Fun)*((int*)*(int*)((int*)&obj+0)+2);
    pFun();

    // calling 2nd virtual function of Drive
    pFun = (Fun)*((int*)*(int*)((int*)&obj+0)+3);
    pFun();

    return 0;
}

OUTPUT:

Base1::fun
Base1::func
Base2::fun
Base2::func
Base3::fun
Base3::func
Drive::Fn
Drive::Fnc