Detail about How VPTR and Virtual table works - MYCPLUS

This topic has 3 replies, 4 voices, and was last updated 16 years, 6 months ago by Adetutu.

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March 10, 2008 at 1:30 am #2072

Participant

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;
}

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:
obj’s Size = 8
obj ‘s Address = 0012FF7C

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

Example 2: Memory Layout of Derived class

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;
}

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;
}

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;
}

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;
}

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

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;
}

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:

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;
}

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:

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;
}

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

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;
}

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

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;
}

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

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;
}

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

March 23, 2008 at 8:04 pm #3343
deepaksancheti
Participant
I am sorry to say…
But you r wrong at the very first example….
It will simply occupy only 4 bytes…..
How come 8 bytes?????
March 31, 2008 at 12:47 am #3344
valaraukar
Participant
int data1 has 4 byte
and
int data2 has 4 byte
then total 8 byte
May 19, 2008 at 6:40 am #3345
Adetutu
Participant
Hi,
Please confirm on which platform you are testing your program. On TurboC3 it has some diffrent output. I hope You are using gcc compiler on linux or solaris

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