Details about VPTR, Virtual table, Virtual Function - MYCPLUS

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January 22, 2008 at 7:47 am #2061

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Assumption: machine is 32-bit .
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

<br />
class Test<br />
{<br />
  public:<br />
    int data1;<br />
    int data2;<br />
    int fun1();<br />
};<br />
int main()<br />
{<br />
  Test obj;<br />
  cout << "obj's Size = " << sizeof(obj) << endl;<br />
  cout << "obj 's Address = " << &#038;obj << endl;<br />
  return 0;<br />
}<br />

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

<br />
class Test<br />
{<br />
public:<br />
  int a;<br />
  int b;<br />
};<br />
class dTest : public Test<br />
{<br />
public:<br />
  int c;<br />
};<br />
int main()<br />
{<br />
  Test obj1;<br />
  cout << "obj1's Size = " << sizeof(obj1) << endl;<br />
  cout << "obj1's Address = " << &#038;obj1 << endl;<br />
  dTest obj2;<br />
  cout << "obj2's Size = "<< sizeof(obj2) << endl;<br />
  cout << "obj2's Address = "<< &#038;obj2 << endl;<br />
  return 0;<br />
}<br />

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.

<br />
class Test<br />
{<br />
public:<br />
  int data;<br />
  virtual void fun1()<br />
  {<br />
    cout << "Test::fun1" << endl;<br />
  }<br />
};<br />
int main()<br />
{<br />
  Test obj;<br />
  cout << "obj's Size = " << sizeof(obj) << endl;<br />
  cout << "obj's Address = " << &#038;obj << endl;<br />
  return 0;<br />
}

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

<br />
class Test<br />
{<br />
public:<br />
  int data;<br />
  virtual void fun1() { cout << "Test::fun1" << endl; }<br />
  virtual void fun2() { cout << "Test::fun2" << endl; }<br />
  virtual void fun3() { cout << "Test::fun3" << endl; }<br />
  virtual void fun4() { cout << "Test::fun4" << endl; }<br />
};<br />
int main()<br />
 {<br />
  Test obj;<br />
  cout << "obj's Size = " << sizeof(obj) << endl;<br />
  cout << "obj's Address = " << &#038;obj << endl;<br />
  return 0;<br />
 }<br />

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:

<br />
class Test<br />
 {<br />
public:<br />
  int a;<br />
  int b;<br />
  Test(int temp1 = 0, int temp2 = 0)<br />
  {<br />
     a=temp1 ;<br />
     b=temp2 ;<br />
  }<br />
  int getA() <br />
  {<br />
     return a;<br />
  }<br />
  int getB() <br />
  {<br />
     return b;<br />
  }<br />
  virtual ~Test();<br />
};<br />
int main()<br />
{<br />
  Test obj(5, 10);<br />
// Changing a and b<br />
  int* pInt = (int*)&obj;<br />
  *(pInt+0) = 100;  <br />
  *(pInt+1) = 200;  <br />
  cout << "a = " << obj.getA() << endl;<br />
  cout << "b = " << obj.getB() << endl;<br />
  return 0;<br />
}<br />

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

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

// 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:

<br />
class Test<br />
{<br />
  virtual void fun1()<br />
  {<br />
     cout << "Test::fun1" << endl;<br />
  }<br />
};<br />
int main()<br />
{<br />
  Test obj;<br />
  cout << "VPTR's Address " << (int*)(&#038;obj+0) << endl;<br />
  cout << "VPTR's Value " << (int*)*(int*)(&#038;obj+0) << endl;<br />
  return 0;<br />
}<br />

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:

<br />
  #include <iostream><br />
using namespace std;<br />
class Test<br />
 {<br />
   virtual void fun1()<br />
   {<br />
    cout << "Test::fun1" << endl;<br />
   }<br />
};<br />
typedef void (*Fun)(void);<br />
int main()<br />
{<br />
  Test obj;<br />
  cout << "VPTR's Address " << (int*)(&#038;obj+0) << endl;<br />
  cout << " VIRTUAL TABLE 's Address " << (int*)*(int*)(&#038;obj+0) << endl; // Value of VPTR<br />
  cout << "Value at first entry of VIRTUAL TABLE " << (int*)*(int*)*(int*)(&#038;obj+0) << endl;<br />
 <br />
  Fun pFun = (Fun)*(int*)*(int*)(&#038;obj+0);   // calling Virtual function<br />
  pFun();<br />
  return 0;<br />
}<br />
</iostream>

#include <iostream>

using namespace std;

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;

}

</iostream>

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

Example 8:

<br />
class Test<br />
{<br />
  virtual void fun1() { cout << "Test::fun1" << endl; }<br />
  virtual void func1() { cout << "Test::func1" << endl; }<br />
};<br />
int main()<br />
 {<br />
  Test obj;<br />
  cout << "VPTR's Address " << (int*)(&#038;obj+0) << endl;<br />
  cout << "VIRTUAL TABLE 's Address"<< (int*)*(int*)(&#038;obj+0) << endl;<br />
  // Calling Virtual table functions<br />
  cout << "Value at 1st entry of VTable " << (int*)*((int*)*(int*)(&#038;obj+0)+0) << endl;<br />
  cout << "Value at 2nd entry of VTable " << (int*)*((int*)*(int*)(&#038;obj+0)+1) << endl;<br />
  return 0;<br />
}<br />

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

<br />
class Test<br />
{<br />
  virtual void fun1() { cout << "Test::fun1" << endl; }<br />
  virtual void func1() { cout << "Test::func1" << endl; }<br />
};<br />
typedef void(*Fun)(void);<br />
int main()<br />
{<br />
  Test obj;<br />
  Fun pFun = NULL;<br />
 <br />
  // calling 1st virtual function<br />
  pFun = (Fun)*((int*)*(int*)(&#038;obj+0)+0);<br />
  pFun();<br />
  // calling 2nd virtual function<br />
  pFun = (Fun)*((int*)*(int*)(&#038;obj+0)+1);<br />
  pFun();<br />
  return 0;<br />
}<br />

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

<br />
class Base1<br />
{<br />
public:<br />
  virtual void fun();<br />
};<br />
class Base2<br />
{<br />
public:<br />
  virtual void fun();<br />
};<br />
class Base3<br />
{<br />
public:<br />
  virtual void fun();<br />
};<br />
class Derive : public Base1, public Base2, public Base3<br />
{<br />
};<br />
int main()<br />
{<br />
  Derive obj;<br />
  cout << "Derive's Size = " << sizeof(obj) << endl;<br />
  return 0;<br />
}<br />

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

<br />
class Base1<br />
{<br />
  virtual void fun1() { cout << "Base1::fun1()" << endl; }<br />
  virtual void func1() { cout << "Base1::func1()" << endl; }<br />
};<br />
class Base2 {<br />
  virtual void fun1() { cout << "Base2::fun1()" << endl; }<br />
  virtual void func1() { cout << "Base2::func1()" << endl; }<br />
};<br />
class Base3 {<br />
  virtual void fun1() { cout << "Base3::fun1()" << endl; }<br />
  virtual void func1() { cout << "Base3::func1()" << endl; }<br />
};<br />
class Derive : public Base1, public Base2, public Base3<br />
{<br />
public:<br />
  virtual void Fn()<br />
  {<br />
  cout << "Derive::Fn" << endl;<br />
  }<br />
  virtual void Fnc()<br />
  {<br />
  cout << "Derive::Fnc" << endl;<br />
  }<br />
};<br />
typedef void(*Fun)(void);<br />
int main()<br />
{<br />
  Derive obj;<br />
  Fun pFun = NULL;<br />
 <br />
  // calling 1st virtual function of Base1<br />
  pFun = (Fun)*((int*)*(int*)((int*)&#038;obj+0)+0);<br />
  pFun();<br />
  // calling 2nd virtual function of Base1<br />
  pFun = (Fun)*((int*)*(int*)((int*)&#038;obj+0)+1);<br />
  pFun();<br />
  // calling 1st virtual function of Base2<br />
  pFun = (Fun)*((int*)*(int*)((int*)&#038;obj+1)+0);<br />
  pFun();<br />
  // calling 2nd virtual function of Base2<br />
  pFun = (Fun)*((int*)*(int*)((int*)&#038;obj+1)+1);<br />
  pFun();<br />
  // calling 1st virtual function of Base3<br />
  pFun = (Fun)*((int*)*(int*)((int*)&#038;obj+2)+0);<br />
  pFun();<br />
  // calling 2nd virtual function of Base3<br />
  pFun = (Fun)*((int*)*(int*)((int*)&#038;obj+2)+1);<br />
  pFun();<br />
  // calling 1st virtual function of Drive<br />
  pFun = (Fun)*((int*)*(int*)((int*)&#038;obj+0)+2);<br />
  pFun();<br />
  // calling 2nd virtual function of Drive<br />
  pFun = (Fun)*((int*)*(int*)((int*)&#038;obj+0)+3);<br />
  pFun();<br />
  return 0;<br />
}<br />

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::Fncadministrator2008-01-27 11:34:41

February 15, 2008 at 12:20 am #3322
bharat123
Participant
Hiin the example 10 of multiple inheritance ,i could’t understand the why the sizeof Derived class is 12 bytes, plz explain why this happening.
waiting for ur reply……………

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