Detail about How VPTR and Virtual table works

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
    

  1. 
class Test
    

    2. 


  2. 
{
    

    3. 

  3. 
public:
    

    4. 

  4. 
int data1;
    

    5. 

  5. 
int data2;
    

    6. 

  6. 
int fun1();
    

    7. 

  7. 
};
    

    8. 

  8.  
    

    

    9. 

  9. 
int main()
    

    10. 

  10. 
{
    

    11. 

  11. 
Test obj;
    

    12. 

  12. 
cout << "obj's Size = " << sizeof(obj) <<>
    

    13. 

  13. 
cout << "obj 's Address = " << &obj <<>
    

    14. 

  14. 
return 0;
    

    15. 

  15. 
}
    

    16.
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

    

  1. 
class Test
    

    2. 


  2. 
{
    

    3. 

  3. 
public:
    

    4. 

  4. 
int a;
    

    5. 

  5. 
int b;
    

    6. 

  6. 
};
    

    7. 

  7.  
    

    

    8. 

  8. 
class dTest : public Test
    

    9. 

  9. 
{
    

    10. 

  10. 
public:
    

    11. 

  11. 
int c;
    

    12. 

  12. 
};
    

    13. 

  13.  
    

    

    14. 

  14. 
int main()
    

    15. 

  15. 
{
    

    16. 

  16. 
Test obj1;
    

    17. 

  17. 
cout << "obj1's Size = " << sizeof(obj1) <<>
    

    18. 

  18. 
cout << "obj1's Address = " << &obj1 <<>
    

    19. 

  19. 
dTest obj2;
    

    20. 

  20. 
cout << "obj2's Size = "<< sizeof(obj2) <<>
    

    21. 

  21. 
cout << "obj2's Address = "<< &obj2 <<>
    

    22. 

  22. 
return 0;
    

    23. 

  23. 
}
    

    24.
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


    

  1. 
class Test
    

    2. 


  2. 
{
    

    3. 

  3. 
public:
    

    4. 

  4. 
int data;
    

    5. 

  5. 
virtual void fun1()
    

    6. 

  6. 
{
    

    7. 

  7. 
cout << "Test::fun1" <<>
    

    8. 

  8. 
}
    

    9. 

  9. 
};
    

    10. 

  10.  
    

    

    11. 

  11. 
int main()
    

    12. 

  12. 
{
    

    13. 

  13. 
Test obj;
    

    14. 

  14. 
cout << "obj's Size = " << sizeof(obj) <<>
    

    15. 

  15. 
cout << "obj's Address = " << &obj <<>
    

    16. 

  16. 
return 0;
    

    17. 

  17. 
}
    

    18.
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
    

  1. 
class Test
    

    2. 


  2. 
{
    

    3. 

  3. 
public:
    

    4. 

  4. 
int data;
    

    5. 

  5. 
virtual void fun1() { cout << "Test::fun1" << class="br0" style="color: gray; font-weight: bold; ">}
    

    6. 

  6. 
virtual void fun2() { cout << "Test::fun2" << class="br0" style="color: gray; font-weight: bold; ">}
    

    7. 

  7. 
virtual void fun3() { cout << "Test::fun3" << class="br0" style="color: gray; font-weight: bold; ">}
    

    8. 

  8. 
virtual void fun4() { cout << "Test::fun4" << class="br0" style="color: gray; font-weight: bold; ">}
    

    9. 

  9. 
};
    

    10. 

  10.  
    

    

    11. 

  11. 
int main()
    

    12. 

  12. 
{
    

    13. 

  13. 
Test obj;
    

    14. 

  14. 
cout << "obj's Size = " << sizeof(obj) <<>
    

    15. 

  15. 
cout << "obj's Address = " << &obj <<>
    

    16. 

  16. 
return 0;
    

    17. 

  17. 
}
    

    18.
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

    

  1. 
class Test
    

    2. 

  2. 
{
    

    3. 

  3. 
public:
    

    4. 

  4. 
int a;
    

    5. 

  5. 
int b;
    

    6. 

  6. 
Test(int temp1 = 0, int temp2 = 0)
    

    7. 

  7. 
{
    

    8. 

  8. 
a=temp1 ;
    

    9. 

  9. 
b=temp2 ;
    

    10. 

  10. 
}
    

    11. 

  11. 
int getA() 
    

    12. 

  12. 
{
    

    13. 

  13. 
return a;
    

    14. 

  14. 
}
    

    15. 

  15. 
int getB() 
    

    16. 

  16. 
{
    

    17. 

  17. 
return b;
    

    18. 

  18. 
}
    

    19. 

  19. 
virtual ~Test();
    

    20. 

  20. 
};
    

    21. 

  21. 

    

    22. 

  22. 
int main()
    

    23. 

  23. 
{
    

    24. 

  24. 
Test obj(5, 10);
    

    25. 

  25. 

    

    26. 

  26. 
// Changing a and b
    

    27. 

  27. 
int* pInt = (int*)&obj;
    

    28. 

  28. 
*(pInt+0) = 100;   
    

    29. 

  29. 
*(pInt+1) = 200;   
    

    30. 

  30. 

    

    31. 

  31. 
cout << "a = " << class="me1">getA() <<>
    

    32. 

  32. 
cout << "b = " << class="me1">getB() <<>
    

    33. 

  33. 
return 0;
    

    34. 

  34. 
}
    

    35.
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

    

  1. 
class Test
    

    2. 


  2. 
{
    

    3. 

  3. 
virtual void fun1()
    

    4. 

  4. 
{
    

    5. 

  5. 
cout << "Test::fun1" <<>
    

    6. 

  6. 
}
    

    7. 

  7. 
};
    

    8. 

  8. 

    

    9. 

  9. 
int main()
    

    10. 

  10. 
{
    

    11. 

  11. 
Test obj;
    

    12. 

  12. 
cout << "VPTR's Address " << (int*)(&obj+0) <<>
    

    13. 

  13. 
cout << "VPTR's Value " << (int*)*(int*)(&obj+0) <<>
    

    14. 

  14. 
return 0;
    

    15. 

  15. 
}
    

    16.
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

    

  1. 
class Test
    

    2. 


  2. 
{
    

    3. 

  3. 
virtual void fun1()
    

    4. 

  4. 
{
    

    5. 

  5. 
cout << "Test::fun1" <<>
    

    6. 

  6. 
}
    

    7. 

  7. 
};
    

    8. 

  8. 
typedef void (*Fun)(void);
    

    9. 

  9. 

    

    10. 

  10. 
int main()
    

    11. 

  11. 
{
    

    12. 

  12. 
Test obj;
    

    13. 

  13. 
cout << "VPTR's Address " << (int*)(&obj+0) <<>
    

    14. 

  14. 
cout << " VIRTUAL TABLE 's Address " << (int*)*(int*)(&obj+0) << class="co1" style="color: green; ">// Value of VPTR
    

    15. 

  15. 
cout << "Value at first entry of VIRTUAL TABLE " << (int*)*(int*)*(int*)(&obj+0) <<>
    

    16. 

  16. 

    

    17. 

  17. 
Fun pFun = (Fun)*(int*)*(int*)(&obj+0);   // calling Virtual function
    

    18. 

  18. 
pFun();
    

    19. 

  19. 
return 0;
    

    20. 

  20. 
}
    

    21.
OUTPUT:
VPTR's Address 0012FF7C
VIRTUAL TABLE 's Address 0046C0EC
Value at first entry of VIRTUAL TABLE 0040100A
Test: fun1


Example 8:

Code: cpp
    

  1. 
class Test
    

    2. 

  2. 
{
    

    3. 

  3. 
virtual void fun1() { cout << "Test::fun1" << class="br0" style="color: gray; font-weight: bold; ">}
    

    4. 

  4. 
virtual void func1() { cout << "Test::func1" << class="br0" style="color: gray; font-weight: bold; ">}
    

    5. 

  5. 
};
    

    6. 

  6. 

    

    7. 

  7. 
int main()
    

    8. 

  8. 
{
    

    9. 

  9. 
Test obj;
    

    10. 

  10. 

    

    11. 

  11. 
cout << "VPTR's Address " << (int*)(&obj+0) <<>
    

    12. 

  12. 
cout << "VIRTUAL TABLE 's Address"<< (int*)*(int*)(&obj+0) <<>
    

    13. 

  13. 

    

    14. 

  14. 
// Calling Virtual table functions
    

    15. 

  15. 
cout << "Value at 1st entry of VTable " << (int*)*((int*)*(int*)(&obj+0)+0) <<>
    

    16. 

  16. 
cout << "Value at 2nd entry of VTable " << (int*)*((int*)*(int*)(&obj+0)+1) <<>
    

    17. 

  17. 

    

    18. 

  18. 
return 0;
    

    19. 

  19. 
}
    

    20.

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
    

  1. 
class Test
    

    2. 

  2. 
{
    

    3. 

  3. 
virtual void fun1() { cout << "Test::fun1" << class="br0" style="color: gray; font-weight: bold; ">}
    

    4. 

  4. 
virtual void func1() { cout << "Test::func1" << class="br0" style="color: gray; font-weight: bold; ">}
    

    5. 

  5. 
};
    

    6. 

  6. 

    

    7. 

  7. 
typedef void(*Fun)(void);
    

    8. 

  8. 

    

    9. 

  9. 
int main()
    

    10. 

  10. 
{
    

    11. 

  11. 
Test obj;
    

    12. 

  12. 
Fun pFun = NULL;
    

    13. 

  13. 

    

    14. 

  14. 
// calling 1st virtual function
    

    15. 

  15. 
pFun = (Fun)*((int*)*(int*)(&obj+0)+0);
    

    16. 

  16. 
pFun();
    

    17. 

  17. 

    

    18. 

  18. 
// calling 2nd virtual function
    

    19. 

  19. 
pFun = (Fun)*((int*)*(int*)(&obj+0)+1);
    

    20. 

  20. 
pFun();
    

    21. 

  21. 

    

    22. 

  22. 
return 0;
    

    23. 

  23. 
}
    

    24.
OUTPUT:

Test::fun1
Test::func1

Example 10: multiple Inheritance

Code: cpp
    

  1. 
class Base1
    

    2. 

  2. 
{
    

    3. 

  3. 
public:
    

    4. 

  4. 
virtual void fun();
    

    5. 

  5. 
};
    

    6. 

  6. 

    

    7. 

  7. 
class Base2
    

    8. 

  8. 
{
    

    9. 

  9. 
public:
    

    10. 

  10. 
virtual void fun();
    

    11. 

  11. 
};
    

    12. 

  12. 

    

    13. 

  13. 
class Base3
    

    14. 

  14. 
{
    

    15. 

  15. 
public:
    

    16. 

  16. 
virtual void fun();
    

    17. 

  17. 
};
    

    18. 

  18. 

    

    19. 

  19. 
class Derive : public Base1, public Base2, public Base3
    

    20. 

  20. 
{
    

    21. 

  21. 
};
    

    22. 

  22. 

    

    23. 

  23. 
int main()
    

    24. 

  24. 
{
    

    25. 

  25. 
Derive obj;
    

    26. 

  26. 
cout << "Derive's Size = " << sizeof(obj) <<>
    

    27. 

  27. 
return 0;
    

    28. 

  28. 
}
    

    29.
OUTPUT:

Derive's Size = 12


Example 11: Calling Virtual Functions in case of Multiple Inheritance
Code: cpp
    

  1. 
class Base1
    

    2. 

  2. 
{
    

    3. 

  3. 
virtual void fun1() { cout << "Base1::fun1()" << class="br0" style="color: gray; font-weight: bold; ">}
    

    4. 

  4. 
virtual void func1() { cout << "Base1::func1()" << class="br0" style="color: gray; font-weight: bold; ">}
    

    5. 

  5. 
};
    

    6. 

  6. 

    

    7. 

  7. 
class Base2 {
    

    8. 

  8. 
virtual void fun1() { cout << "Base2::fun1()" << class="br0" style="color: gray; font-weight: bold; ">}
    

    9. 

  9. 
virtual void func1() { cout << "Base2::func1()" << class="br0" style="color: gray; font-weight: bold; ">}
    

    10. 

  10. 
};
    

    11. 

  11. 

    

    12. 

  12. 
class Base3 {
    

    13. 

  13. 
virtual void fun1() { cout << "Base3::fun1()" << class="br0" style="color: gray; font-weight: bold; ">}
    

    14. 

  14. 
virtual void func1() { cout << "Base3::func1()" << class="br0" style="color: gray; font-weight: bold; ">}
    

    15. 

  15. 
};
    

    16. 

  16. 

    

    17. 

  17. 
class Derive : public Base1, public Base2, public Base3
    

    18. 

  18. 
{
    

    19. 

  19. 
public:
    

    20. 

  20. 
virtual void Fn()
    

    21. 

  21. 
{
    

    22. 

  22. 
cout << "Derive::Fn" <<>
    

    23. 

  23. 
}
    

    24. 

  24. 
virtual void Fnc()
    

    25. 

  25. 
{
    

    26. 

  26. 
cout << "Derive::Fnc" <<>
    

    27. 

  27. 
}
    

    28. 

  28. 
};
    

    29. 

  29. 

    

    30. 

  30. 
typedef void(*Fun)(void);
    

    31. 

  31. 

    

    32. 

  32. 
int main()
    

    33. 

  33. 
{
    

    34. 

  34. 
Derive obj;
    

    35. 

  35. 
Fun pFun = NULL;
    

    36. 

  36. 

    

    37. 

  37. 
// calling 1st virtual function of Base1
    

    38. 

  38. 
pFun = (Fun)*((int*)*(int*)((int*)&obj+0)+0);
    

    39. 

  39. 
pFun();
    

    40. 

  40. 

    

    41. 

  41. 
// calling 2nd virtual function of Base1
    

    42. 

  42. 
pFun = (Fun)*((int*)*(int*)((int*)&obj+0)+1);
    

    43. 

  43. 
pFun();
    

    44. 

  44. 

    

    45. 

  45. 
// calling 1st virtual function of Base2
    

    46. 

  46. 
pFun = (Fun)*((int*)*(int*)((int*)&obj+1)+0);
    

    47. 

  47. 
pFun();
    

    48. 

  48. 

    

    49. 

  49. 
// calling 2nd virtual function of Base2
    

    50. 

  50. 
pFun = (Fun)*((int*)*(int*)((int*)&obj+1)+1);
    

    51. 

  51. 
pFun();
    

    52. 

  52. 

    

    53. 

  53. 
// calling 1st virtual function of Base3
    

    54. 

  54. 
pFun = (Fun)*((int*)*(int*)((int*)&obj+2)+0);
    

    55. 

  55. 
pFun();
    

    56. 

  56. 

    

    57. 

  57. 
// calling 2nd virtual function of Base3
    

    58. 

  58. 
pFun = (Fun)*((int*)*(int*)((int*)&obj+2)+1);
    

    59. 

  59. 
pFun();
    

    60. 

  60. 

    

    61. 

  61. 
// calling 1st virtual function of Drive
    

    62. 

  62. 
pFun = (Fun)*((int*)*(int*)((int*)&obj+0)+2);
    

    63. 

  63. 
pFun();
    

    64. 

  64. 

    

    65. 

  65. 
// calling 2nd virtual function of Drive
    

    66. 

  66. 
pFun = (Fun)*((int*)*(int*)((int*)&obj+0)+3);
    

    67. 

  67. 
pFun();
    

    68. 

  68. 

    

    69. 

  69. 
return 0;
    

    70. 

  70. 
}
    

    71.

OUTPUT:

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