Details about VPTR, Virtual table, Virtual Function

[shalizy]

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 />

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<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 />

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 />

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<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 />

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 /> }

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<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 /> }

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 />

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<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 />

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 />

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<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 />

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 />

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<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 />

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 />

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<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 />

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>

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<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>

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 />

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<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 />

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 />

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<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 />

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 />

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<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 />

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 />

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<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 />

OUTPUT:
Base1::fun
Base1::func
Base2::fun
Base2::func
Base3::fun
Base3::func
Drive::Fn
Drive::Fncadministrator2008-01-27 11:34:41

[bharat123]

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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