C++
                       OBJECT ORIENTED PROGRAMMING
                              POLYMORPHISM
                                               -Sanchit Karve
                                                born2c0de@hotmail.com



REQUIREMENTS:
-> Should know Inheritance in C++

That's it. But in the end I explain how the compiler implements polymorphism and
so knowledge of Assembly Language will be a great help in understanding
Polymorphism. Nevertheless I will still try to make the assembly explanation as
simple as possible.So even if you don't know Assembly,try understanding it.




I.                         WHAT IS POLYMORPHISM?


Polymorphism is in short the ability to call different functions by just using
one type of function call.It is a lot useful since it can group classes and
their functions together.Polymorphism is the most important part of Object-
Oriented Programming. Some people feel that if they have an idea of what classes
are they have stepped in the object-oriented world.But this is not true.
Polymorphism is the core of object-oriented programming and if anybody stops
here he's missing out the best part of Object Oriented Programming(OOP).All this
may seem a lot hard to understand but read on......you'll understand better as
you keep reading.


Let us now try to understand polymorphism with the help of an example.Suppose we
want to draw a picture consisting of circles,squares,lines and triangles.So we
can make a class Shape and create an instance of it like this:
     Shape *s[100];

Now all the addresses of the objects of the other classes(line,circle etc.) are
stored in the Shape Array.And then to draw the Picture all we have to do is this:
  for(int i=0;i<=100;i++)
      s[i]->draw();

Now as the loop runs different draw functions of each class is called. This is
great because:
i. Functions from different classes are executed through the same function call.
i. The Array s[] has been defined to contain shape pointers and not square or
   triangle pointers.

This can be done by using Virtual Functions.



II.                        VIRTUAL FUNCTIONS????????

The Literal Meaning of Virtual means to appear like something while in reality
it is something else ie. when virtual functions are used, a program appears to
call a function of one class but actually it may be calling a function from
another class.In the previous example draw() is a virtual function since it
calls different draw functions from different classes by using the same function
call draw();


Now how do we know which version of draw() would be called during execution?
Which draw() function would get used depends on the contents of s[i].But for
this polymorphic approach to work we must satisfy the following conditions:
->The Base class must contain a draw() function which is declared virtual.
->All other classes(line,circle etc.) should be derived from the base class.


Well, all this may be hard to understand in just one go so we'll start using
programs that'll help us understand better.Here's the First One.

#include <iostream>

using namespace std;

class base                          //Base Class
{
public:
   void func()
   {
     cout<<"In base::func()\n";
   }
};

class d1:public base                       // Derived Class 1
{
public:
   void func()
   {
    cout<<"In d1::func()\n";
   }
};

class d2:public base     // Derived Class 2
{
public:
   void func()
   {
    cout<<"In d2::func()\n";
   }
};

int main()
{
  d1 d;
  base *b=&d;
  b->func();
  d2 e;
  b=&e;
  b->func();
  return 0;
}


Run this program and you would see that the output would be:
In base::func()
In base::func()

Shouldn't this statement give an error? (b=&e;) No. Since the compiler allows a
pointer of a base class to accept addresses of derived class objects.This is
known as UPCASTING.Here the Compiler looks at the type of pointer b and since it
belongs to the base class it calls the base class function.

But now, let's make a slight modification in our program. Precede the
declaration of func() in the base class with the keyword virtual so that it
looks like this:

virtual void func()
{
  cout<<"In base::func()\n";
}


Now Compile and Run the Program. Now the Output is:
In d1::func()
In d2::func()

This time the Compiler looks at the contents of the pointer instead of it's type.
Hence since addresses of objects of d1 and d2 classes are stored in *b the
respective func() is called.But this way how does the compiler know which
function to compile when it doesn't know which object's address 'b' might
contain?Which version does the compiler call?

Actually even the compiler does not know which function to call at compile-time.
Hence it decides which function to call at run-time with the help of a table
called VTABLE. Using this table the compiler finds what object is pointed by
the pointer b and then calls the appropriate function. VTABLE is explained later.

The method by which the compiler decides which function to call at run-time is
known as late-binding or dynamic-binding. It slows down the program but makes it
a lot more flexible.

III.                    PURE VIRTUAL FUNCTIONS


Now we realise that since the base class virtual function never gets called
anyway we'd better keep it's body blank.But there's a better way to do this.
We can change the virtual function func() in the base class to the following:
virtual void func()=0;

The =0 is not an assignment operator here but it is just a way of telling the
compiler that the function has no body.But there is another side of this. An
object of a class which contains a pure virtual function cannot be created.It
seems logical enough ie. If you have classes triangle,square,circle derived from
shape class we wouldn't want to make an object of the shape class.Hence the
shape class should be provided with a pure virtual function.If you even try to
create an object of a class containing a pure virtual function the compiler
would report an error even pointing out which pure virtual function prevents you
from creating an object.


IV.                         HOW VIRTUAL FUNCTIONS WORK

Using Virtual Functions is just one part of polymorphism and knowing how they
work completes the other half.When the keyword 'virtual' is inserted in the
declaration of the function the compiler inserts all mechanisms in the program
to use Virtual Functions. Each Class has a VTABLE that stores the functions that
it can access and Each class contains a VPTR which can access the VTABLE.Look at
this program and the table below it and you will understand the VTABLE and the
VPTR.


#include <stdio.h>


class item
{
 public:
    virtual void price()
    {
      printf("In item::price()\n");
    }
    virtual void type()
    {
      printf("In item::type()\n");
    }
    void display();
};
void item::display(){printf("In item::display()\n");}

class microwave:public item
{
public:
   void price()
   {
     printf("Microwave::Price()\n");
   }

   void type()
   {
     printf("Microwave::type()\n");
   }
};

class computer:public item
{
 public:
    void price()
    {
       printf("Compuer::Price()\n");
    }
};

class radio:public item
{
public:
   void type()
   {
     printf("radio::type()\n");
   }
};

int main()
{
 microwave m1;
 computer c1;
 radio r1;

 item *i=&m1;
 i->price();
 i->type();
 printf("\n");

 i=&c1;
 i->price();
 i->type();
 i->display();
 printf("\n");

 i=&r1;
 i->price();
 i->type();
 printf("\n");

 microwave m2;
 i=&m2;
 i->price();
 i->type();
 i->display();
 printf("\n");
  return 0;
}

The Output of this Program would be:

Microwave::Price()
Microwave::type()

Compuer::Price()
In item::type()
In item::display()

In item::price()
radio::type()

Microwave::Price()
Microwave::type()
In item::display()

Now here is how the VTABLE of each class Looks Like:
-------------------------------------------------------------------------------|
ITEM POINTERS   |     OBJECTS         |                 VTABLES                |
-------------------------------------------------------------------------------|
i-------------> |    item{VPTR}---->  |        &item::price()                  |
               |                     |        &item::type()                   |
-------------------------------------------------------------------------------|
               |                     |                                        |
i-------------> |microwave m1,m2{VPTR}|        &microwave::price()             |
               |                     |        &microwave::type()              |
-------------------------------------------------------------------------------|
               |                     |                                        |
i-------------> | computer c1{VPTR}-> |        &computer::price()              |
               |                     |        &item::type()                   |
-------------------------------------------------------------------------------|
               |                     |                                        |
i-------------->| radio r1{VPTR}----> |        &item::price()                  |
               |                     |        &radio::type()                  |
-------------------------------------------------------------------------------|


First the VPTR Pointer is initialised to it's proper VTABLE by the contructor
which is automatically done by the compiler.When a Virtual Function is being
called the VPTR looks up the VTABLE and calls the virtual function.If the
function is not present in the VTABLE [like here display()] then the function of
the base class is called. So everywhere where the display function is called
item::display() is called everytime.No matter how many objects of a class are
created they all point to the same VTABLE of the class.


V.                    ARE VIRTUAL FUNCTIONS OPTIONAL?

Normal Function calls are called by the Assembly instruction 'call' while
virtual functions require complex instructions. This takes up code space as well
as execution time.

Virtual Functions reduce the code's speed. Some languages like SmallTalk perform
Late Binding everytime a function is called and hence SmallTalk Programs aren't
fast enough. But C++ is a Superset of C where Efficiency is important and hence
C++ allows both static binding as well as late binding. The default convention
used is static binding so that there is no loss in speed.

Don't stop using Virtual Functions in your classes just because they reduce
execution speed. Infact it makes it easier to manage and code and it's
advantages are more than it's disadvantages. So wherever possible use Virtual
Functions in your classes.


VI.                             MISCELLENEOUS

1)  If a Virtual Function is called within a derived classes constructor or
   destructor then the derived function is always called.

2)  If b is a base class pointer and d is a derived class pointer then b=d will
   copy only the base class contents and remove the derived class contents.This
   is known as Object Slicing and should be avoided.

3)  For a Virtual Function to work the function must be present in tha base
   class even though it is declared virtual in the derived class.

4)  Virtual Destructors can also be used and they allow execution of the derived
   destructor first and then it calls itself.
   

VII.                           VIRTUAL BASE CLASSES


Consider a situation when a 'base' class has two classes derived from it.For
example derived1 and  derived2.
Suppose we create another class which derives itself from both the derived
classes ie. derived3.Now suppose a member function of derived3 wants to access
data or functions in a base class.Since derived1 and derived2 are are derived
from base each inherits a copy of base.This copy is referred to as a subobject.
Now when derived3 refers to the data in the base class, which of the two copies
should it access? The compiler notices this ambiguous situation and reports an
error. To get rid of this we should make derived1 and derived2 as virtual base
classes.This is shown in the following program.

#include <iostream>

using namespace std;

class base
{
   protected:
   int data;
  
   public:
   base()
   {
    data=10;
   }
};

class derived1 : virtual public base
{};

class derived2 : virtual public base
{};

class derived3 : public derived1,public derived2
{
   public:
   int getdata()
   {
      return data;
   }
};

int main()
{
  derived3 d3;
  int val=d3.getdata();
  cout<<val<<endl;
  return 0;
}

Using the keyword virtual in the two classes derived1 and derived2 makes them
share a single subobject of the base class hence eliminating all ambiguity since
there is only one subobject for derived3 to access.Hence derived1 and derived2
are known as virtual base classes.



VIII.                       VTABLE COMPILATION PROOF
 THIS PROGRAM IS COMPILED BY BORLAND C++ 5.02
Lot of people know that the compiler uses a VTABLE to implement Virtual
Functions but few get to see the actual code that the compiler generates.To save
space I shall Compile the same program which uses the item,microwave,computer
and radio class and explain how the compiler implements Virtual Functions. I
have used IDA Pro to disassemble this program. Here is the disassembled listing
followed by the explanation. I have included line numbers so that I can just
refer to the line numbers while explaining how the program works.


;                           THE FUNCTION  main()
_main  proc near

m2_VPTR  = dword    ptr -10h
r1_VPTR  = dword    ptr -0Ch
c1_VPTR  = dword    ptr -8
m1_VPTR  = dword    ptr -4
argc  = dword    ptr  8
argv  = dword    ptr  0Ch
envp  = dword    ptr  10h

1        push    ebp
2  mov    ebp, esp
3  add    esp, 0FFFFFFF0h; 16 bytes allocated on    the stack
4  push    ebx; Register Saved
5  mov    [ebp+m1_VPTR], offset item_VTABLE
6  mov    [ebp+m1_VPTR], offset microwve_VTABLE
7  mov    [ebp+c1_VPTR], offset item_VTABLE
8  mov    [ebp+c1_VPTR], offset computer_VTABLE
9  mov    [ebp+r1_VPTR], offset item_VTABLE
10  mov    [ebp+r1_VPTR], offset radio_VTABLE
11  lea    ebx, [ebp+m1_VPTR]
12  push    ebx; this*    pushed
13  mov    eax, [ebx]
14  call    dword ptr [eax]; Calls    microwve_price
15  pop    ecx; 4 bytes freed    used up    by this*
16  push    ebx; this*    pushed
17  mov    edx, [ebx]
18  call    dword ptr [edx+4]; Calls microwve_type
19  pop    ecx; 4 bytes freed    used up    by this*
20  push    offset newline; __va_args
21  call    _printf
22  pop    ecx; 4 bytes cleared used up by argument
23  lea    ebx, [ebp+c1_VPTR]
24  push    ebx; this*    pushed
25  mov    eax, [ebx]
26  call    dword ptr [eax]
27  pop    ecx
28  push    ebx
29  mov    edx, [ebx]
30  call    dword ptr [edx+4]
31  pop    ecx
32  push    ebx
33  call    item_display
34  pop    ecx
35  push    offset newline1; __va_args
36  call    _printf
37  pop    ecx
38  lea    ebx, [ebp+r1_VPTR]
39  push    ebx
40  mov    eax, [ebx]
41  call    dword ptr [eax]
42  pop    ecx
43  push    ebx
44  mov    edx, [ebx]
45  call    dword ptr [edx+4]
46  pop    ecx
47  push    offset newline2; __va_args
48  call    _printf
49  pop    ecx
50  mov    [ebp+m2_VPTR], offset item_VTABLE
51  mov    [ebp+m2_VPTR], offset microwve_VTABLE
52  lea    ebx, [ebp+m2_VPTR]
53  push    ebx
54  mov    eax, [ebx]
55  call    dword ptr [eax]
56  pop    ecx
57  push    ebx
58  mov    edx, [ebx]
59  call    dword ptr [edx+4]
60  pop    ecx
61  push    ebx
62  call    item_display
63  pop    ecx
64  push    offset newline3; __va_args
65  call    _printf
66  pop    ecx
67  pop    ebx
68  mov    esp, ebp
69  pop    ebp
70  retn
_main  endp

item_price    proc near            ; Function item::price()
1  push    ebp
2  mov    ebp, esp
3  push    offset aInItemPrice;__va_args
4  call    _printf
5  pop    ecx
6  pop    ebp
7  retn
item_price    endp


radio_type    proc near              ;   Function radio::type()
1  push    ebp
2  mov    ebp, esp
3  push    offset aRadioType; __va_args
4  call    _printf
5  pop    ecx
6  pop    ebp
7  retn
radio_type    endp


computer_price    proc near              ;  Function computer::price()
1  push    ebp
2  mov    ebp, esp
3  push    offset aCompuerPrice; __va_args
4  call    _printf
5  pop    ecx
6  pop    ebp
7  retn
computer_price    endp


item_type    proc near              ; Function item::type()

1  push    ebp
2  mov    ebp, esp
3  push    offset aInItemType; __va_args
4  call    _printf
5  pop    ecx
6  pop    ebp
7  retn
item_type    endp


microwve_price    proc near                ;Function microwave::price()
1  push    ebp
2  mov    ebp, esp
3  push    offset aMicrowavePrice; __va_args
4  call    _printf
5  pop    ecx
6  pop    ebp
7  retn
microwve_price    endp


microwve_type    proc near                ;Function microwave::type()
1  push    ebp
2  mov    ebp, esp
3  push    offset aMicrowaveType;    __va_args
4  call    _printf
5  pop    ecx
6  pop    ebp
7  retn
microwve_type    endp


;                             THE  DATA   SECTION


aInItemDisplay    db 'In item::display()',0Ah,0
newline  db 0Ah,0
newline1    db 0Ah,0
newline2    db 0Ah,0
newline3    db 0Ah,0

aInItemPrice    db 'In item::price()',0Ah,0
aRadioType    db 'radio::type()',0Ah,0
aCompuerPrice    db 'Compuer::Price()',0Ah,0
aInItemType    db 'In item::type()',0Ah,0
aMicrowavePrice    db 'Microwave::Price()',0Ah,0
aMicrowaveType    db 'Microwave::type()',0Ah,0

;                            THE   VTABLES

radio_VTABLE    dd offset item_price
 dd offset radio_type

computer_VTABLE    dd offset computer_price
 dd offset item_type

item_VTABLE    dd offset item_price
 dd offset item_type

microwve_VTABLE    dd offset microwve_price
 dd offset microwve_type
 
 


----------------------------EXPLANATION-----------------------------------------

For those who know nothing about Assembly but still dared to read till here
should remember that in Assembly code anything after the semicolon ';' is a
Remark. So between code I have inserted remarks so that it is easier to read.

Since we did not include the constructor in our code the compiler automatically
inserts it between the main() section.Hence we can see in LINE 5. that m1_VPTR
is first initialised to the VTABLE of item. Then in the next LINE m1_VPTR points
to the address of the VTABLE of Microwave class.The same process continues till
LINE 9.

Now in LINE 13. EAX contains the address pointed to by EBX ie. microwave_VTABLE.
In LINE 14. it calls the function which is located at the address of eax ie.
microwave::price(). This is the call of a virtual function.Look through the data
Section and see how the VTABLES are set.

In LINE 33. there is a straight-forward call to item::display() and this is how
a non-virtual function is called. Now compare the calling process of a non-
virtual function to that of a virtual function. You will notice that the calling
of a virtual function includes the following process:
1) Initialise the VPTR to that of the Base class VTABLE
2) Set the VPTR to the Derived class VTABLE
3) Load the address of the VTABLE in a Register
4) Call the Function located at the address pointed to by the VTABLE

But you require just one step to call a non-virtual function ie. the call
instruction followed by the function name.

You should notice that the other microwave object also points to the same VTABLE
but has a different VPTR. Each Class has it's own VTABLE and is shared among all
VPTR's of the Same class.

In LINE 30. the instruction  call dword ptr [edx+4] calls the function located
at the (address+4 bytes) pointed by the EDX Register. This means that the 2nd
Function in the VTABLE is called.

Notice that in the VTABLE of the radio class the address of item::price() is
present.I hope you understand and appreciate the implementation of virtual
functions by the compiler.

This is the end of our tutorial. I hope you use Virtual Functions in your
programs since it is easier to code your programs using these functions. If you
have any doubts regarding Polymorphism feel free to contact me via email at
born2c0de@hotmail.com