Pure Virtual Function & Abstract Base Class

• If we want to provide only interface for the derived class then we use the concept of abstract base class.
• If we create abstract base class then we can not create object of that base class.
• Writing pure virtual function means writing (=0) at the end of a prototype of a function without any defination.
• When we write at least a single pure virtual function inside the class then that class is considered as a abstract base class.
• A class having pure virtual function only says what to do does not say how to do.
• When we inherit that abstract base class then the derived class must implement the pure virtual functions are itself became an abstract class. Means a pure virtual function forces inherited classes to provide a definition for it.
• We can create an abstract class when only want to manipulate a set of classes through a common interface,but the common interface doesn’t need to have an implementation.
• We can create pure virtual function using the syntax as virtual int Fun() = 0;
• By using the above syntax we can tell the compiler that reserve the slot for this function in VTABLE, but not to put any address in that perticular slot.
• Because of which the VTABLE is incomplete. Even if only one function in class is declared as a pure virtual the VTABLE is incomplete.
• When we create object of abstract base class, the VTABLE is incomplete and by looking at it compiler gives an error message.
• It’s possible to provide a definition for a pure virtual function in the base class. But still it is not allowed to create the object of abstract class. and in this case also VTABLE is also incomplete.
• We can not provide the inline definition for the pure virtual functions means we can not provide the definition for the pure virtual function inside the class.
• If a class is deriving an abstract class, it is compulsion that the given derived class must provide a defination of a pure virtual function which is defined in the abstract base class.
• If derived class does not want to provide a defination of pure virtual function then writing a prototype of a function is necessary (=0 must be there at the end of the prototype).
• If there is no such a prototype in a derived class then compiler gives an error.
• By providing the definition for the pure virtual functions we can call that function inside the derived class functions.
• We can also provide a partial defination for pure virtual function but still entry of VTABLE is empty.
• If we want to provide such partial defination then that function must be defined outside the class.
• And this partially provided defination must be called by a derived class otherwise this defination is of no use.

 

Example:

class base

{

public:

virtual void demo()=0;

};

void base:: demo()

{

printf(“in pure virtual function”);

}

class derived : public base {

public:

void demo()

{

base::demo();

printf(“Inside the derived class\n”);

}

};

int main()

{

derived obj;

base *d = &obj;

d->demo();

return 0;

}

Output:

in pure virtual function.

Inside the derived class.

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