differnce b/w pure virtual and virtual.......

[umair ali]

is pure virtual function has more memory than virtual function?????

[weaknessforcats]

First, what is a virtual function?

Virtual functions are used to implement polymorphism in C++. With polymorphism, you have a hierarchy of classes and the derived class objects are to be used only with a base class pointer. So, if you have a base class pointer pointing at a derived class object, and you call a base class method, the virtual keyword diverts the call to the derived class method because you really have a derived class object.

Obviously, in the example above, there is a base class method with the same name and arguments as the derived class method. The compiler as a choice: either call the base class method or call the derived class method.

So what do you do when you always want to call the derived class method and never call the base class method. Like maybe the base class method is just a function prototype of what the derived class should implement?

Enter the pure virtual function. The =0 only means that you cannot call the function using an object of the class that declares it.

You are permitted to call the pure virtual function from the derived class since the derived class is not the class declaring the pure virtual function. This is a nifty to stash code that is common to all derived objects rather than duplicating the common code in each derived class.

There is nothing about memory here. Memory is not part of C++ but is part of how C++ is implemented by the compiler. Commonly, a class with virtual functions has an associated virtual function table, or vtable, that the compiler has constructed to keep all these virtual calls straight. This table has the addresses of the correct virtual functions to call using an object of this class. The address of this table is kept inside the object so you will see objects like this are larger, by the size of an address, from objects that do not have virtual functions.

[Sherin]

Virtual functions:

1) It has its definition in base class.

2) Virtual Function is declared with keyword 'virtual' at the declaration.
Ex : virtual return_type function_name(function arguments);

3) All derived classes may or may not override, virtual function

4) These are hierarchical in nature so it does not affect if any derived class does not inherit it.

5) No concept of abstract classes.



Pure Virtual functions:

1) It has no definition in base class

2) Pure Virtual Function is declared as
Ex : virtual return_type function_name(function arguments) = 0;

3) All derived classes must override,pure virtual function.

4) Gives compilation error if derived classes are not inherited.

5) If class is pure virtual function, then class is declared as abstract class

[Banfa]

Pure Virtual functions:

1) "It has no definition in base class"
Not strictly true, it normally doesn't have a definition in the base class but you can give it a definition there if you choose; however this definition cannot be given as part of the declaration i.e.

class Example
{
    virtual int exFun(int param) = 0
    {
      // Code
    }
}

is invalid. Also it can only be called explicitly from a base class. Finally I have never managed to think of a situation where this feature is useful.

3) "All derived classes must override,pure virtual function."
Again strictly not true but if they don't override it the function remains pure virtual in the derived class and the derived class is abstract although it itself could then be inherited and the pure virtual function overridden.

4) "Gives compilation error if derived classes are not inherited."
Not quite sure what you mean here, do you mean that if you fail to override a pure virtual function in a derived class but still try to instantiate the derived class you get a compilation error? This is true because the derived class is still abstract and you can't instantiate abstract classes.