[c++] Abstract Class vs Interface in C++

Possible Duplicate:
How do you declare an interface in C++?

This is a general question about C++. As you know, there is no clear distinction between interface and abstract class in C++ unlike Java and C#. When would it be more preferrable to use an interface instead of an abstract class in C++? Could you give some examples?

Pure Virtual Functions are mostly used to define:

a) abstract classes

These are base classes where you have to derive from them and then implement the pure virtual functions.

b) interfaces

These are 'empty' classes where all functions are pure virtual and hence you have to derive and then implement all of the functions.

Pure virtual functions are actually functions which have no implementation in base class and have to be implemented in derived class.

Please don't put members into an interface; though it's correct in phrasing. Please don't "delete" an interface.

class IInterface() 
{ 
   Public: 
   Virtual ~IInterface(){}; 
   … 
} 

Class ClassImpl : public IInterface 
{ 
    … 
} 

Int main() 
{ 

  IInterface* pInterface = new ClassImpl(); 
  … 
  delete pInterface; // Wrong in OO Programming, correct in C++.
}

An abstract class would be used when some common implementation was required. An interface would be if you just want to specify a contract that parts of the program have to conform too. By implementing an interface you are guaranteeing that you will implement certain methods. By extending an abstract class you are inheriting some of it's implementation. Therefore an interface is just an abstract class with no methods implemented (all are pure virtual).

interface were primarily made popular by Java.
Below are the nature of interface and its C++ equivalents:

1. interface can contain only body-less abstract methods; C++ equivalent is pure virtual methods, though they can/cannot have body
2. interface can contain only static final data members; C++ equivalent is static const data members which are compile time constants
3. Multiple interface can be implemented by a Java class, this facility is needed because a Java class can inherit only 1 class; C++ supports multiple inheritance straight away with help of virtual keyword when needed

Because of point 3 interface concept was never formally introduced in C++. Still one can have a flexibility to do that.

Besides this you can refer Bjarne's FAQ on this topic.