Abstract class vs interface

[hornet07]

Can someone explain why you would use an interface instead of an abstract class? I can't quite see the point, as the interface doesn't do anything the abstract class coudln't do. Please show snippets of code so I can understand the difference better.
Thanks.

[shadowCODE]

Definitions Disclaimer: These are with respect to OOP in C++..
Sorry for the long answer.

Abstract class
An abstract class is, conceptually, a class that cannot be instantiated and is usually implemented as a class that has one or more pure virtual (abstract) functions.

A pure virtual function is one which must be overridden by any concrete (i.e., non-abstract) derived class. This is indicated in the declaration with the syntax " = 0" in the member function's declaration.

class AbstractClass {
public:
  virtual void AbstractMemberFunction() = 0; // Pure virtual function makes
                                             // this class Abstract class.
  virtual void NonAbstractMemberFunction1(); // Virtual function.
 
  void NonAbstractMemberFunction2();
};

In general an abstract class is used to define an implementation and is intended to be inherited from by concrete classes. It's a way of forcing a contract between the class designer and the users of that class

Interface
An interface has no implementation.
An interface class contains only a virtual destructor and pure virtual functions.
An interface class is a class that specifies the polymorphic interface i.e. pure virtual function declarations into a base class. The programmer using a class hierarchy can then do so via a base class that communicates only the interface of classes in the hierarchy.

class shape   // An interface class
{
  public:
    virtual ~shape();
    virtual void move_x(int x) = 0;
    virtual void move_y(int y) = 0;
    virtual void draw() = 0;
//...
};

Every interface class should have a virtual destructor. Virtual destructor makes sure that when a shape is deleted polymorphically, correct destructor of the derived class is invoked.

Differences
1 - interfaces can have no state or implementation
2 - a class that implements an interface must provide an implementation of all the method of that interface
3 - abstract classes may contain state (data members) and/or implementation (methods)
4 - abstract classes can be inherited without implementing the abstract methods (though such a derived class is abstract itself)
5 -interfaces may be multiple-inherited, abstract classes may not (this is probably the key concrete reason for interfaces to exist separately from abtract classes - they permit an implementation of multiple inheritance that removes many of the problems of general MI).
6- If you anticipate creating multiple versions of your component, create an abstract class. Abstract classes provide a simple and easy way to version your components. By updating the base class, all inheriting classes are automatically updated with the change. Interfaces, on the other hand, cannot be changed once created. If a new version of an interface is required, you must create a whole new interface.
7-If the functionality you are creating will be useful across a wide range of disparate objects, use an interface. Abstract classes should be used primarily for objects that are closely related, whereas interfaces are best suited for providing common functionality to unrelated classes.

[hornet07]

Thanks for the clear explanation :)