What is the abstract feature in c plus plus?

Answer 1

Abstraction generally refers to abstract classes. An abstract class is one that cannot itself be instantiated, but can be derived from. Abstract classes are conceptual classes rather than concrete classes, and are intended to provide a common interface to the concrete classes derived from them. A class becomes abstract when it contains one or more pure-virtual methods, which must be implemented in the derived classes otherwise they become abstract classes themselves. A concrete class is one that provides a complete implementation for is abstract base classes, or inherits implementations from base classes other than those that originally declared the function to be pure-virtual.

An example of an abstract class is Shape. All shapes can be drawn, therefore Shape will have a Draw() method. However, it cannot implement this method unless it knows what type of shape it actually is. Therefore the Draw() method should be declared pure-virtual, thus rendering the Shape class to be abstract. You can then derive concrete shapes from the Shape class, such as Circle and Square classes, which will each provide the actual implementation for the Draw() method. Since Square and Circle are types of Shape, you can create collections of Shape objects and call their Draw() methods without regard to their actual type. If the object is really a Square, then calling Shape.Draw() will actually invoke Square.Draw(). There is no need to determine the actual type of the shape because every shape has a common interface.

This is really no different to overriding virtual methods in non-abstract classes. The main difference is that one would not be expected to instantiate a Shape (because it is abstract), other than through derivation, and the pure-virtual methods MUST be implemented by the derived classes, whereas normal virtual methods are simply expected to be overridden but it is not a requirement to do so unless you require an enhancement to the base class method.

Abstract classes typically have few if any member variables and little or no implementation. Ultimately, the class exists purely to provide a common interface to all its derivatives, and only functionality and member variables that are common to all its derivatives should be placed within it. Even if the abstract class provides a complete implementation for all its pure-virtual methods, you cannot instantiate an abstract class -- it must be derived from -- and all pure-virtual methods must still be implemented in the derivatives, even if only to call the base class methods explicitly.