Explain concept of Association and Aggregation in object-oriented approach. What is Abstract class? Why Abstract classes are used in C++?

Association and Aggregation in Object-Oriented Approach:

Association:

Association is a relationship between two or more objects that indicates a connection or link. It represents the "has-a" relationship, where one object contains or uses another object. Associations can be one-to-one, one-to-many, or many-to-many.

• One-to-One Association: A one-to-one association indicates that each object in one class is associated with exactly one object in another class. For example, a Student object may be associated with a single Address object.

• One-to-Many Association: A one-to-many association indicates that each object in one class can be associated with multiple objects in another class. For example, a Department object may be associated with multiple Employee objects.

• Many-to-Many Association: A many-to-many association indicates that each object in one class can be associated with multiple objects in another class, and vice versa. For example, a Project object may be associated with multiple Employee objects, and each Employee object may be associated with multiple Project objects.

Associations can be implemented using various mechanisms, such as pointers, references, or composition.

Aggregation:

Aggregation is a specialized form of association that represents a "whole-part" relationship. It is used to model scenarios where one object (the whole) is composed of other objects (the parts). Aggregation implies a strong relationship between the whole and its parts, where the parts are dependent on the whole for their existence and meaning.

For example, a Car object may be composed of multiple Engine, Wheel, and Door objects. The Car object is the whole, and the Engine, Wheel, and Door objects are its parts. The parts cannot exist independently of the whole.

Aggregation can be implemented using various mechanisms, such as composition, containment, or delegation.

Abstract Class:

An abstract class is a class that contains one or more pure virtual functions. A pure virtual function is a function that has no implementation and is declared with a zero function body.

Abstract classes are used in C++ to:

• Define a common interface for a group of related classes.
• Promote code reusability and maintainability.
• Allow for polymorphic behavior through inheritance.

When you inherit from an abstract class, you must provide implementations for all of its pure virtual functions. If you do not, the class inheriting from the abstract class will also be abstract.

Abstract classes are useful when you want to define a common interface for a group of related classes. For example, you might have a base class called Shape that defines a common interface for all shapes. This interface might include pure virtual functions for calculating the area and perimeter of a shape.

You could then create derived classes from the Shape class, such as Circle, Square, and Triangle. These derived classes would provide implementations for the pure virtual functions defined in the Shape class.

This design allows you to write code that works with any type of shape, without having to worry about the specific details of each shape.

Examples of Abstract Classes in C++:

• The iostream library contains several abstract classes, such as istream and ostream. These classes define the basic interface for input and output streams.

• The vector class template is an abstract class that defines the interface for a dynamic array.

• The list class template is an abstract class that defines the interface for a doubly linked list.

Why Abstract Classes are Used in C++:

Abstract classes are used in C++ for the following reasons:

• To define a common interface for a group of related classes.
• To promote code reusability and maintainability.
• To allow for polymorphic behavior through inheritance.

By using abstract classes, you can create a more flexible and extensible program that is easier to maintain.