Abstract classes and interfaces in C++

Abstract classes and interfaces are two important concepts in object-oriented programming (OOP) languages like C++. They provide a way to define common behavior and structure for classes while allowing for flexibility and extensibility in the design of a software system. In this article, we will explore the concepts of abstract classes and interfaces in C++.

Abstract classes

An abstract class is a class that cannot be instantiated and serves as a blueprint for other classes. It contains pure virtual functions, which are implemented by derived classes. Abstract classes are used when you want to define a common interface that multiple classes must implement, but there should never be an object of the abstract class itself.

To create an abstract class, you must declare at least one pure virtual function in the class, denoted by the virtual keyword followed by = 0. For example:

class AbstractShape {
public:
    virtual void draw() = 0;  // Pure virtual function
    virtual double area() = 0;  // Pure virtual function
};

In the above example, AbstractShape is an abstract class that defines two pure virtual functions, draw() and area(). Any class that derives from AbstractShape is required to implement these functions.

Interfaces

In addition to abstract classes, C++ also supports interfaces. An interface is similar to an abstract class, but with only pure virtual functions and no member variables. Interfaces define a contract that must be adhered to by any class that implements them.

To create an interface in C++, you define a class with only pure virtual functions, much like an abstract class, but without any member variables. Here's an example:

class Shape {
public:
    virtual void draw() = 0;  // Pure virtual function
    virtual double area() const = 0;  // Pure virtual function
};

In the above example, Shape is an interface that defines the behavior common to different shapes. Any class that implements this interface is required to provide an implementation for the draw() and area() functions.

Differences between abstract classes and interfaces

The main difference between abstract classes and interfaces lies in their usage and implementation. While abstract classes can have member variables and concrete functions in addition to pure virtual functions, interfaces only contain pure virtual functions and cannot have any member variables. Additionally, a class can implement multiple interfaces, but it can only inherit from a single abstract class.

Abstract classes are useful when you have a common set of member variables or functions that can be shared among derived classes, whereas interfaces are useful when you want to define a contract that multiple classes must strictly adhere to, without any implementation details.

Conclusion

Abstract classes and interfaces are powerful tools for designing flexible and extensible software systems in C++. They allow you to define common behaviors and structures that can be implemented by multiple classes. Abstract classes provide a way to share member variables and non-pure-virtual functions among derived classes, whereas interfaces define a contract that must be followed by implementing classes. Understanding and using abstract classes and interfaces can greatly enhance the modularity and flexibility of your C++ programs.