Pure Virtual function and Abstract Class in C++

What is Abstract Class in C++?

• An abstract class in C++ is a class that cannot be instantiated directly, meaning you cannot create objects of an abstract class.
• Instead, it serves as a template for other classes by defining a common interface and behavior that derived classes must adhere to.

Key Characteristics of an Abstract Class in C++

Pure Virtual Function

• Pure virtual function is a special type of virtual function that is declared in an abstract base class but has no implementation in that class.
• An abstract class typically contains one or more pure virtual functions, which are declared using the virtual keyword followed by = 0.
• These functions have no implementation in the abstract class and must be implemented by any class that derives from it.

Inability to Instantiate

Derived Class Implementation

• Derived classes that inherit from an abstract class must provide concrete implementations for all pure virtual functions defined in the abstract base class.
• Not adhering to this will lead to a compilation error.

Abstract Class in C++ Syntax

• class AbstractClassName: Defines the abstract class with a specific name (AbstractClassName in this example).
• public: Specifies the access control for the class members.
• virtual returnType functionName(parameters) = 0: Declares a pure virtual function within the abstract class.
• The = 0 at the end of the declaration indicates that this is a pure virtual function with no implementation.

Abstract Class in C++ Example

• Shape is an abstract base class with a pure virtual function calculateArea(). Objects of the Shape class cannot be instantiated.
• Two derived classes, Circle and Rectangle, inherit from Shape and provide concrete implementations of the calculateArea() function.

Constructor in Abstract class?

• Yes, an abstract class in C++ can have a constructor. Abstract classes can have constructors just like any other class.
• When you create an object of a derived class, the constructor of the abstract base class is automatically called to set up the base class part of the object. for example

Conclusion

• Abstract classes cannot be instantiated and are meant to be inherited from.
• Pure virtual functions have no implementation in the abstract class and must be implemented by derived classes.
• Abstract classes and pure virtual functions promote code modularity, extensibility, and polymorphism in object-oriented programming.