Unit V - Virtual Functions and Polymorphism in C++

Topic 1: Virtual Functions

• A virtual function is a member function in a base class that can be overridden in a derived class.
• Declared using the virtual keyword in the base class.
• Enables dynamic binding, where the function called is determined at runtime based on the object's actual type, not the pointer or reference type.
• Example: A base class Shape with a virtual draw() function can be overridden by derived classes like Circle or Rectangle.

Theoretical Explanation

Virtual functions enable polymorphism, allowing derived classes to override base class functions, with the correct function called at runtime based on the object’s type (dynamic binding). Key subtopics include:

• Normal Member Functions Accessed with Pointers: When a base class pointer points to a derived class object, calling a non-virtual function invokes the base class version (static binding, resolved at compile-time).
• Virtual Member Functions Accessed with Pointers: Declaring a function as virtual in the base class ensures the derived class’s version is called when accessed via a base class pointer or reference (dynamic binding).
• Pure Virtual Functions: Declared as virtual void function() = 0;, making the base class abstract (non-instantiable). Derived classes must override these functions.
• override specifier (C++11 and later):

While not strictly required, using the override keyword when redefining a virtual function in a derived class is good practice. It explicitly indicates that the function is intended to override a base class virtual function and helps the compiler catch potential errors (e.g., mismatched signatures).

Syntax:

class Base{
public:
    virtual return_type functionName(){
        // Function body
    }
};

class Derived : public Base{
public:
    
    // Overriden virtual function of the base class
    return_type fnctionName(){
        // Function body
    }
}

Polymorphism

The word polymorphism means having many forms. A real-life example of polymorphism is a person who at the same time can have different characteristics. A man at the same time is a father, a husband, and an employee. So, the same person exhibits different behaviour in different situations. This is called polymorphism.

In C++, polymorphism concept can be applied to functions and operators. A single function can work differently in different situations. Similarly, an operator works different when used in different context.

Types of Polymorphism

Polymorphism in C++ can be classified into two types: