RUN-TIME POLYMORPHISM IN C++

Run-time polymorphism 

Run-time polymorphism means the method call is resolved at run time, not at compile time.

Use

• Base class pointer
• Points to the derived class object
• Calls the virtual function

 

Key Concepts

1 Inheritance

Run-time polymorphism works only with inheritance.

class Base { };

class Derived : public Base { };

 

2 Virtual Function

A virtual function is a member function of a class that is declared using the keyword virtual and is overridden in a derived class.

virtual void show();

 

3 Base Class Pointer

A base class pointer can point to a derived class object.

Base *b;

b = new Derived();

 

Need Run-Time Polymorphism

• To achieve dynamic behavior
• To support flexibility and extensibility
• Used heavily in:
• GUI systems
• Game engines
• Operating systems
• Real-world modeling

 

Process of Run-Time Polymorphism (Internally )

Virtual Table (V-Table)

• Compiler creates a V-Table
• V-Table stores addresses of virtual functions
• Each object contains a V-Pointer (VPTR)

At runtime, correct function is selected using VPTR.

 

Rules of Run-Time Polymorphism

1. Must use inheritance
2. Function must be virtual
3. Function call must be through a base class pointer
4. Function overriding must have the same signature
5. Happens at run time

 

Example : Using Virtual Function

#include <iostream>

using namespace std;

 

class Base {

public:

    virtual void show() {

        cout << "This is Base class show function" << endl;

    }

};

 

class Derived : public Base {

public:

    void show() {

        cout << "This is Derived class show function" << endl;

    }

};

 

int main() {

    Base *b;

    Derived d;

    b = &d;

    b->show();

    return 0;

}

Output

This is a derived class showing a function

 

Difference: Compile-time vs Run-time Polymorphism

Feature	Compile-time	Run-time
Binding	Early	Late
Speed	Faster	Slightly slower
Example	Overloading	Virtual functions
Flexibility	Less	More

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