Runtime polymorphism in C++

Introduction

Runtime polymorphism lets a program decide which function to use while it is running, not before. This helps make programs flexible and easy to change.

When you want different objects to respond differently to the same function call.

When you have a base class pointer or reference but want to call derived class functions.

When you want to write code that works with many types of objects without changing it.

When you want to add new behaviors without changing existing code.

When you want to use interfaces or abstract classes to design your program.

Syntax

C++

class Base {
public:
    virtual void show() {
        // base class version
    }
};

class Derived : public Base {
public:
    void show() override {
        // derived class version
    }
};

The keyword virtual before a function in the base class tells C++ to use runtime polymorphism.

The override keyword in the derived class helps catch mistakes and shows you are replacing the base function.

Examples

This example shows a base class Animal with a virtual function sound(). The derived class Dog changes the sound.

C++

class Animal {
public:
    virtual void sound() {
        std::cout << "Some sound" << std::endl;
    }
};

class Dog : public Animal {
public:
    void sound() override {
        std::cout << "Bark" << std::endl;
    }
};

Here, a pointer of type Animal* points to a Dog object. Calling sound() runs the Dog version because of runtime polymorphism.

C++

Animal* pet = new Dog();
pet->sound();

Sample Program

This program creates two shapes: a circle and a square. Both are accessed through pointers of the base class Shape. When draw() is called, the program decides at runtime which version to use, showing runtime polymorphism.

C++

#include <iostream>

class Shape {
public:
    virtual void draw() {
        std::cout << "Drawing a shape" << std::endl;
    }
};

class Circle : public Shape {
public:
    void draw() override {
        std::cout << "Drawing a circle" << std::endl;
    }
};

class Square : public Shape {
public:
    void draw() override {
        std::cout << "Drawing a square" << std::endl;
    }
};

int main() {
    Shape* shape1 = new Circle();
    Shape* shape2 = new Square();

    shape1->draw();  // Calls Circle's draw
    shape2->draw();  // Calls Square's draw

    delete shape1;
    delete shape2;
    return 0;
}

Important Notes

Always declare base class functions as virtual to enable runtime polymorphism.

Use override in derived classes to avoid mistakes and improve code clarity.

Remember to delete objects created with new to avoid memory leaks.

Summary

Runtime polymorphism lets programs choose the right function during execution.

Use virtual functions in base classes and override them in derived classes.

This helps write flexible and reusable code that works with many object types.

Practice

1. What is the main purpose of virtual functions in C++ runtime polymorphism?

easy

A. To allocate memory for objects dynamically

B. To make functions run faster by avoiding function calls

C. To prevent any function from being overridden in derived classes

D. To allow derived classes to override base class methods and decide at runtime which method to call

Solution

1. Step 1: Understand virtual functions role

   Virtual functions allow a base class to declare a function that derived classes can override.

2. Step 2: Explain runtime decision

   At runtime, the program decides which overridden function to call based on the actual object type.

3. Final Answer:

   To allow derived classes to override base class methods and decide at runtime which method to call -> Option D

4. Quick Check:

   Virtual functions enable runtime method selection [OK]

Hint: Virtual means choose function at runtime, not compile time [OK]

Common Mistakes:

• Thinking virtual speeds up function calls
• Believing virtual prevents overriding
• Confusing virtual with memory allocation

2. Which of the following is the correct syntax to declare a virtual function in a base class?

easy

A. void display() virtual;

B. void virtual display();

C. virtual void display();

D. virtual: void display();

Solution

1. Step 1: Recall virtual function syntax

   The keyword virtual comes before the return type in the function declaration.

2. Step 2: Match correct syntax

   virtual void display(); correctly places virtual before void display();

3. Final Answer:

   virtual void display(); -> Option C

4. Quick Check:

   Virtual keyword precedes return type [OK]

Hint: Virtual always goes before return type in declaration [OK]

Common Mistakes:

• Placing virtual after function name
• Using virtual as a label or modifier incorrectly
• Missing semicolon after declaration

3. What will be the output of the following code?

class Base {
public:
    virtual void show() { std::cout << "Base"; }
};
class Derived : public Base {
public:
    void show() override { std::cout << "Derived"; }
};
int main() {
    Base* ptr = new Derived();
    ptr->show();
    delete ptr;
    return 0;
}

medium

A. Derived

B. Base

C. Compilation error

D. Runtime error

Solution

1. Step 1: Identify virtual function usage

   The show() function in Base is virtual and overridden in Derived.

2. Step 2: Understand pointer type and object type

   The pointer is of type Base* but points to a Derived object, so the overridden show() in Derived is called at runtime.

3. Final Answer:

   Derived -> Option A

4. Quick Check:

   Virtual function calls derived version [OK]

Hint: Virtual functions call derived method via base pointer [OK]

Common Mistakes:

• Expecting base class method output
• Confusing pointer type with object type
• Ignoring virtual keyword effect

4. Identify the error in the following code snippet related to runtime polymorphism:

class Animal {
public:
    void sound() { std::cout << "Animal sound"; }
};
class Dog : public Animal {
public:
    void sound() { std::cout << "Bark"; }
};
int main() {
    Animal* a = new Dog();
    a->sound();
    delete a;
    return 0;
}

medium

A. Incorrect pointer type declaration

B. Missing virtual keyword in base class function

C. Derived class function should be static

D. Memory leak due to missing delete

Solution

1. Step 1: Check base class function declaration

   The base class function sound() is not declared virtual, so no runtime polymorphism occurs.

2. Step 2: Effect on function call

   Because sound() is not virtual, calling a->sound() calls the base class version, not the derived one.

3. Final Answer:

   Missing virtual keyword in base class function -> Option B

4. Quick Check:

   Virtual needed for runtime polymorphism [OK]

Hint: Base function must be virtual for polymorphism [OK]

Common Mistakes:

• Assuming override works without virtual
• Thinking pointer type causes error
• Ignoring virtual keyword importance

5. Given the classes below, what will be the output when ptr->identify() is called?

class Shape {
public:
    virtual void identify() { std::cout << "Shape"; }
};
class Circle : public Shape {
public:
    void identify() override { std::cout << "Circle"; }
};
class ColoredCircle : public Circle {
public:
    void identify() override { std::cout << "Colored Circle"; }
};
int main() {
    Shape* ptr = new ColoredCircle();
    ptr->identify();
    delete ptr;
    return 0;
}

hard

A. Colored Circle

B. Circle

C. Shape

D. Compilation error due to multiple overrides

Solution

1. Step 1: Trace virtual function overrides

   identify() is virtual in Shape, overridden in Circle, and again in ColoredCircle.

2. Step 2: Determine actual object type at runtime

   The pointer ptr is of type Shape* but points to a ColoredCircle object, so the most derived override runs.

3. Final Answer:

   Colored Circle -> Option A

4. Quick Check:

   Most derived override called at runtime [OK]

Hint: Runtime calls most derived override in inheritance chain [OK]

Common Mistakes:

• Expecting base or intermediate class output
• Thinking multiple overrides cause errors
• Ignoring virtual override chain