Function overriding in C++

Introduction

Function overriding lets a child class change how a function works that it got from its parent. This helps make programs flexible and easy to change.

When you want a child class to do something different from the parent class for the same action.

When you have a general behavior in a parent class but need specific behaviors in child classes.

When you want to reuse code but still customize parts of it in child classes.

Syntax

C++

class Parent {
public:
    virtual void functionName() {
        // parent version
    }
};

class Child : public Parent {
public:
    void functionName() override {
        // child version
    }
};

The virtual keyword in the parent class means the function can be overridden.

The override keyword in the child class helps catch mistakes if the function does not match the parent.

Examples

Here, Dog changes the sound function to print "Bark" instead of "Some 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;
    }
};

Circle overrides draw to show a message specific to circles.

C++

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

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

Sample Program

This program shows how the start function is overridden in the Car class. When we call start through a Vehicle pointer, the right version runs depending on the actual object.

C++

#include <iostream>

class Vehicle {
public:
    virtual void start() {
        std::cout << "Vehicle starting" << std::endl;
    }
};

class Car : public Vehicle {
public:
    void start() override {
        std::cout << "Car starting with key" << std::endl;
    }
};

int main() {
    Vehicle* v1 = new Vehicle();
    Vehicle* v2 = new Car();

    v1->start();  // Calls Vehicle's start
    v2->start();  // Calls Car's start because of overriding

    delete v1;
    delete v2;
    return 0;
}

Important Notes

Without virtual in the parent, the child version won't be called through a parent pointer.

Use override to help the compiler check your overriding is correct.

Summary

Function overriding lets child classes change parent class functions.

Use virtual in parent and override in child for safe overriding.

This helps make flexible and reusable code.

Practice

1. What is function overriding in C++?

easy

A. A child class provides its own version of a parent class's virtual function.

B. A function with the same name but different parameters in the same class.

C. A function that is called automatically when an object is created.

D. A function that cannot be changed once defined.

Solution

1. Step 1: Understand function overriding concept

   Function overriding means a child class changes the behavior of a function defined in its parent class.

2. Step 2: Identify correct description

   A child class provides its own version of a parent class's virtual function. correctly describes overriding as providing a new version of a parent's virtual function in the child class.

3. Final Answer:

   A child class provides its own version of a parent class's virtual function. -> Option A

4. Quick Check:

   Function overriding = child changes parent's virtual function [OK]

Hint: Overriding means child changes parent's virtual function [OK]

Common Mistakes:

• Confusing overriding with overloading
• Thinking overriding happens without virtual keyword
• Believing overriding applies to non-virtual functions

2. Which of the following is the correct syntax to override a function in C++11 and later?

easy

A. void func() virtual override;

B. override void func();

C. void func() override;

D. virtual void func() override;

Solution

1. Step 1: Recall override syntax in child class

   In C++11+, the child class function uses the same signature and adds the 'override' keyword after the function declaration.

2. Step 2: Identify correct syntax

   virtual void func() override; shows the correct syntax: 'virtual void func() override;'. The 'virtual' keyword can be used in the child class but is optional; however, it is valid and common to include it.

3. Final Answer:

   virtual void func() override; -> Option D

4. Quick Check:

   Child function uses virtual and override keywords correctly [OK]

Hint: Use 'override' after function signature in child class; 'virtual' is optional but allowed [OK]

Common Mistakes:

• Adding 'virtual' keyword after function name
• Placing 'override' before function name
• Omitting 'override' keyword entirely

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* b = new Derived();
    b->show();
    delete b;
    return 0;
}

medium

A. Derived

B. Base

C. Compilation error

D. Runtime error

Solution

1. Step 1: Understand virtual function behavior

   The pointer 'b' is of type Base* but points to a Derived object. Because 'show' is virtual, the Derived version is called.

2. Step 2: Trace the function call

   Calling b->show() invokes Derived::show(), printing "Derived".

3. Final Answer:

   Derived -> Option A

4. Quick Check:

   Virtual function calls child's version [OK]

Hint: Virtual functions call child's version via base pointer [OK]

Common Mistakes:

• Expecting base class output
• Forgetting to make base function virtual
• Confusing pointer type with object type

4. Identify the error in the following code snippet:

class Parent {
public:
    void display() { std::cout << "Parent"; }
};
class Child : public Parent {
public:
    void display() override { std::cout << "Child"; }
};

medium

A. Child's display() should not use override keyword.

B. Parent's display() must be declared virtual to override.

C. display() must return int, not void.

D. No error, code is correct.

Solution

1. Step 1: Check virtual keyword in parent

   For overriding to work, the base class function must be declared virtual.

2. Step 2: Analyze override keyword usage

   Child uses 'override' but Parent's display() is not virtual, causing a compilation error.

3. Final Answer:

   Parent's display() must be declared virtual to override. -> Option B

4. Quick Check:

   Base function must be virtual for override [OK]

Hint: Base function must be virtual to allow override [OK]

Common Mistakes:

• Ignoring virtual keyword in base class
• Thinking override works without virtual
• Assuming override keyword is optional

5. Given the classes below, what will be the output of obj->print();?

class A {
public:
    virtual void print() { std::cout << "A"; }
};
class B : public A {
public:
    void print() override { std::cout << "B"; }
};
class C : public B {
public:
    void print() override { std::cout << "C"; }
};

int main() {
    A* obj = new C();
    obj->print();
    delete obj;
    return 0;
}

hard

A. A

B. B

C. C

D. Compilation error

Solution

1. Step 1: Understand pointer and object types

   Pointer 'obj' is of type A*, but it points to an object of class C.

2. Step 2: Follow virtual function overriding chain

   Since print() is virtual and overridden in B and C, the most derived override (C::print) is called.

3. Final Answer:

   C -> Option C

4. Quick Check:

   Virtual call resolves to most derived override [OK]

Hint: Virtual calls use most derived override at runtime [OK]

Common Mistakes:

• Expecting base class output
• Confusing pointer type with object type
• Ignoring virtual function behavior