Method overriding in C++ - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a virtual function in the base class.

C++

class Base {
public:
    virtual void [1]() {
        std::cout << "Base function" << std::endl;
    }
};

Drag options to blanks, or click blank then click option'

Aoutput

Bdisplay

Cprint

Dshow

Attempts:

3 left

💡 Hint

Common Mistakes

Using a different method name than the one overridden in the derived class.

The method name display is used as the virtual function in the base class.

2fill in blank

medium

Complete the code to override the base class method in the derived class.

C++

class Derived : public Base {
public:
    void [1]() override {
        std::cout << "Derived function" << std::endl;
    }
};

Drag options to blanks, or click blank then click option'

Adisplay

Bprint

Cshow

Doutput

Attempts:

3 left

💡 Hint

Common Mistakes

Using a different method name, which does not override the base method.

The derived class overrides the display method from the base class.

3fill in blank

hard

Fix the error in the code by completing the method declaration to enable overriding.

C++

class Base {
public:
    [1] void display() {
        std::cout << "Base function" << std::endl;
    }
};

Drag options to blanks, or click blank then click option'

Adisplay

Bvirtual void display

Coverride display

Dvirtual

Attempts:

3 left

💡 Hint

Common Mistakes

Omitting the virtual keyword, which disables overriding.

The method must be declared with the virtual keyword to allow overriding.

4fill in blank

hard

Fill both blanks to correctly override the base class method and call it from the derived class.

C++

class Derived : public Base {
public:
    void [1]() override {
        Base::[2]();
        std::cout << "Derived override" << std::endl;
    }
};

Drag options to blanks, or click blank then click option'

Adisplay

Bshow

Dprint

Attempts:

3 left

💡 Hint

Common Mistakes

Using different method names that do not match the base class.

The derived class overrides display and calls the base class display method.

5fill in blank

hard

Fill all three blanks to create a base class with a virtual method, override it in the derived class, and call the method via a base pointer.

C++

class Base {
public:
    [1] void display() {
        std::cout << "Base display" << std::endl;
    }
};

class Derived : public Base {
public:
    void [2]() override {
        std::cout << "Derived display" << std::endl;
    }
};

int main() {
    Base* ptr = new Derived();
    ptr->[3]();
    delete ptr;
    return 0;
}

Drag options to blanks, or click blank then click option'

Avirtual

Bdisplay

Doverride

Attempts:

3 left

💡 Hint

Common Mistakes

Not marking the base method as virtual, so overriding does not work.

Forgetting the override keyword in the derived class.

Calling a wrong method name via pointer.

The base class method is declared virtual. The derived class method uses override. The method called via pointer is display.

Practice

1. What is the main purpose of method overriding in C++?

easy

A. To create a new method with a different name in the child class

B. To allow a child class to provide a specific implementation of a method already defined in its parent class

C. To prevent a method from being used in the child class

D. To copy all methods from the parent class to the child class without changes

Solution

1. Step 1: Understand method overriding concept

   Method overriding allows a child class to change how a method works that it inherits from its parent.

2. Step 2: Identify the correct purpose

   The child class provides its own version of the method, customizing behavior while keeping the same method name.

3. Final Answer:

   To allow a child class to provide a specific implementation of a method already defined in its parent class -> Option B

4. Quick Check:

   Method overriding = child changes parent method [OK]

Hint: Overriding means child changes parent's method behavior [OK]

Common Mistakes:

• Confusing overriding with method overloading
• Thinking overriding creates new method names
• Believing overriding copies methods without changes

2. Which of the following is the correct way to declare a method for overriding in C++?

easy

A. override void display();

B. void display() override;

C. void virtual display();

D. virtual void display();

Solution

1. Step 1: Recall syntax for virtual methods

   In C++, the parent class method must be marked with virtual to allow overriding.

2. Step 2: Check each option

   virtual void display(); correctly uses virtual void display();. void display() override; uses override incorrectly in declaration, it is used in child method definition. Options C and D have wrong syntax.

3. Final Answer:

   virtual void display(); -> Option D

4. Quick Check:

   Parent method uses virtual keyword [OK]

Hint: Parent method must be virtual for overriding [OK]

Common Mistakes:

• Placing override keyword in parent method declaration
• Using wrong order of keywords
• Omitting virtual keyword in parent

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

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

medium

A. Derived

B. Runtime error

C. Compilation error

D. Base

Solution

1. Step 1: Understand virtual function behavior

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

2. Step 2: Trace the call ptr->show()

   At runtime, the program calls Derived::show(), printing "Derived".

3. Final Answer:

   Derived -> Option A

4. Quick Check:

   Virtual method calls child's version [OK]

Hint: Virtual calls use actual object method, not pointer type [OK]

Common Mistakes:

• Expecting Base output due to pointer type
• Confusing override keyword effect
• Ignoring virtual keyword

4. Identify the error in the following code snippet related to method overriding:

class Parent {
public:
    void greet() { std::cout << "Hello from Parent" << std::endl; }
};
class Child : public Parent {
public:
    void greet() override { std::cout << "Hello from Child" << std::endl; }
};

medium

A. Parent class method greet() should be marked virtual

B. Child class method greet() should be marked virtual

C. override keyword is not allowed in C++

D. No error, code is correct

Solution

1. Step 1: Check parent method declaration

   For overriding to work properly, the parent method must be declared virtual. Here, it is not.

2. Step 2: Analyze override keyword usage

   The child method uses override, which requires the parent method to be virtual. Since parent method is not virtual, this causes a compilation error.

3. Final Answer:

   Parent class method greet() should be marked virtual -> Option A

4. Quick Check:

   Parent method must be virtual for override [OK]

Hint: Parent method must be virtual if child uses override [OK]

Common Mistakes:

• Thinking child method must be virtual
• Ignoring need for virtual in parent
• Believing override keyword is optional

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

class Animal {
public:
    virtual void display() { std::cout << "Animal" << std::endl; }
};
class Dog : public Animal {
public:
    void display() override { std::cout << "Dog" << std::endl; }
};
class Puppy : public Dog {
public:
    void display() override { std::cout << "Puppy" << std::endl; }
};

int main() {
    Animal* ptr = new Puppy();
    ptr->display();
    delete ptr;
    return 0;
}

hard

A. Animal

B. Dog

C. Puppy

D. Compilation error due to multiple overrides

Solution

1. Step 1: Understand multi-level overriding

   Each class overrides display() with its own version. The pointer is of type Animal* but points to a Puppy object.

2. Step 2: Determine which method is called

   Because display() is virtual, the most derived override (Puppy::display) is called at runtime.

3. Final Answer:

   Puppy -> Option C

4. Quick Check:

   Virtual calls deepest override in inheritance chain [OK]

Hint: Virtual calls deepest override in inheritance chain [OK]

Common Mistakes:

• Expecting Dog or Animal output due to pointer type
• Thinking multiple overrides cause errors
• Ignoring virtual keyword effect