Base and derived classes in C++ - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a base class named Animal.

C++

class [1] {
public:
    void speak() {
        std::cout << "Animal sound" << std::endl;
    }
};

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

ADog

BAnimal

CCat

DBird

Attempts:

3 left

💡 Hint

Common Mistakes

Using a derived class name instead of the base class name.

Misspelling the class name.

The base class is named Animal. This is the class from which others can inherit.

2fill in blank

medium

Complete the code to declare a derived class Dog that inherits from Animal.

C++

class Dog : public [1] {
public:
    void speak() {
        std::cout << "Woof!" << std::endl;
    }
};

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

AAnimal

BCat

CBird

DFish

Attempts:

3 left

💡 Hint

Common Mistakes

Using a class name that is not the base class.

Forgetting the public keyword.

The derived class Dog inherits from the base class Animal using public inheritance.

3fill in blank

hard

Fix the error in the code to call the base class speak() method inside the derived class Dog's speak() method.

C++

class Dog : public Animal {
public:
    void speak() {
        [1]::speak();
        std::cout << "Woof!" << std::endl;
    }
};

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

ADog

BBase

CAnimal

DParent

Attempts:

3 left

💡 Hint

Common Mistakes

Using the derived class name instead of the base class name.

Using incorrect class names like Base or Parent.

To call the base class method, use the base class name Animal followed by the scope resolution operator.

4fill in blank

hard

Fill both blanks to create a constructor in the derived class Cat that calls the base class constructor with a name parameter.

C++

class Cat : public Animal {
public:
    Cat(std::string [1]) : [2]([1]) {}
};

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

Aname

BAnimal

CDog

DCat

Attempts:

3 left

💡 Hint

Common Mistakes

Using different names for the parameter and argument.

Calling the wrong class constructor.

The constructor parameter is name. The base class constructor Animal is called with this parameter.

5fill in blank

hard

Fill all three blanks to override the speak() method in the derived class Bird and call the base class speak() method.

C++

class Bird : public [1] {
public:
    void speak() override {
        [2]::[3]();
        std::cout << "Tweet!" << std::endl;
    }
};

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

AAnimal

BBird

Cspeak

DDog

Attempts:

3 left

💡 Hint

Common Mistakes

Using the derived class name instead of the base class name.

Using incorrect method names.

The derived class Bird inherits from Animal. To call the base class method, use Animal::speak().

Practice

1. What is the main purpose of a base class in C++?

easy

A. To store only private data members

B. To create objects directly without inheritance

C. To override functions in derived classes

D. To hold common features shared by derived classes

Solution

1. Step 1: Understand base class role

   A base class is designed to hold common features that multiple derived classes can share.

2. Step 2: Compare options

   To hold common features shared by derived classes correctly states the purpose. Other options describe derived class behavior or incorrect roles.

3. Final Answer:

   To hold common features shared by derived classes -> Option D

4. Quick Check:

   Base class = common features [OK]

Hint: Base class holds shared features for reuse [OK]

Common Mistakes:

• Confusing base class with derived class
• Thinking base class cannot have data members
• Assuming base class is only for private data

2. Which of the following is the correct syntax to declare a derived class Car inheriting from base class Vehicle in C++?

easy

A. class Car : Vehicle {};

B. class Car : public Vehicle {};

C. class Car extends Vehicle {};

D. class Car inherits Vehicle {};

Solution

1. Step 1: Recall C++ inheritance syntax

   In C++, a derived class inherits from a base class using a colon and access specifier like public.

2. Step 2: Check options

   class Car : public Vehicle {}; uses correct syntax: class Car : public Vehicle {}. Others use incorrect keywords or omit access specifier.

3. Final Answer:

   class Car : public Vehicle {} -> Option B

4. Quick Check:

   Inheritance syntax = colon + access specifier [OK]

Hint: Use colon and public keyword for inheritance [OK]

Common Mistakes:

• Omitting access specifier like public
• Using wrong keywords like inherits or extends
• Forgetting colon before base class name

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

class Base {
public:
    void show() { std::cout << "Base"; }
};
class Derived : public Base {
public:
    void show() { std::cout << "Derived"; }
};
int main() {
    Derived d;
    d.show();
    return 0;
}

medium

A. Derived

B. Base

C. Compilation error

D. No output

Solution

1. Step 1: Understand method overriding in derived class

   The derived class Derived has its own show() method which hides the base class method.

2. Step 2: Check which method is called

   Calling d.show() on a Derived object calls the derived class method, printing "Derived".

3. Final Answer:

   Derived -> Option A

4. Quick Check:

   Derived method overrides base method [OK]

Hint: Derived class method hides base method if same name [OK]

Common Mistakes:

• Assuming base class method runs without virtual keyword
• Expecting compilation error due to method name clash
• Thinking both outputs print

4. Identify the error in this code snippet:

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

medium

A. Missing virtual keyword in base class method

B. Incorrect pointer type for inheritance

C. Derived class method should be static

D. No error, code runs fine

Solution

1. Step 1: Understand polymorphism with base pointers

   Calling display() via a base class pointer calls base method unless it is virtual.

2. Step 2: Check for virtual keyword

   The base class method display() lacks virtual, so ptr->display() calls base version, not derived.

3. Final Answer:

   Missing virtual keyword in base class method -> Option A

4. Quick Check:

   Virtual needed for polymorphic calls [OK]

Hint: Add virtual to base method for polymorphism [OK]

Common Mistakes:

• Thinking pointer type is wrong
• Believing derived method runs without virtual
• Assuming static keyword fixes this

5. Given these classes:

class Animal {
public:
    virtual std::string sound() { return "Unknown"; }
};
class Dog : public Animal {
public:
    std::string sound() override { return "Bark"; }
};
class Cat : public Animal {
public:
    std::string sound() override { return "Meow"; }
};

Which code snippet correctly creates a list of animals and prints their sounds?

hard

A. std::vector<Animal> animals = {Dog(), Cat()}; for(auto& a : animals) std::cout << a.sound() << ' ';

B. std::vector<Dog> animals = {Dog(), Cat()}; for(auto& a : animals) std::cout << a.sound() << ' ';

C. std::vector animals = {new Dog(), new Cat()}; for(auto a : animals) std::cout << a->sound() << ' ';

D. std::vector<Animal> animals = {Dog(), Cat()}; for(auto a : animals) std::cout << a.sound() << ' ';

Solution

1. Step 1: Understand polymorphism with collections

   To preserve derived class behavior, store pointers to base class, not objects by value.

2. Step 2: Analyze options

   std::vector animals = {new Dog(), new Cat()}; for(auto a : animals) std::cout << a->sound() << ' '; uses std::vector<Animal*> with pointers to Dog and Cat, calling sound() polymorphically.

3. Final Answer:

   std::vector<Animal*> animals = {new Dog(), new Cat()}; for(auto a : animals) std::cout << a->sound() << ' '; -> Option C

4. Quick Check:

   Use base class pointers for polymorphic containers [OK]

Hint: Use base class pointers to keep derived behavior [OK]

Common Mistakes:

• Storing objects by value causes slicing
• Using wrong container type for polymorphism
• Forgetting to use pointers or references