Pure virtual functions in C++ - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

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

C++

class Base {
public:
    virtual void display() [1];
};

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

A= 0

B= 1

C{}

D;

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting to add '= 0' makes the function not pure virtual.

Using '{}' instead of '= 0' defines a normal virtual function.

A pure virtual function is declared by assigning 0 to the function declaration, like virtual void display() = 0;.

2fill in blank

medium

Complete the code to make Base an abstract class by adding a pure virtual function show.

C++

class Base {
public:
    virtual void [1]() = 0;
};

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

Aprint

Bshow

Cdisplay

Drun

Attempts:

3 left

💡 Hint

Common Mistakes

Using a function name different from the one asked.

Adding a function body instead of '= 0;'.

The function name can be any valid identifier. Here, show is used as the pure virtual function name.

3fill in blank

hard

Fix the error in the derived class by correctly overriding the pure virtual function display.

C++

class Derived : public Base {
public:
    void [1]() override {
        // implementation
    }
};

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

Adisplay

Brun

Cprint

Dshow

Attempts:

3 left

💡 Hint

Common Mistakes

Using a different function name than the base class pure virtual function.

Forgetting to override the pure virtual function causes the derived class to be abstract.

The derived class must override the pure virtual function with the exact same name to provide implementation.

4fill in blank

hard

Fill both blanks to declare a pure virtual function calculate that returns an int.

C++

class Calculator {
public:
    virtual [1] [2]() = 0;
};

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

Aint

Bvoid

Ccalculate

Ddisplay

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping return type and function name.

Using wrong function names.

The function return type is int and the function name is calculate.

5fill in blank

hard

Fill all three blanks to define a pure virtual function process that takes an int parameter and returns bool.

C++

class Processor {
public:
    virtual [1] [2]([3] value) = 0;
};

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

Abool

Bprocess

Cint

Dvoid

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up return type and parameter type.

Using wrong function names.

The function returns bool, is named process, and takes an int parameter.

Practice

1. What is the main purpose of a pure virtual function in C++?

easy

A. To force derived classes to implement the function

B. To allow creating objects of the base class

C. To provide a default implementation in the base class

D. To make the function private

Solution

1. Step 1: Understand pure virtual function role

   A pure virtual function is declared with = 0 and has no body in the base class.

2. Step 2: Effect on derived classes

   Derived classes must provide their own implementation of this function, or they become abstract too.

3. Final Answer:

   To force derived classes to implement the function -> Option A

4. Quick Check:

   Pure virtual function = forces implementation [OK]

Hint: Pure virtual means must override in child class [OK]

Common Mistakes:

• Thinking pure virtual allows base class objects
• Assuming pure virtual provides default code
• Confusing pure virtual with private functions

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

easy

A. void draw() = 0;

B. virtual void draw() = 0;

C. virtual void draw() {}

D. void virtual draw() = 0;

Solution

1. Step 1: Recall pure virtual function syntax

   Pure virtual functions use the keyword virtual and end with = 0;.

2. Step 2: Check each option

   virtual void draw() = 0; matches the correct syntax: virtual void draw() = 0;. Others have wrong order, missing virtual, or provide a body.

3. Final Answer:

   virtual void draw() = 0; -> Option B

4. Quick Check:

   Syntax = virtual + = 0 [OK]

Hint: Pure virtual = virtual + = 0 at end [OK]

Common Mistakes:

• Placing 'virtual' after return type incorrectly
• Adding function body {} for pure virtual
• Omitting 'virtual' keyword

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

#include <iostream>
class Shape {
public:
  virtual void draw() = 0;
};

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

int main() {
  Circle c;
  c.draw();
  return 0;
}

medium

A. No output

B. Compilation error: cannot instantiate abstract class

C. Circle drawn

D. Runtime error

Solution

1. Step 1: Analyze class Shape

   Shape has a pure virtual function draw(), so Shape is abstract and cannot be instantiated.

2. Step 2: Analyze class Circle

   Circle overrides draw() with its own implementation, so Circle is concrete and can be instantiated.

3. Step 3: Check main function

   Circle object c is created and c.draw() prints "Circle drawn".

4. Final Answer:

   Circle drawn -> Option C

5. Quick Check:

   Derived class implements pure virtual = prints output [OK]

Hint: Derived class must implement to create objects [OK]

Common Mistakes:

• Trying to instantiate abstract base class
• Forgetting to override pure virtual function
• Expecting no output due to pure virtual

4. Identify the error in this code snippet:

class Animal {
public:
  virtual void sound() = 0;
};

class Dog : public Animal {
};

int main() {
  Dog d;
  d.sound();
  return 0;
}

medium

A. No error, code runs fine

B. Animal class should not have pure virtual functions

C. Dog object cannot be created because main is missing

D. Dog class does not implement pure virtual function sound()

Solution

1. Step 1: Check pure virtual function in Animal

   Animal declares sound() as pure virtual, so Animal is abstract.

2. Step 2: Check Dog class implementation

   Dog inherits Animal but does not implement sound(), so Dog remains abstract and cannot be instantiated.

3. Step 3: Analyze main function

   Trying to create Dog object d causes compilation error because Dog is abstract.

4. Final Answer:

   Dog class does not implement pure virtual function sound() -> Option D

5. Quick Check:

   Abstract class must implement all pure virtual functions [OK]

Hint: Derived must implement all pure virtual functions [OK]

Common Mistakes:

• Assuming base class cannot have pure virtual functions
• Thinking missing main causes error here
• Believing code runs without overriding pure virtual

5. Given the following classes, which statement is true?

#include <iostream>
class Base {
public:
  virtual void f() = 0;
  virtual void g() { std::cout << "Base g" << std::endl; }
};

class Derived : public Base {
public:
  void f() override { std::cout << "Derived f" << std::endl; }
  void g() override { std::cout << "Derived g" << std::endl; }
};

int main() {
  Base* b = new Derived();
  b->f();
  b->g();
  delete b;
  return 0;
}

hard

A. Output is:
Derived f
Derived g

B. Output is:
Base f
Base g

C. Compilation error because Base has pure virtual function

D. Runtime error due to calling pure virtual function

Solution

1. Step 1: Understand Base class

   Base has pure virtual f() and virtual g() with implementation.

2. Step 2: Analyze Derived class

   Derived overrides both f() and g() with its own implementations.

3. Step 3: Check main function calls

   Pointer b of type Base* points to Derived object. Virtual calls invoke Derived versions of f() and g().

4. Final Answer:

   Output is: Derived f Derived g -> Option A

5. Quick Check:

   Virtual functions call derived overrides [OK]

Hint: Virtual calls use derived class methods [OK]

Common Mistakes:

• Expecting base class methods to run
• Thinking pure virtual causes compile error here
• Confusing pure virtual with runtime error