Abstract classes in C++ - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare an abstract class with a pure virtual function.

C++

class Shape {
public:
    virtual void draw() [1];
};

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

A()

B{}

C;

D= 0

Attempts:

3 left

💡 Hint

Common Mistakes

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

Using empty braces {} defines a normal function, not pure virtual.

In C++, a pure virtual function is declared by assigning = 0 to the function declaration inside the class.

2fill in blank

medium

Complete the code to prevent instantiation of the abstract class.

C++

Shape s; // Error: cannot instantiate abstract class

class Shape {
public:
    virtual void draw() = [1];
};

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

A0

B1

Cnull

Dvoid

Attempts:

3 left

💡 Hint

Common Mistakes

Using = 1 or other values instead of = 0.

Trying to instantiate an abstract class causes a compile error.

Assigning = 0 to a virtual function makes the class abstract, so it cannot be instantiated.

3fill in blank

hard

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

C++

class Circle : public Shape {
public:
    void draw() [1] {
        // draw circle
    }
};

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

A()

B() override

C(int) override

D(void) override

Attempts:

3 left

💡 Hint

Common Mistakes

Changing the parameter list causes the function not to override.

Omitting override keyword is allowed but not recommended.

The derived class must override the pure virtual function with the exact signature and use override keyword for clarity.

4fill in blank

hard

Fill both blanks to declare an abstract class with a pure virtual destructor and a pure virtual function.

C++

class Interface {
public:
    virtual ~Interface() [1];
    virtual void execute() [2];
};

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

A= 0

B{}

C;

D()

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting to define a virtual destructor causes undefined behavior.

Using empty braces {} instead of = 0 does not make the function pure virtual.

Both the destructor and the function are declared pure virtual by assigning = 0.

5fill in blank

hard

Fill all three blanks to implement a derived class that overrides all pure virtual functions and destructor.

C++

class Derived : public Interface {
public:
    ~Derived() [1] override {}
    void execute() [2] override {
        // implementation
    }
    void extra() [3] {
        // extra method
    }
};

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

A()

D(int)

Attempts:

3 left

💡 Hint

Common Mistakes

Changing parameter lists breaks overriding.

Forgetting override keyword can cause silent errors.

All overridden functions must match the base class signature exactly, which is () with no parameters.

Practice

1. What is true about an abstract class in C++?

easy

A. It can be instantiated like any other class.

B. It cannot be instantiated directly.

C. It must have no member functions.

D. It cannot have any data members.

Solution

1. Step 1: Understand abstract class instantiation rules

   Abstract classes have at least one pure virtual function, so they cannot create objects directly.

2. Step 2: Check other options

   The other options are incorrect because abstract classes can have member functions and data members.

3. Final Answer:

   It cannot be instantiated directly. -> Option B

4. Quick Check:

   Abstract class = no direct objects [OK]

Hint: Abstract classes always have pure virtual functions [OK]

Common Mistakes:

• Thinking abstract classes can create objects
• Believing abstract classes cannot have data members
• Confusing abstract classes with interfaces

2. Which of the following is the correct way to declare a pure virtual function in a C++ abstract class?

easy

A. virtual void func() = 0;

B. void func() = 0;

C. virtual void func();

D. void func() {};

Solution

1. Step 1: Recall pure virtual function syntax

   Pure virtual functions are declared with 'virtual' keyword and '= 0' at the end.

2. Step 2: Analyze options

   virtual void func() = 0; matches the correct syntax. void func() = 0; misses 'virtual'. virtual void func(); is a normal virtual function. void func() {}; is a normal function with a body.

3. Final Answer:

   virtual void func() = 0; -> Option A

4. Quick Check:

   Pure virtual = virtual + '= 0' [OK]

Hint: Pure virtual always ends with '= 0;' [OK]

Common Mistakes:

• Omitting 'virtual' keyword
• Adding function body to pure virtual
• Confusing normal virtual with pure virtual

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

#include <iostream>
class Base {
public:
    virtual void show() = 0;
};
class Derived : public Base {
public:
    void show() { std::cout << "Hello"; }
};
int main() {
    Derived d;
    d.show();
    return 0;
}

medium

A. No output

B. Compilation error

C. Hello

D. Runtime error

Solution

1. Step 1: Check abstract class and derived implementation

   Base has a pure virtual function 'show'. Derived overrides it with a concrete implementation.

2. Step 2: Analyze main function output

   Derived object 'd' calls 'show()', which prints "Hello".

3. Final Answer:

   Hello -> Option C

4. Quick Check:

   Derived implements pure virtual, so prints output [OK]

Hint: Derived must implement pure virtual to avoid errors [OK]

Common Mistakes:

• Expecting compilation error without implementation
• Thinking pure virtual functions run by themselves
• Confusing output with no output

4. Identify the error in this code snippet:

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

class Concrete : public Abstract {
public:
    void display() { std::cout << "Hi"; }
};

int main() {
    Abstract a;
    Concrete c;
    c.display();
    return 0;
}

medium

A. Cannot instantiate abstract class 'Abstract' directly.

B. Missing implementation of display() in Concrete.

C. Syntax error in class declaration.

D. No error, code runs fine.

Solution

1. Step 1: Check instantiation of abstract class

   Abstract class 'Abstract' has a pure virtual function, so cannot create object 'a'.

2. Step 2: Verify other parts

   Concrete implements display(), so no error there. Syntax is correct.

3. Final Answer:

   Cannot instantiate abstract class 'Abstract' directly. -> Option A

4. Quick Check:

   Abstract objects not allowed [OK]

Hint: Never create objects of abstract classes [OK]

Common Mistakes:

• Trying to instantiate abstract class
• Ignoring pure virtual function rules
• Assuming implementation missing in derived

5. You want to design a shape system where every shape must provide its area calculation. Which is the best way to enforce this using abstract classes in C++?

hard

A. Create a class Shape with no functions and add area() only in derived classes.

B. Create a class Shape with a normal function double area() that returns 0, and override it in derived classes.

C. Create a class Shape with a virtual function double area() with empty body.

D. Create an abstract class Shape with a pure virtual function double area() = 0; and derive specific shapes implementing area().

Solution

1. Step 1: Understand requirement for enforcing area() implementation

   Using a pure virtual function in an abstract class forces all derived classes to implement area().

2. Step 2: Evaluate options

   Create an abstract class Shape with a pure virtual function double area() = 0; and derive specific shapes implementing area(). uses pure virtual function, which is the best way. Create a class Shape with a normal function double area() that returns 0, and override it in derived classes. returns 0 by default, which may hide missing implementations. Create a class Shape with no functions and add area() only in derived classes. does not enforce area() in base. Create a class Shape with a virtual function double area() with empty body. has virtual but not pure, so no enforcement.

3. Final Answer:

   Create an abstract class Shape with a pure virtual function double area() = 0; and derive specific shapes implementing area(). -> Option D

4. Quick Check:

   Pure virtual enforces implementation [OK]

Hint: Use pure virtual to force derived class methods [OK]

Common Mistakes:

• Using normal virtual instead of pure virtual
• Not enforcing method in base class
• Returning default values hiding missing overrides