Abstract classes in C++ - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an abstract class in C++?

An abstract class is a class that cannot be instantiated on its own and usually contains at least one pure virtual function. It serves as a blueprint for other classes.

beginner

What is a pure virtual function?

A pure virtual function is a function declared by assigning 0 in its declaration (e.g., virtual void func() = 0;). It must be overridden by derived classes.

beginner

Why can't you create an object of an abstract class?

Because abstract classes have pure virtual functions without implementations, the compiler prevents creating objects to ensure derived classes provide those implementations.

beginner

How do you declare a pure virtual function in C++?

By adding = 0 at the end of the function declaration inside the class, for example: virtual void draw() = 0;.

intermediate

What happens if a derived class does not override all pure virtual functions?

The derived class also becomes abstract and cannot be instantiated until it overrides all pure virtual functions from its base classes.

Which keyword is used to declare a pure virtual function in C++?

A = 0

B virtual

C abstract

D override

A pure virtual function is declared by adding '= 0' at the end of its declaration.

Can you create an object of an abstract class?

A Yes, always

B No, never

C Only if it has no pure virtual functions

D Only if derived classes exist

Abstract classes cannot be instantiated because they have pure virtual functions without implementations.

What must a derived class do if its base class has a pure virtual function?

A Make the function private

B Ignore the pure virtual function

C Delete the pure virtual function

D Override the pure virtual function

Derived classes must override all pure virtual functions to become concrete and instantiable.

What happens if a derived class does not override a pure virtual function?

A It causes a runtime error

B It compiles without errors

C It becomes an abstract class

D It overrides automatically

If a derived class does not override all pure virtual functions, it remains abstract and cannot be instantiated.

Which of these is true about abstract classes?

A They can have implemented functions

B They cannot have any functions

C They must be final classes

D They are always empty

Abstract classes can have both implemented and pure virtual functions.

Explain what an abstract class is and why it is useful in C++.

Think about classes that define rules but don't create objects directly.

Describe how pure virtual functions enforce behavior in derived classes.

Consider how a base class can require derived classes to provide specific functions.

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