Types of inheritance in C++

Introduction

Inheritance lets one class get features from another class. It helps reuse code and organize programs better.

When you want a new class to have all features of an existing class plus some new ones.

When you want to model real-world relationships like a Dog is a kind of Animal.

When you want to share common code among several classes.

When you want to extend or modify behavior of existing classes.

Syntax

C++

class BaseClass {
  // base class members
};

class DerivedClass : access_specifier BaseClass {
  // derived class members
};

access_specifier is usually public, protected, or private.

Public inheritance means 'is-a' relationship.

Examples

Dog inherits from Animal. Dog can eat and bark.

C++

class Animal {
public:
  void eat() {}
};

// Single inheritance
class Dog : public Animal {
public:
  void bark() {}
};

Class C inherits from both A and B.

C++

class A {};
class B {};

// Multiple inheritance
class C : public A, public B {};

C inherits from B, which inherits from A.

C++

class A {};
class B : public A {};
class C : public B {};  

// Multilevel inheritance

Multiple classes inherit from the same base class.

C++

class A {};
class B : public A {};
class C : public A {};

// Hierarchical inheritance
// Both B and C inherit from A

Sample Program

This program shows single inheritance. Dog class inherits eat() from Animal and adds bark().

C++

#include <iostream>
using namespace std;

// Base class
class Animal {
public:
  void eat() {
    cout << "Eating" << endl;
  }
};

// Derived class
class Dog : public Animal {
public:
  void bark() {
    cout << "Barking" << endl;
  }
};

int main() {
  Dog d;
  d.eat();  // inherited from Animal
  d.bark(); // own method
  return 0;
}

Important Notes

Use public inheritance to keep base class methods accessible in derived class.

Private inheritance hides base class methods from outside.

Multiple inheritance can cause conflicts if base classes have same method names.

Summary

Inheritance lets a class reuse code from another class.

Types include single, multiple, multilevel, and hierarchical inheritance.

Use inheritance to model real-world 'is-a' relationships.

Practice

1. Which type of inheritance involves a single derived class inheriting from one base class in C++?

easy

A. Hierarchical inheritance

B. Multiple inheritance

C. Multilevel inheritance

D. Single inheritance

Solution

1. Step 1: Understand inheritance types

   Single inheritance means one class inherits from exactly one base class.

2. Step 2: Match definition to options

   Only Single inheritance describes a single derived class inheriting from one base class.

3. Final Answer:

   Single inheritance -> Option D

4. Quick Check:

   One derived, one base = Single inheritance [OK]

Hint: One derived from one base is single inheritance [OK]

Common Mistakes:

• Confusing multiple inheritance with single inheritance
• Thinking multilevel means one base class only
• Mixing hierarchical with single inheritance

2. Which of the following is the correct syntax for multiple inheritance in C++?

easy

A. class Derived : public Base1 & Base2 {};

B. class Derived : public Base1, public Base2 {};

C. class Derived : Base1 Base2 {};

D. class Derived inherits Base1, Base2 {};

Solution

1. Step 1: Recall multiple inheritance syntax

   In C++, multiple inheritance uses commas to separate base classes with access specifiers.

2. Step 2: Check each option

   class Derived : public Base1, public Base2 {}; uses correct syntax: 'class Derived : public Base1, public Base2 {};'. Others have syntax errors.

3. Final Answer:

   class Derived : public Base1, public Base2 {}; -> Option B

4. Quick Check:

   Comma separates base classes with access specifiers [OK]

Hint: Use commas and access specifiers for multiple inheritance [OK]

Common Mistakes:

• Omitting commas between base classes
• Using '&' instead of commas
• Writing 'inherits' keyword which doesn't exist in C++

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

class A { public: void show() { std::cout << "A"; } };
class B : public A { public: void show() { std::cout << "B"; } };
class C : public B {};
int main() {
  C obj;
  obj.show();
  return 0;
}

medium

A. B

B. A

C. C

D. Compilation error

Solution

1. Step 1: Understand inheritance chain

   Class C inherits from B, which inherits from A. B overrides show() method.

2. Step 2: Determine which show() is called

   Calling obj.show() on C calls B's show() because C inherits B's version and does not override it.

3. Final Answer:

   B -> Option A

4. Quick Check:

   Derived class method called if overridden [OK]

Hint: Derived class method overrides base method unless not overridden [OK]

Common Mistakes:

• Assuming base class method runs instead of overridden
• Expecting 'C' output though C has no show()
• Thinking code causes compilation error

4. Identify the error in this C++ code snippet involving inheritance:

class Base { public: int x; };
class Derived : Base {};
int main() {
  Derived d;
  d.x = 1;
  return 0;
}

medium

A. Main function missing return type

B. Base class must be declared inside Derived

C. Missing access specifier before Base in Derived class

D. Derived class cannot be empty

Solution

1. Step 1: Check inheritance syntax

   In C++, inheritance requires an access specifier like public, private, or protected before base class name.

2. Step 2: Identify the error

   Derived : Base {} lacks access specifier; should be Derived : public Base {}.

3. Final Answer:

   Missing access specifier before Base in Derived class -> Option C

4. Quick Check:

   Inheritance needs access specifier [OK]

Hint: Always specify public/private/protected before base class [OK]

Common Mistakes:

• Omitting access specifier in inheritance
• Thinking empty class body is error
• Confusing return type in main function

5. Given the following classes:

class Animal { public: void eat() { std::cout << "Eating"; } };
class Mammal : public Animal {};
class Bird : public Animal {};
class Bat : public Mammal, public Bird {};
int main() {
  Bat b;
  b.eat();
  return 0;
}

What problem arises and how to fix it?

hard

A. Ambiguity error due to multiple inheritance; fix with virtual inheritance

B. No problem; code runs fine

C. Syntax error in Bat class declaration

D. eat() method must be overridden in Bat

Solution

1. Step 1: Analyze inheritance structure

   Bat inherits Mammal and Bird, both inherit Animal, causing two Animal copies in Bat.

2. Step 2: Identify ambiguity

   Calling b.eat() is ambiguous because Bat has two Animal parts; compiler error occurs.

3. Step 3: Fix with virtual inheritance

   Use virtual inheritance for Mammal and Bird from Animal to share one Animal base.

4. Final Answer:

   Ambiguity error due to multiple inheritance; fix with virtual inheritance -> Option A

5. Quick Check:

   Diamond problem fixed by virtual inheritance [OK]

Hint: Use virtual inheritance to solve diamond ambiguity [OK]

Common Mistakes:

• Ignoring diamond problem ambiguity
• Thinking code compiles without error
• Assuming method override fixes ambiguity