Inheritance in C++

Learn Inheritance in C++ with simple examples, syntax, types, and access specifiers. A complete beginner to advanced guide for interviews and exams.

Inheritance in C++ is an Object-Oriented Programming (OOP) concept where one class (called the derived class) acquires the properties and behavior (variables and functions) of another class (called the base class).

In simple terms:
Inheritance allows you to reuse existing code and build new features on top of it.

Basic Syntax


class BaseClass {
// data members and member functions
};

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

Example :

class BaseClass {
public:
    int a;

    void display() {
        cout << "This is base class" << endl;
    }
};

class DerivedClass : public BaseClass {
public:
    void show() {
        cout << "This is derived class" << endl;
    }
};

Example Program

#include <iostream>
using namespace std;

class Animal {
public:
    void eat() {
        cout << "Animal eats" << endl;
    }
};

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

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

Output:

Animal eats
Dog barks

How Inheritance Works in C++

1.Base Class is Created

The base class contains common properties and functions.

class Animal {
public:
    void eat() {
        cout << "Animal eats" << endl;
    }
};

2.Derived Class Inherits Base Class

The derived class uses : to inherit from the base class.

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

3.Object of Derived Class is Created

When you create an object of the derived class, it can access:

Its own methods
Inherited methods from base class

int main() {
    Dog d;
    d.eat();   // inherited function
    d.bark();  // own function
}

Internal Working of Inheritance

Memory Sharing

The derived class object contains base class data + its own data
No duplication of code

Conceptually:

Dog object = Animal properties + Dog properties

Function Access

Derived class can directly use public and protected members
Cannot access private members of base class directly

Constructor Execution Order

When object is created:

Base class constructor runs first
Derived class constructor runs next

class A {
public:
    A() {
        cout << "Base Constructor" << endl;
    }
};

class B : public A {
public:
    B() {
        cout << "Derived Constructor" << endl;
    }
};

Output:

Base Constructor
Derived Constructor

Destructor Order (Reverse)

When object is destroyed:

Derived class destructor runs
Base class destructor runs

Function Overriding

Derived class can override base class function

class A {
public:
    void show() {
        cout << "Base";
    }
};

class B : public A {
public:
    void show() {
        cout << "Derived";
    }
};

Access Specifiers in C++ Inheritance

In inheritance, access specifiers define how the base class members are accessible in the derived class.

Types of Access Specifiers

`public`

Accessible everywhere
Can be used inside and outside the class

`protected`

Accessible within the class and derived class
Not accessible outside

`private`

Accessible only within the same class
Not accessible in derived class directly

Syntax with Access Specifier

class DerivedClass : access_specifier BaseClass {
    // code
};

Example:

class B : public A

How Access Specifiers Work in Inheritance

Inheritance Type	Base `public` →	Base `protected` →	Base `private` →
`public`	public	protected	❌ not accessible
`protected`	protected	protected	❌ not accessible
`private`	private	private	❌ not accessible

Example Code

#include <iostream>
using namespace std;

class A {
public:
    int x = 10;

protected:
    int y = 20;

private:
    int z = 30;
};

class B : public A {
public:
    void show() {
        cout << x << endl; // ✅ public → accessible
        cout << y << endl; // ✅ protected → accessible
        // cout << z; ❌ private → not accessible
    }
};

int main() {
    B obj;
    obj.show();
    cout << obj.x; // ✅ public accessible outside
    // cout << obj.y; ❌ not accessible outside
}

Key Points

public → accessible everywhere
protected → accessible in child class only
private → not accessible in child class

Default inheritance:

class → private inheritance
struct → public inheritance

Real-Life Working Example

Think of:

Parent Class: Vehicle
Child Class: Car

Car automatically gets:

speed
engine

But also adds:

air conditioning
music system

Why Inheritance is Used in C++

Benefits

Code reusability
Reduces duplication
Easy maintenance
Supports polymorphism
Improves readability

Important Points

Inheritance uses : operator
Base class constructor executes first
Private members are not directly accessible
Supports multiple types (single, multiple, multilevel, etc.)
Used for “is-a” relationship (Dog is an Animal)

What is the use of `:` in C++ Inheritance?

In C++, the colon : is used to specify inheritance.

It tells the compiler that:

“This class is inheriting from another class.”

Basic Syntax

class DerivedClass : public BaseClass {
};

Here:

: → means inherit from
public → access specifier
BaseClass → parent class

How `:` Works

When you write:

class B : public A {
};

It means:

Class B gets properties and functions of class A
B becomes child (derived class)
A becomes parent (base class)

Example

#include <iostream>
using namespace std;

class A {
public:
    void show() {
        cout << "Base class" << endl;
    }
};

class B : public A {
};

int main() {
    B obj;
    obj.show();  // inherited using :
}

Without : → inheritance will NOT happen

Without `:` (No Inheritance)

class B {
};

Now:

B has no relation with A
Cannot access A’s functions

Real Meaning of `:`

Think of it like:

: = “is derived from”
: = “inherits from”

Important Uses of `:`

1. To inherit a class

class Child : public Parent

2. Multiple inheritance

class C : public A, public B

3. Access control during inheritance

class B : private A

Types of Inheritance in C++

1. Single Inheritance

One base class → One derived class

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

2. Multiple Inheritance

One derived class inherits from multiple base classes

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

3. Multilevel Inheritance

Chain of inheritance

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

4. Hierarchical Inheritance

One base class → multiple derived classes

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

5. Hybrid Inheritance

Combination of multiple types (complex structure)

Access Specifiers in Inheritance

Inheritance Type	Public Members	Protected Members	Private Members
public	public	protected	not accessible
protected	protected	protected	not accessible
private	private	private	not accessible

Example:

class A {
protected:
    int x;
};

class B : public A {
public:
    void set() {
        x = 10; // accessible
    }
};

Key Concepts

1. Code Reusability

Reuse existing class code → reduces redundancy

2. Method Overriding

Derived class can redefine base class function

class A {
public:
    void show() {
        cout << "A";
    }
};

class B : public A {
public:
    void show() {
        cout << "B";
    }
};

3. Constructor in Inheritance

Base class constructor runs first

class A {
public:
    A() {
        cout << "Base constructor" << endl;
    }
};

class B : public A {
public:
    B() {
        cout << "Derived constructor" << endl;
    }
};

4. Destructor Order

Destructor runs in reverse order

5. `protected` Keyword

Allows access in derived classes but not outside

6. `super` concept in C++

C++ uses base class name instead of super

class B : public A {
public:
    void show() {
        A::show(); // calling base class function
    }
};

Advantages of Inheritance

✔ Code reuse
✔ Easy maintenance
✔ Extensibility
✔ Improves readability
✔ Supports polymorphism

Disadvantages

❌ Tight coupling between classes
❌ Can increase complexity
❌ Improper use leads to errors

Real-Life Example

Think of:

Vehicle (Base Class)
Car, Bike (Derived Classes)

All share common features like speed, engine, but also have unique features.

Interview Questions

What is inheritance in C++?
Difference between inheritance and polymorphism?
Types of inheritance?
What is multiple inheritance?
What is diamond problem?
Difference between public, private, protected inheritance?
Constructor order in inheritance?
What is virtual inheritance?

Summary

Inheritance is a powerful feature in C++ that allows one class to inherit properties and behavior from another class, making code reusable, scalable, and easier to manage.

Frequently Asked Questions (FAQs) on Inheritance in C++

1️⃣ What is inheritance in C++?

Inheritance is an OOP concept where a derived class acquires properties and functions of a base class.

2️⃣ Why is inheritance used in C++?

It is used for:

Code reusability
Reducing duplication
Easy maintenance
Extending existing functionality

3️⃣ What is a base class and derived class?

Base class → Parent class
Derived class → Child class that inherits from base class

4️⃣ What does `:` mean in inheritance?

The : operator is used to define inheritance between classes.

5️⃣ What are the types of inheritance in C++?

Single
Multiple
Multilevel
Hierarchical
Hybrid

6️⃣ What is single inheritance?

One derived class inherits from one base class.

7️⃣ What is multiple inheritance?

A derived class inherits from more than one base class.

8️⃣ What is multilevel inheritance?

A class is derived from another derived class (chain structure).

9️⃣ What is hierarchical inheritance?

Multiple classes inherit from the same base class.

🔟 What is hybrid inheritance?

Combination of multiple inheritance types.

1️⃣1️⃣ What are access specifiers in inheritance?

They control visibility of base class members:

public
protected
private

1️⃣2️⃣ Can private members be inherited?

Yes, but they are not directly accessible in derived class.

1️⃣3️⃣ What is function overriding?

When a derived class provides its own implementation of a base class function.

1️⃣4️⃣ What is the order of constructor execution?

Base class constructor executes first
Then derived class constructor

1️⃣5️⃣ What is the diamond problem?

It occurs in multiple inheritance when a class inherits from two classes that have a common base class, causing ambiguity.

Solution: Virtual inheritance

For detailed understanding of Platform Devices and Drivers on Linux, refer to the Linux documentation on Platform Devices and Drivers .

Basic Syntax

Example :

Example Program

How Inheritance Works in C++

1.Base Class is Created

2.Derived Class Inherits Base Class

3.Object of Derived Class is Created

Internal Working of Inheritance

Memory Sharing

Function Access

Constructor Execution Order

Destructor Order (Reverse)

Function Overriding

Access Specifiers in C++ Inheritance

Types of Access Specifiers

public

protected

private

Syntax with Access Specifier

How Access Specifiers Work in Inheritance

Example Code

Key Points

Real-Life Working Example

Why Inheritance is Used in C++

Benefits

Important Points

What is the use of : in C++ Inheritance?

Basic Syntax

How : Works

Example

Without : (No Inheritance)

Real Meaning of :

Important Uses of :

1. To inherit a class

2. Multiple inheritance

3. Access control during inheritance

Types of Inheritance in C++

1. Single Inheritance

2. Multiple Inheritance

3. Multilevel Inheritance

4. Hierarchical Inheritance

5. Hybrid Inheritance

Access Specifiers in Inheritance

Key Concepts

1. Code Reusability

2. Method Overriding

3. Constructor in Inheritance

4. Destructor Order

5. protected Keyword

6. super concept in C++

Advantages of Inheritance

Disadvantages

Real-Life Example

Interview Questions

Summary

Frequently Asked Questions (FAQs) on Inheritance in C++

1️⃣ What is inheritance in C++?

2️⃣ Why is inheritance used in C++?

3️⃣ What is a base class and derived class?

4️⃣ What does : mean in inheritance?

5️⃣ What are the types of inheritance in C++?

6️⃣ What is single inheritance?

7️⃣ What is multiple inheritance?

8️⃣ What is multilevel inheritance?

9️⃣ What is hierarchical inheritance?

🔟 What is hybrid inheritance?

1️⃣1️⃣ What are access specifiers in inheritance?

1️⃣2️⃣ Can private members be inherited?

1️⃣3️⃣ What is function overriding?

1️⃣4️⃣ What is the order of constructor execution?

1️⃣5️⃣ What is the diamond problem?

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`public`

`protected`

`private`

What is the use of `:` in C++ Inheritance?

How `:` Works

Without `:` (No Inheritance)

Real Meaning of `:`

Important Uses of `:`

5. `protected` Keyword

6. `super` concept in C++

4️⃣ What does `:` mean in inheritance?