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When you’re learning object-oriented programming in C++, two important concepts you’ll come across are Abstract Class and Interface in Cpp. These might sound a bit technical at first, but don’t worry — we’ll break everything down in a simple way.

Are you new to object-oriented programming and trying to understand the difference between an Abstract Class and Interface in Cpp? This beginner-friendly guide on Abstract Class and Interface in C++ is exactly what you need! In this detailed article, we explain what abstract classes are, how pure virtual functions work, and how C++ uses abstract classes to simulate interface behavior.

You’ll learn:

• The definition and purpose of abstract classes in C++
• How to declare and use pure virtual functions
• The concept of interfaces in C++ and how to implement them
• Key differences between abstract classes and interfaces
• Real-world examples and code snippets for hands-on learning

Whether you’re a student, a programming beginner, or someone preparing for C++ interviews, this comprehensive explanation of Abstract Class and Interface in C++ will strengthen your understanding and help you write clean, modular, and reusable code.

Abstract Class and Interface in Cpp

What is an Abstract Class?

An abstract class is a class that cannot be directly used to create objects. It’s like a blueprint that other classes must follow.

It is mainly used when:

• You want to define a common structure or behavior for all derived classes.
• You want to force derived classes to implement some specific functions.

How to define an abstract class?

In C++, a class becomes abstract if it has at least one pure virtual function.

class Animal {
public:
    virtual void makeSound() = 0; // Pure virtual function
};

What is a Pure Virtual Function?

A pure virtual function is a function that has no body in the base class and must be overridden in derived classes.

Syntax:

virtual returnType functionName() = 0;

Example of Abstract Class:

#include <iostream>
using namespace std;

class Animal {
public:
    virtual void makeSound() = 0;  // pure virtual function
};

class Dog : public Animal {
public:
    void makeSound() {
        cout << "Woof!" << endl;
    }
};

int main() {
    Dog d;
    d.makeSound();  // Output: Woof!
}

🔸 You can’t create an object of Animal because it’s an abstract class.
🔸 You must override makeSound() in the derived class.

What is an Interface?

In C++, interface is not a keyword like in Java or C#. But we can achieve interface-like behavior using abstract classes.

An interface:

• Only contains pure virtual functions.
• Doesn’t have any data members or function definitions.

Example of Interface in C++:

class Printable {
public:
    virtual void print() = 0;  // pure virtual function
};

Any class that implements Printable must define the print() function.

Full Example of Interface:

#include <iostream>
using namespace std;

class Printable {
public:
    virtual void print() = 0;
};

class Document : public Printable {
public:
    void print() {
        cout << "Printing Document..." << endl;
    }
};

int main() {
    Document doc;
    doc.print();  // Output: Printing Document...
}

Key Differences: Abstract Class and Interface in cpp

Feature	Abstract Class	Interface-like (in C++)
Can have data members?	Yes	No
Can have function body?	Yes (non-pure virtual functions)	No (only pure virtual functions)
Can have constructors?	Yes	No (interfaces don’t need them)
Used for	Shared code + enforcing rules	Only enforcing rules (contract)

When to Use What?

• Use abstract class when you want to share code and behavior among related classes.
• Use an interface-style class when you want to define a contract (a set of rules that must be followed) without any implementation.

Summary of Abstract Class and Interface in cpp

• Abstract class = class with at least one pure virtual function.
• You can’t create objects of abstract classes.
• Interfaces in C++ are implemented using abstract classes with only pure virtual functions.
• Abstract classes can have both complete and incomplete functions; interfaces only have incomplete ones.

Frequently Asked Questions (FAQ) – Abstract Class and Interface in cpp

Q1: What is an abstract class in C++?
A: An abstract class in C++ is a class that contains at least one pure virtual function. It cannot be used to create objects directly and is meant to be inherited by other classes that implement its pure virtual functions.

Q2: What is a pure virtual function?
A: A pure virtual function is a function declared in a base class with no definition. It uses the = 0 syntax and must be overridden by derived classes. Example:

virtual void speak() = 0;

Q3: Can we create an object of an abstract class in C++?
A: No, you cannot create an object of an abstract class. You must inherit it in a derived class and implement all pure virtual functions before creating an object.

Q4: What is an interface in C++?
A: C++ does not have a built-in interface keyword like Java, but you can create interface-like behavior using an abstract class that contains only pure virtual functions and no data members.

Q5: What is the difference between an abstract class and an interface in C++?
A:

• An abstract class can have both pure and non-pure (regular) member functions and can include data members.
• An interface (in C++ terms) contains only pure virtual functions and no data or implementation.

Q6: Can an abstract class have constructors in C++?
A: Yes, abstract classes can have constructors. However, since you can’t create an object of an abstract class, the constructor is only called when a derived class is instantiated.

Q7: Can a class implement multiple interfaces in C++?
A: Yes, C++ supports multiple inheritance, so a class can inherit from multiple abstract classes (interfaces), allowing it to implement multiple sets of behaviors.

Q8: When should I use an abstract class vs. an interface in C++?
A:

• Use an abstract class when you want to provide default/shared implementation along with rules.
• Use an interface-style abstract class when you only want to enforce certain function definitions (pure contract).

Q9: Can an abstract class have a destructor in C++?
A: Yes, and it’s good practice to declare the destructor as virtual in an abstract class to ensure proper cleanup when deleting derived objects.

Q10: Is it possible to inherit an abstract class without implementing all pure virtual functions?
A: Yes, but then the derived class also becomes abstract. To make it concrete (instantiable), all pure virtual functions must be implemented.

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