Abstract Classes

An abstract class is a class that cannot be instantiated directly.

A class becomes abstract when it has at least one pure virtual function.

Purpose

Abstract classes are used to define a common interface for derived classes.

They can contain:

• Pure virtual functions.
• Normal virtual functions.
• Normal member functions.
• Data members.
• Constructors.
• Destructors.

Complete Example

#include <iostream>
#include <string>
using namespace std;

class Payment {
protected:
    double amount;

public:
    Payment(double paymentAmount) : amount(paymentAmount) {}
    virtual ~Payment() = default;

    virtual void process() const = 0;

    void showAmount() const {
        cout << "Amount: " << amount << endl;
    }
};

class CardPayment : public Payment {
public:
    CardPayment(double amount) : Payment(amount) {}

    void process() const override {
        cout << "Processing card payment" << endl;
    }
};

int main() {
    CardPayment payment(1500);

    payment.showAmount();
    payment.process();

    return 0;
}

Output:

Amount: 1500
Processing card payment

Abstract Class vs Interface

C++ does not have a separate interface keyword like Java.

A C++ class can behave like an interface if it mainly contains pure virtual functions.

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

Common Mistake

This is illegal:

// Payment payment(1000); // error: Payment is abstract

This is legal:

Payment* paymentPtr;

Pointers and references to abstract classes are allowed.

Viva Answer

An abstract class is a class that has at least one pure virtual function and cannot be instantiated directly. It is used as a base class to define a common interface for derived classes.

Quick Check

1. What makes a class abstract?
2. Can an abstract class have normal member functions?
3. Can we create a pointer to an abstract class?