📘 Lesson · Lesson 72

Pure Virtual & Abstract Class

Abstract Classes

💡 Note

A pure virtual function (= 0) has no body and must be overridden. A class with one becomes an abstract class (cannot be instantiated).

Example

C++

#include <iostream>
using namespace std;
class Shape {
public:
    virtual double area() = 0;   // pure virtual
};
class Circle : public Shape {
    double r;
public:
    Circle(double r) : r(r) {}
    double area() override { return 3.14 * r * r; }
};
int main() {
    Shape* s = new Circle(5);
    cout << s->area();   // 78.5
    return 0;
}

Output:
78.5

Summary

A pure virtual function (=0) forces child classes to implement it.
An abstract class has at least one pure virtual and cannot be instantiated.

Abstract Classes

💡 Note

A pure virtual function (= 0) has no body and must be overridden. A class with one becomes an abstract class (cannot be instantiated).

Example

C++

#include <iostream>
using namespace std;
class Shape {
public:
    virtual double area() = 0;   // pure virtual
};
class Circle : public Shape {
    double r;
public:
    Circle(double r) : r(r) {}
    double area() override { return 3.14 * r * r; }
};
int main() {
    Shape* s = new Circle(5);
    cout << s->area();   // 78.5
    return 0;
}

Output:
78.5

सारांश

A pure virtual function (=0) forces child classes to implement it.
An abstract class has at least one pure virtual and cannot be instantiated.

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