Abstract Class in C++

• A class that has at least one pure virtual function.

• Cannot be instantiated (you cannot create objects directly).

• Can contain:

  • Normal methods (concrete) ✅

  • Virtual methods ✅

  • Pure virtual methods (abstract) ✅

  • Data members ✅

Example:

CopyCopy#include <iostream>
using namespace std;

class Shape {   // Abstract class
public:
    virtual void draw() = 0;   // pure virtual -> must be overridden
    void info() { cout << "This is a shape\n"; } // normal method
};

class Circle : public Shape {
public:
    void draw() override { cout << "Drawing Circle\n"; }
};

int main() {
    // Shape s; ❌ Error: cannot instantiate abstract class
    Shape* shape = new Circle();
    shape->draw();   // "Drawing Circle"
    shape->info();   // "This is a shape"
    delete shape;
}

👉 Shape is abstract because it has a pure virtual method.
👉 Circle is concrete because it implements draw().

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Interface in C++ (Simulated)

In Java, interface means only abstract methods (and constants).
In C++, we simulate interfaces using abstract classes with only pure virtual methods.

Example:

CopyCopy#include <iostream>
using namespace std;

// Interface simulation
class Drawable {
public:
    virtual void draw() = 0;   // pure virtual
    virtual void resize() = 0; // pure virtual
};

class Circle : public Drawable {
public:
    void draw() override { cout << "Circle Drawn\n"; }
    void resize() override { cout << "Circle Resized\n"; }
};

int main() {
    Drawable* d = new Circle();
    d->draw();   // Circle Drawn
    d->resize(); // Circle Resized
    delete d;
}

👉 Here Drawable is like an interface:

• No concrete implementation.

• Just method declarations (= 0).

Difference Between Abstract Class and Interface in C++