Inheritance & Polymorphism

#include <iostream>
#include <vector>
#include <memory>
using namespace std;

// Abstract base class
class Shape {
protected:
    string color;
public:
    Shape(const string &color) : color(color) {}
    virtual ~Shape() = default;  // Virtual destructor (essential!)

    // Pure virtual function (makes class abstract)
    virtual double area() const = 0;
    virtual void draw() const = 0;

    // Non-virtual: shared implementation
    string getColor() const { return color; }
};

class Circle : public Shape {
    double radius;
public:
    Circle(double r, const string &color = "red")
        : Shape(color), radius(r) {}

    double area() const override { return 3.14159 * radius * radius; }
    void draw() const override {
        cout << "Drawing " << color << " circle (r=" << radius << ")" << endl;
    }
};

class Rectangle : public Shape {
    double width, height;
public:
    Rectangle(double w, double h, const string &color = "blue")
        : Shape(color), width(w), height(h) {}

    double area() const override { return width * height; }
    void draw() const override {
        cout << "Drawing " << color << " rectangle (" << width << "x" << height << ")" << endl;
    }
};

int main() {
    // Polymorphism: base pointer holds derived objects
    vector<unique_ptr<Shape>> shapes;
    shapes.push_back(make_unique<Circle>(5.0));
    shapes.push_back(make_unique<Rectangle>(4.0, 6.0));
    shapes.push_back(make_unique<Circle>(3.0, "green"));

    // Calls correct draw() and area() for each type
    for (const auto &shape : shapes) {
        shape->draw();
        cout << "Area: " << shape->area() << endl;
    }

    // Shape s;  // ERROR: cannot instantiate abstract class

    return 0;
}