#include <iostream>
#include <cmath>
#include <memory>

// Here's a basic interface for geometric shapes.
class Geometry {
public:
    virtual double area() const = 0;
    virtual double perim() const = 0;
    virtual ~Geometry() = default;
};

// For our example we'll implement this interface on
// `Rect` and `Circle` classes.
class Rect : public Geometry {
public:
    Rect(double w, double h) : width(w), height(h) {}
    
    double area() const override {
        return width * height;
    }
    
    double perim() const override {
        return 2*width + 2*height;
    }
    
private:
    double width, height;
};

class Circle : public Geometry {
public:
    Circle(double r) : radius(r) {}
    
    double area() const override {
        return M_PI * radius * radius;
    }
    
    double perim() const override {
        return 2 * M_PI * radius;
    }
    
private:
    double radius;
};

// If a function takes a pointer or reference to the interface type,
// we can pass any object that implements that interface.
// Here's a generic `measure` function taking advantage of this
// to work on any `Geometry`.
void measure(const Geometry& g) {
    std::cout << "Area: " << g.area() << std::endl;
    std::cout << "Perimeter: " << g.perim() << std::endl;
}

int main() {
    Rect r(3, 4);
    Circle c(5);

    // The `Circle` and `Rect` classes both
    // implement the `Geometry` interface so we can use
    // instances of these classes as arguments to `measure`.
    measure(r);
    measure(c);

    return 0;
}