Constexpr Virtual Functions for Compile-Time Class Hierarchies

#include <iostream>
 
class Shape {
public:
    constexpr virtual double area() const = 0;
    constexpr virtual ~Shape() = default;
};
 
class Rectangle : public Shape {
public:
    constexpr Rectangle(double w, double h) : width_(w), height_(h) {}
    constexpr double area() const override { return width_ * height_; }
private:
    double width_, height_;
};
 
class Circle : public Shape {
public:
    constexpr explicit Circle(double r) : radius_(r) {}
    constexpr double area() const override { return 3.141592653589793 * radius_ * radius_; }
private:
    double radius_;
};
 
constexpr double total_area(const Shape& s1, const Shape& s2) {
    return s1.area() + s2.area();
}
 
int main() {
    constexpr Rectangle rect(4.0, 5.0);
    constexpr Circle circ(3.0);
    constexpr double combined = total_area(rect, circ);
    
    static_assert(combined > 48.0 && combined < 49.0, "Compile-time calculation failed");
    std::cout << "Compile-time evaluated dynamic area: " << combined << '\n';
}