Curiously Recurring Template Pattern

In C++, a Curiously Recurring Template Pattern is an idiomatic approach to achieving Static Polymorphism, where a class $X$ derives from a class Template instantiation using $X$ itself as the template argument.

template <typename Derived>
struct Shape {
    double area() const {
        return static_cast<const Derived*>(this)->area_impl();
    }
};
 
struct Circle : Shape<Circle> {
    double radius;
    double area_impl() const { return 3.14159 * radius * radius; }
};
 
struct Square : Shape<Square> {
    double side;
    double area_impl() const { return side * side; }
};