CRTP: Static Polymorphism in C++

template <typename Derived>
class Shape {
public:
    void draw() const {
        static_cast<const Derived*>(this)->draw_impl();
    }
};

class Circle : public Shape<Circle> {
public:
    void draw_impl() const { /* 画圆 */ }
};

class Square : public Shape<Square> {
public:
    void draw_impl() const { /* 画正方形 */ }
};

// 使用
template <typename T>
void render(const Shape<T>& shape) {
    shape.draw();  // 编译期确定具体类型
}

template <typename Derived>
class Comparable {
public:
    bool operator!=(const Derived& other) const {
        return !(static_cast<const Derived&>(*this) == other);
    }
};

class Point : public Comparable<Point> {
    int x, y;
public:
    bool operator==(const Point& other) const {
        return x == other.x && y == other.y;
    }
};
// Point 自动获得 != 运算符