C++: What is CRTP (Curiously Recurring Template Pattern)?

1// Base class takes Derived as template parameter
2template<typename Derived>
3class Base {
4public:
5    void interface() {
6        // Cast to derived and call implementation
7        static_cast<Derived*>(this)->implementation();
8    }
9};
10
11// Derived passes itself as template argument
12class Derived : public Base<Derived> {
13public:
14    void implementation() {
15        std::cout << "Derived implementation\n";
16    }
17};
18
19int main() {
20    Derived d;
21    d.interface();  // Calls Derived::implementation
22}

1template<typename Derived>
2class Shape {
3public:
4    void draw() const {
5        static_cast<const Derived*>(this)->draw_impl();
6    }
7};
8
9class Circle : public Shape<Circle> {
10public:
11    void draw_impl() const {
12        std::cout << "Drawing circle\n";
13    }
14};

1template<typename Derived>
2class Singleton {
3public:
4    static Derived& getInstance() {
5        static Derived instance;
6        return instance;
7    }
8};
9
10class Logger : public Singleton<Logger> {
11    // ...
12};