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

1// CRTP mixin for counting objects
2template<typename Derived>
3class Counter {
4    static int count;
5public:
6    Counter() { ++count; }
7    Counter(const Counter&) { ++count; }
8    ~Counter() { --count; }
9    static int getCount() { return count; }
10};
11
12template<typename Derived>
13int Counter<Derived>::count = 0;
14
15class Dog : public Counter<Dog> {};
16class Cat : public Counter<Cat> {};
17
18Dog d1, d2;
19std::cout << Dog::getCount();  // 2
20
21Cat c1;
22std::cout << Cat::getCount();  // 1
23
24// Static polymorphism (no virtual)
25template<typename Derived>
26class Shape {
27public:
28    void draw() {
29        static_cast<Derived*>(this)->drawImpl();
30    }
31};
32
33class Circle : public Shape<Circle> {
34public:
35    void drawImpl() { std::cout << "Circle\n"; }
36};