Curiously Recurring Template Pattern (CRTP)

In which a subclass derives from a class template instantiated using itself (subclass) as the template argument.

// Base template, `Base`:
template <typename T>  // T will be a derived class.
class Base {};

// Derived class instantiates T using `Derived` itself:
class Derived : public Base<Derived> {};

Note how Derived refers to itself before it is fully defined (brings to mind recursion).

CRTP can be useful for static polymorphism. But before that, let’s look at general (or dynamic) polymorphism in C++:

class Animal {
public:
  virtual std::string sound() = 0;
  void hello() { std::cout << sound() << "\n"; }
};

class Dog : public Animal {
  std::string sound() override { return "woof"; }
};

class Cat : public Animal {
  std::string sound() override { return "meow"; }
};

int main() {
  Dog d;
  d.hello();

  Cat c;
  c.hello();
}

Define a base class Animal, and two subclasses Dog and Cat which overrides the implementation of sound.

This is known as dynamic dispatch. At runtime, the implementation of the member function sound is looked up via the vtable for the object (Dog or Cat in our example). This dynamic lookup comes with a performance cost, since the lookup happens at runtime.

CRTP allows us to move this dynamic polymorphism to compile time:

template <typename T>
class Clock {
  public:
    void hello() {  // hello is declared here, but not instantiated
      std::cout << static_cast<T*>(this)->sound() << "\n";
    }
};

class Tick : public Clock<Tick> {
public:
  std::string sound() { return "tick"; }
};

class Tock : public Clock<Tock> {
public:
  std::string sound() { return "tock"; }
};

int main() {
  Tick tick;  // hello is instantiated here, T is known, sound() is defined.
  tick.hello();

  Tock tock;
  tock.hello();
}

Instead of a base class we have a class template Clock¹. The body of hello is not instantiated when Clock is defined, but rather only when the template is used (recall that templates cost nothing, until used). So, the body of hello can cast this to the derived class (template argument)T, and call member functions of the derived class.

Now, the call to sound is done at compile time (there is no more virtual member function).

The Wikipedia page for CRTP lists many more interesting examples and uses.

Object counter

The object counter class gives us some guidelines for when to use CRTP:

• you want code sharing between classes
• but need to keep base classes unique because, e.g. you want static member variables to be separate

Using dynamic polymorphism, if you had used:

struct counter { ... };
class X : counter {};
class Y : counter {};

The static member variable will be shared for X and Y.

But since each template class is unique, the static member variables will not be shared.

Printer

Each time you call a base class member function, the current class in the chain of method invocations becomes the base class - we forget who we are. CRTP can help us to remember the derived class.