Alok Save Alok Save - 1 month ago 7x
C++ Question

What is the curiously recurring template pattern (CRTP)?

Can anyone please provide a good explanation for

with a code example?

Please do not ask me to refer a book, I already do have the books and refer them but I usually find the explanations/examples that you guys come up with at SO much more suitable and practical in understanding the concept and lot of other important subtle things that go with a concept.


In short, CRTP is when a class A has a base class which is a template specialization for the class A itself. E.g.

template <class T> class X{...};
class A : public X<A> {...};

It is curiously recurring, isn't it? :)

Now, what does this give you? This actually gives the X template the ability to be a base class for its specializations.

For example, you could make a generic singleton class (simplified version) like this

template <class ActualClass> 
class Singleton
   static ActualClass& GetInstance()
      if(p == nullptr)
         p = new ActualClass;
      return *p; 

   static Actualclass* p;
   Singleton(Singleton const &);
   Singleton& operator = (Singleton const &); 
template <class T>
T* Singleton<T>::p = nullptr;

Now, in order to make an arbitrary class A a singleton you should do this

class A: public Singleton<A>
   //Rest of functionality 

So you see? The singleton template assumes that its specialization for any type X will be inherited from singleton<X> and thus will have all its(public, protected) members accessible, including the GetInstance! There are other useful uses of CRTP. For example, if you want to count all instances that currently exist for your class, but want to encapsulate this logic in a separate template (the idea for a concrete class is quite simple - have a static variable, increment in ctors, decrement in dtors). Try to do it as an excercise!

Yet another useful example, for boost(I am not sure how they have implemented it, but CRTP will do too). Imagine you want to provide only operator < for your classes but automatically operator == for them!

you could do it like this:

template<class Derived>
class Equality

template <class Derived>
bool operator == (Equality<Derived> const& op1, Equality<Derived> const & op2)
    Derived const& d1 = static_cast<Derived const&>(op1);//you assume this works     
    //because you know that the dynamic type will actually be your template parameter.
    //wonderful, isnit it?
    Derived const& d2 = static_cast<Derived const&>(op2); 
    return !(d1 < d2) && !(d2 < d1);//assuming derived has operator <

Now you can use it like this

struct Apple:public Equality<Apple> 
    int size;

bool operator < (Apple const & a1, Apple const& a2)
    return a1.size < a2.size;

now, you haven't provided explicitly operator == for apple? But you have it! You can write

int main()
    Apple a1;
    Apple a2; 

    a1.size = 10;
    a2.size = 10;
    if(a1 == a2) //the compiler won't complain! 

This could seem that you would write less if you just wrote operator == for Apple, but imagine that the Equality template would provide not only == but >, >=, <= etc. And you could use these definitions for multiple classes, reusing the code!

CRTP is a wonderful thing :) HTH