What is the curiously recurring template pattern (CRTP)?

Without referring to a book, can anyone please provide a good explanation for CRTP (curiously recurring template pattern) with a code example?

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

#include <iostream>

template <class T>
class Singleton
{
public:
     static T* GetInstance() {
         if ( p == nullptr ) p = new T();
         return p;
     }
protected:
     Singleton() = default;

     Singleton(Singleton const &) = delete;
     Singleton &operator=(const Singleton &) = delete;

private:
     static T *p;
};
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> 
{ 
friend Singleton;
private:
    A() = default;
};
A *a0= A::GetInstance();

Howevry, CRTP is not necessary in this case, see as follow:

class C 
{ 
friend Singleton<C>; 
private: C() = default;
};
C *c1= Singleton<C>::GetInstance();

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 exercise!

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, isn't it?
    Derived const& d2 = static_cast<Derived const&>(op2); 
    return !(d1 < d2) && !(d2 < d1);//assuming derived has operator <
}

or implement within the template scope without casting

template<class T>
class Equality
{
    friend bool operator == (const T& op1, const T& op2)
    { 
        return !(op1 < op2) && !(op2 < op1); 
    }
};

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