CRTP

CRTP

The acronym CRTP stands for the C++ idiom Curiously Recurring Template Pattern. It is a technique in C++ in which a Derived class derives from a class template Base. The key is that Base has Derived as a template argument.

Let’s have a look at an example:

template<class T> 
class Base{
   ...
};

class Derived: public Base<Derived>{ 
   ...
};

CRTP enables static polymorphism.

Typical use-case

There are two typical use-cases for CRTP: Mixins and static polymorphism.

Mixins

Mixins are a popular concept in the design of classes used to mix in new code. Therefore, it’s a technique often used in Python to change the behavior of a class by using multiple inheritances. In contrast to C++, it is legal in Python to have more than one definition of a method in a class hierarchy. Python simply uses the first method in the Method Resolution Order (MRO).

We can implement mixins in C++ by using CRTP. A prominent example is the class std::enable_shared_from_this. Using this class, we can create objects that return an std::shared_ptr with themselves. We have to derive the public class MySharedClass from std::enable_shared_from_this. Now, our MySharedClass has a method called shared_from_this.

An additional common use-case for mixins is a class that we want to extend with the capability that their instances support the comparison for equality and inequality.

Static polymorphism

Static polymorphism is quite similar to dynamic polymorphism. But contrary to dynamic polymorphism with virtual methods, the dispatch of the method calls will take place at compile-time. Now, we are at the center of the CRTP idiom.

class ShareMe: public std::enable_shared_from_this<ShareMe>{
  std::shared_ptr<ShareMe> getShared(){
    return shared_from_this();
  } 
};

• std::enable_shared_from_this creates a shared _ptr for an object.
• std::enable_shared_from_this: is the base class of the object.
• shared_from_this: returns the shared object.

Further information

• CRTP

• Mixins

• Method Resolution Order

---

In the next lesson, we’ll look at a couple of examples of CRTP.