奇异递归模板模式（Curiously Recurring Template Pattern，CRTP）

The Curiously Recurring Template Pattern (CRTP) is a C++ idiom whose name was coined by James Coplien in 1995, in early C++ template code.

The “C” in CRTP made it travel the years in the C++ community by being this: a Curiosity. We often find definitions of what CRTP is, and it is indeed an intriguing construct.

But what’s even more interesting is what CRTP means in code, as in what you can express and achieve by using it, and this is the point of this series.

If you have used CRTP in your own code then you certainly know what it is useful for. In this case you may know most of what’s written in this series of posts (though you may want to give a quick glance to episode #2, just to make sure we’re in line), and you can just skip over to episode #3 where I propose a generic helper for CRTP, that I found helpful when using it in code.

The CRTP, episode One: Definition
The CRTP, episode Two: What it can bring to your code
The CRTP, episode Three: Implementation

What CRTP is

The CRTP consists in:

inheriting from a template class,
use the derived class itself as a template parameter of the base class.

This is what it looks like in code:

template <typename T>
class Base
{...
};class Derived : public Base<Derived>
{...
};

The purpose of doing this is using the derived class in the base class. From the perspective of the base object, the derived object is itself, but downcasted. Therefore the base class can access the derived class by static_casting itslef into the derived class.

template <typename T>
class Base
{
public:void doSomething(){T& derived = static_cast<T&>(*this);//use derived...}
};

Note that contrary to typical casts to derived class, we don’t use dynamic_cast here. A dynamic_cast is used when you want to make sure at run-time that the derived class you are casting into is the correct one. But here we don’t need this guarantee: the Base class is designed to be inherited from by its template parameter, and by nothing else. Therefore it takes this as an assumption, and a static_cast is enough.

What could go wrong

If two classes happen to derive from the same CRTP base class we likely get to undefined behaviour when the CRTP will try to use the wrong class:

class Derived1 : public Base<Derived1>
{...
};class Derived2 : public Base<Derived1> // bug in this line of code
{...
};

There is a solution to prevent this, that has been proposed by Marek Kurdej in the comments section. It consists in adding a private constructor in the base class, and making the base class friend with the templated class:

template <typename T>
class Base
{
public:// ...
private:Base(){};friend T;
};

Indeed, the constructors of the derived class have to call the constructor of the base class (even if you don’t write it explicitly in the code, the compiler will do his best to do it for you). Since the constructor in the base class is private, no one can access it except the friend classes. And the only friend class is… the template class! So if the derived class is different from the template class, the code doesn’t compile. Neat, right?

Another risk with CRTP is that methods in the derived class will hide methods from the base class with the same name. As explained in Effective C++ Item 33, the reason for that is that these methods are not virtual. Therefore you want to be careful not to have identical names in the base and derived classes:

class Derived : public Base<Derived>
{
public:void doSomething(); // oops this hides the doSomething methods from the base class !
}

The first time I was shown CRTP my initial reaction was: “wait, I didn’t get it”. Then I saw it a couple of other times and I got it. So if you didn’t get how it works, just re-read section 1 a couple of times, and that should do it (if it doesn’t just get in touch and I’ll be happy to talk with you about it).

To tell you the truth, I started by writing a huge blog post about CRTP, which would have been daunting to read completely I think. For that reason I decided to split it up into several logical parts, which constitute the episodes of this series. This post was relatively short, but was necessary to put the basics in place.

Next up: how the CRTP can be useful to your code.