Skip to main content

Curiously Recurring Template Pattern (CRTP)

It is a C++ idiom in which inheritance and template type parameters are cleverly integrated to generate flexible types at compile-time. In CRTP, a template class inherits from its own template parameter. For example,

template <typename Base>
class MixinOne : public Base {};

This technique is also one of the ways of using "Mixins". Here class Mixin helps to "mix in" some useful functionality into any class type Base. For example, class Mixin can make class Base a singleton. Complete code example is here. In this case of CRTP, a hierarchy (a new class plus inheritance relationship) is created when Mixin template is instantiated. Thus hierarchy creation is differed till the instantiation of the template. This is pretty cool!

Moreover, CRTP can appear in a different fashion in which a class passes itself as a template parameter to its base class. For example,

class Derived: public MixinTwo<Derived> {};

The non-template class Derived inherits interface/structure (hopefully customized for itself) from Mixin class. In this case of CRTP, the class hierarchy is "created" when Derived class is coded unlike in the earlier case where, class hierarchy is "created" when template is instantiated.

This curious name to the idiom was given by James Coplien. Also see this for a complete example of the technique.

Comments

Anonymous said…
I can understand how the pattern works, but man it is one screwd-up programming technique, template programmers should really get out more and live life.
Anonymous said…
Kindly delete that above comment and this one to whomever owns this blog, that post was made by a coworker who is stumped at a programming problem, please, thanks.
Unknown said…
The article

Building More Flexible Types With Mixins
Applying the Curiously Recurring Template Pattern
By Christopher Diggins

http://devnet.developerpipeline.com/documents/s=9843/cuj0601diggins/

is now at:

http://www.ddj.com/cpp/184402056

Cheers

Popular Content

Unit Testing C++ Templates and Mock Injection Using Traits

Unit testing your template code comes up from time to time. (You test your templates, right?) Some templates are easy to test. No others. Sometimes it's not clear how to about injecting mock code into the template code that's under test. I've seen several reasons why code injection becomes challenging. Here I've outlined some examples below with roughly increasing code injection difficulty. Template accepts a type argument and an object of the same type by reference in constructor Template accepts a type argument. Makes a copy of the constructor argument or simply does not take one Template accepts a type argument and instantiates multiple interrelated templates without virtual functions Lets start with the easy ones. Template accepts a type argument and an object of the same type by reference in constructor This one appears straight-forward because the unit test simply instantiates the template under test with a mock type. Some assertion might be tested in

Multi-dimensional arrays in C++11

What new can be said about multi-dimensional arrays in C++? As it turns out, quite a bit! With the advent of C++11, we get new standard library class std::array. We also get new language features, such as template aliases and variadic templates. So I'll talk about interesting ways in which they come together. It all started with a simple question of how to define a multi-dimensional std::array. It is a great example of deceptively simple things. Are the following the two arrays identical except that one is native and the other one is std::array? int native[3][4]; std::array<std::array<int, 3>, 4> arr; No! They are not. In fact, arr is more like an int[4][3]. Note the difference in the array subscripts. The native array is an array of 3 elements where every element is itself an array of 4 integers. 3 rows and 4 columns. If you want a std::array with the same layout, what you really need is: std::array<std::array<int, 4>, 3> arr; That's quite annoying for

Covariance and Contravariance in C++ Standard Library

Covariance and Contravariance are concepts that come up often as you go deeper into generic programming. While designing a language that supports parametric polymorphism (e.g., templates in C++, generics in Java, C#), the language designer has a choice between Invariance, Covariance, and Contravariance when dealing with generic types. C++'s choice is "invariance". Let's look at an example. struct Vehicle {}; struct Car : Vehicle {}; std::vector<Vehicle *> vehicles; std::vector<Car *> cars; vehicles = cars; // Does not compile The above program does not compile because C++ templates are invariant. Of course, each time a C++ template is instantiated, the compiler creates a brand new type that uniquely represents that instantiation. Any other type to the same template creates another unique type that has nothing to do with the earlier one. Any two unrelated user-defined types in C++ can't be assigned to each-other by default. You have to provide a