---
title: Curiously Recurring Template Pattern
language: en
category: Structural
tag:
- Extensibility
- Instantiation
---

## Name / classification

Curiously Recurring Template Pattern

## Also known as

Recursive Type Bound, Recursive Generic

## Intent

Allow derived components to inherit certain functionalities from a base component that are compatible with the derived type.

## Explanation

Real-world example

> For a mixed martial arts promotion that is planning an event, ensuring that the fights are organized between athletes
> of the same weight class is crucial. This prevents mismatches between fighters of significantly different sizes, such
> as a heavyweight facing off against a bantamweight.

In plain words

> Make certain methods within a type to accept arguments specific to its subtypes.

Wikipedia says

> The curiously recurring template pattern (CRTP) is an idiom, originally in C++, in which a class X
> derives from a class template instantiation using X itself as a template argument.

**Programmatic example**

Let's define the generic interface Fighter

```java
public interface Fighter<T> {

  void fight(T t);

}
```

The MMAFighter class is used to instantiate fighters distinguished by their weight class

```java
public class MmaFighter<T extends MmaFighter<T>> implements Fighter<T> {

  private final String name;
  private final String surname;
  private final String nickName;
  private final String speciality;

  public MmaFighter(String name, String surname, String nickName, String speciality) {
    this.name = name;
    this.surname = surname;
    this.nickName = nickName;
    this.speciality = speciality;
  }

  @Override
  public void fight(T opponent) {
    LOGGER.info("{} is going to fight against {}", this, opponent);
  }

  @Override
  public String toString() {
    return name + " \"" + nickName + "\" " + surname;
  }
```

The followings are some subtypes of MmaFighter

```java
class MmaBantamweightFighter extends MmaFighter<MmaBantamweightFighter> {

  public MmaBantamweightFighter(String name, String surname, String nickName, String speciality) {
    super(name, surname, nickName, speciality);
  }

}

public class MmaHeavyweightFighter extends MmaFighter<MmaHeavyweightFighter> {

  public MmaHeavyweightFighter(String name, String surname, String nickName, String speciality) {
    super(name, surname, nickName, speciality);
  }

}
```

A fighter is allowed to fight an opponent of the same weight classes, if the opponent is of a different weight class
there is an error

```java
MmaBantamweightFighter fighter1 = new MmaBantamweightFighter("Joe", "Johnson", "The Geek", "Muay Thai");
MmaBantamweightFighter fighter2 = new MmaBantamweightFighter("Ed", "Edwards", "The Problem Solver", "Judo");
fighter1.fight(fighter2); // This is fine

MmaHeavyweightFighter fighter3 = new MmaHeavyweightFighter("Dave", "Davidson", "The Bug Smasher", "Kickboxing");
MmaHeavyweightFighter fighter4 = new MmaHeavyweightFighter("Jack", "Jackson", "The Pragmatic", "Brazilian Jiu-Jitsu");
fighter3.fight(fighter4); // This is fine too

fighter1.fight(fighter3); // This will raise a compilation error
```

## Class diagram

![alt text](./etc/crtp.png "CRTP class diagram")

## Applicability

Use the Curiously Recurring Template Pattern when

* You have type conflicts when chaining methods in an object hierarchy
* You want to use a parameterized class method that can accept subclasses of the class as arguments, allowing it to be applied to objects that inherit from the class
* You want certain methods to work only with instances of the same type, such as for achieving mutual comparability.

## Tutorials

* [The NuaH Blog](https://nuah.livejournal.com/328187.html)
* Yogesh Umesh Vaity answer to [What does "Recursive type bound" in Generics mean?](https://stackoverflow.com/questions/7385949/what-does-recursive-type-bound-in-generics-mean)

## Known uses

* [java.lang.Enum](https://docs.oracle.com/en/java/javase/17/docs/api/java.base/java/lang/Enum.html)

## Credits

* [How do I decrypt "Enum<E extends Enum\<E>>"?](http://www.angelikalanger.com/GenericsFAQ/FAQSections/TypeParameters.html#FAQ106)
* Chapter 5 Generics, Item 30 in [Effective Java](https://www.amazon.com/gp/product/0134685997/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0134685997&linkCode=as2&tag=javadesignpat-20&linkId=4e349f4b3ff8c50123f8147c828e53eb)