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Curiously Recurring Template Pattern

StructuralCode simplificationExtensibilityGenericIdiomInstantiationPolymorphismRecursionAbout 3 min

Also known as

  • CRTP
  • Mixin Inheritance
  • Recursive Type Bound
  • Recursive Generic
  • Static Polymorphism

Intent

Curiously Recurring Template Pattern (CRTP) is used to achieve a form of static polymorphism by having a class template derive from a template instantiation of its own class, allowing method overriding and polymorphic behavior at compile time rather than at runtime.

Explanation

Real-world example

Consider a scenario where a library system manages various types of media: books, DVDs, and magazines. Each media type has specific attributes and behaviors, but they all share common functionality like borrowing and returning.

Using the Curiously Recurring Template Pattern (CRTP), you can create a base template class MediaItem that includes these common methods. Each specific media type (e.g., Book, DVD, Magazine) would then inherit from MediaItem using itself as a template parameter. This allows each media type to customize the common functionality without the overhead of virtual methods.

For example, Book would inherit from MediaItem<Book>, allowing the library system to use polymorphic behavior at compile-time, ensuring that each media type implements the necessary methods efficiently. This approach provides the benefits of polymorphism and code reuse while maintaining high performance and type safety.

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

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.

Let's define the generic interface Fighter.

public interface Fighter<T> {

    void fight(T t);

}

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

@Slf4j
@Data
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;

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

The followings are some subtypes of MmaFighter.

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, an error is raised.

public static void main(String[] args) {

    MmaBantamweightFighter fighter1 = new MmaBantamweightFighter("Joe", "Johnson", "The Geek", "Muay Thai");
    MmaBantamweightFighter fighter2 = new MmaBantamweightFighter("Ed", "Edwards", "The Problem Solver", "Judo");
    fighter1.fight(fighter2);

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

Program output:

08:42:34.048 [main] INFO crtp.MmaFighter -- MmaFighter(name=Joe, surname=Johnson, nickName=The Geek, speciality=Muay Thai) is going to fight against MmaFighter(name=Ed, surname=Edwards, nickName=The Problem Solver, speciality=Judo)
08:42:34.054 [main] INFO crtp.MmaFighter -- MmaFighter(name=Dave, surname=Davidson, nickName=The Bug Smasher, speciality=Kickboxing) is going to fight against MmaFighter(name=Jack, surname=Jackson, nickName=The Pragmatic, speciality=Brazilian Jiu-Jitsu)

Applicability

  • When you need to extend the functionality of a class through inheritance but prefer compile-time polymorphism to runtime polymorphism for efficiency reasons.
  • When you want to avoid the overhead of virtual functions but still achieve polymorphic behavior.
  • In template metaprogramming to provide implementations of functions or policies that can be selected at compile time.
  • 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

Known uses

  • Implementing compile-time polymorphic interfaces in template libraries.
  • Enhancing code reuse in libraries where performance is critical, like in mathematical computations, embedded systems, and real-time processing applications.
  • Implementation of the Cloneable interface in various Java libraries.

Consequences

Benefits:

  • Elimination of virtual function call overhead, enhancing performance.
  • Safe reuse of the base class code without the risks associated with multiple inheritances.
  • Greater flexibility and extensibility in compile-time polymorphism scenarios.

Trade-offs:

  • Increased complexity in understanding and debugging due to the interplay of templates and inheritance.
  • Can lead to code bloat because each instantiation of a template results in a new class.
  • Less flexibility compared to runtime polymorphism as the behavior must be determined entirely at compile time.

Credits

MIT licensed
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