Trait (computer programming)

In computer programming, a trait is a concept used in object-oriented programming, which represents a set of methods that can be used to extend the functionality of a class.[1][2]


Traits both provide a set of methods that implement behaviour to a class, and require that the class implement a set of methods that parameterize the provided behaviour.

For inter-object communication (and sharing between objects), traits are somewhat between an object-oriented protocol (interface) and a mixin. An interface may define one or more behaviors via method signatures, while a trait defines behaviors via full method definitions: i.e., it includes the body of the methods. In contrast, mixins include full method definitions and may also carry state through member variable, while traits usually don't.

Hence an object defined as a trait is created as the composition of methods, which can be used by other classes without requiring multiple inheritance. In case of a naming collision, when more than one trait to be used by a class has a method with the same name, the programmer must explicitly disambiguate which one of those methods will be used in the class; thus manually solving the diamond problem of multiple inheritance. This is different from other composition methods in object-oriented programming, where conflicting names are automatically resolved by scoping rules.

Whereas mixins can be composed only using the inheritance operation, traits offer a much wider selection of operations, including:[3][4]

Traits are composed in the following ways:

Supported languages

Traits come originally from the programming language Self[5] and are supported by the following programming languages:



This example uses a trait to enhance other classes:

// The template
trait TSingleton
    private static $_instance = null;

    public static function getInstance()
        if (null === self::$_instance)
            self::$_instance = new self();

        return self::$_instance;

class FrontController
    use TSingleton;

// Can also be used in already extended classes
class WebSite extends SomeClass
    use TSingleton;

This allows simulating aspects of multiple inheritance:

trait TBounding
    public $x, $y, $width, $height;

trait TMoveable
    public function moveTo($x, $y)
        // ...

trait TResizeable
    public function resize($newWidth, $newHeight)
        // ...

class Rectangle
    use TBounding, TMoveable, TResizeable;

    public function fillColor($color)
        // ...

See also


  1. 1 2 3 Schärli, Nathanael; Ducasse, Stéphane; Nierstrasz, Oscar; Black, Andrew P. (2003). "Traits: Composable Units of Behaviour" (PDF). Proceedings of the European Conference on Object-Oriented Programming (ECOOP). Lecture Notes in Computer Science. Springer-Verlag. 2743: 248–274.
  2. Ducasse, Stéphane; Nierstrasz, Oscar; Schärli, Nathanael; Wuyts, Roel; Black, Andrew P. (March 2006). "Traits: A mechanism for fine-grained reuse.". ACM Transactions on Programming Languages and Systems. 28 (2): 331–388. doi:10.1145/1119479.1119483.
  3. Fisher, Kathleen; Reppy, John (2003). "Statically typed traits" (PDF). University of Chicago. Archived (PDF) from the original on May 17, 2004.
  4. Fisher, Kathleen; Reppy, John (2004). A typed calculus of traits (PDF). 11th Workshop on Foundations of Object-oriented Programming. University of Chicago.
  5. Curry, Gael; Baer, Larry; Lipkie, Daniel; Lee, Bruce (1982). Traits: An approach to multiple-inheritance subclassing. SIGOA Conference on Office Information Systems. Philadelphia, Pennsylvania, USA: ACM Press. pp. 1–9.
  6. Van Cutsem, Tom; Bergel, Alexandre; Ducasse, Stéphane; De Meuter, Wolfgang (2009). Adding State and Visibility Control to Traits Using Lexical Nesting (PDF). European Conference on Object-Oriented Programming (ECOOP 2009). Lecture Notes in Computer Science. 5653. Springer-Verlag. pp. 220–243. ISBN 978-3-642-03012-3. doi:10.1007/978-3-642-03013-0_11
  7. "iterator_traits<Iterator>". Standard Template Library. SGI.
  8. Myers, Nathan C. (June 1995). "Traits: a new and useful template technique". C++ Report. Retrieved January 23, 2016.
  9. Abrahams, David. "Generic Programming Techniques: Traits". Boost C++ Libraries. Retrieved January 23, 2016.
  10. "Traits". The D Language Reference. Digital Mars. Retrieved January 23, 2016.
  11. "std.traits". The D Language — Library Reference. Digital Mars. Retrieved January 23, 2016.
  12. "Classes". The D Language Reference. Digital Mars. Retrieved January 23, 2016.
  13. Steele, Guy; Maessen, Jan-Willem (June 11, 2006). "Fortress Programming Language Tutorial" (PDF). Sun Microsystems. Retrieved January 23, 2016.
  14. "Object Orientation: Traits". The Groovy Programming Language. Retrieved January 23, 2016.
  15. "Default Methods". The Java Tutorials. Oracle. Retrieved January 23, 2016.
  16. Bono, Viviana; Mensa, Enrico; Naddeo, Marco (September 2014). Trait-oriented Programming in Java 8. International Conference on Principles and Practices of Programming on the Java Platform: virtual machines, languages, and tools (PPPJ ’14). Kraków, Poland.
  17. "Java 8 default methods as traits: safe?". Quora. February 23, 2015. Retrieved January 24, 2016.
  18. Forslund, Emil (February 3, 2016). "Definition of the Trait Pattern in Java". Age of Java. Retrieved February 3, 2016.
  19. Seliger, Peter (April 11, 2014). "The Many Talents of JavaScript". Retrieved January 23, 2015.
  20. "Traits.js: Traits for JavaScript". Retrieved January 23, 2016.
  21. Van Cutsem, Tom; Miller, Mark S. (2012). "Robust Trait Composition for Javascript" (PDF). Science of Computer Programming: Special Issue on Advances in Dynamic Languages. Retrieved January 23, 2016.
  22. "CocktailJS". Retrieved January 23, 2016.
  23. "Interfaces". Kotlin Reference. JetBrains. Retrieved January 23, 2016.
  24. Breslav, Andrey (May 29, 2015). "Kotlin M12 is out!". Kotlin Blog. JetBrains. Retrieved January 23, 2016.
  25. "Traits". Lasso Language Guide. LassoSoft. January 6, 2014. Retrieved January 23, 2016.
  26. chromatic (April 30, 2009). "The Why of Perl Roles". Retrieved January 23, 2016.
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  28. Marr, Stefan (January 9, 2011). "Request for Comments: Horizontal Reuse for PHP". wiki. The PHP Group. Retrieved January 31, 2011.
  29. Perä, Teppo. "py3traits Documentation". Retrieved January 23, 2016.
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  32. "Traits". The Racket Reference. Retrieved January 23, 2016.
  33. David Naseby (February 14, 2004). "Traits in Ruby". Ruby Naseby. Retrieved January 23, 2016.
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External links

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