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{{more citations needed|date=November 2022}}
In [[computer programming]], a '''trait''' is a [[programming language|language]] concept that represents a set of [[Method (computer programming)|methods]] that can be used to extend the functionality of a [[Class (computer science)|class]].<ref name="schaerli-ecoop-2003">{{cite
==Rationale==
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In case of a [[naming collision]] between methods provided by different traits, the programmer must explicitly disambiguate which one of those methods will be used in the class; thus manually solving the ''[[Multiple inheritance#The diamond problem|diamond problem]]'' of [[multiple inheritance]]. This is different from other composition methods in object-oriented programming, where conflicting names are automatically resolved by [[Scope (computer science)|scoping rules]].
Operations which can be performed with traits include:<ref>{{cite
|first1 = Kathleen |last1 = Fisher | author1-link = Kathleen Fisher
|first2 = John |last2 = Reppy
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* [[AmbientTalk]]: Combines the properties of Self traits (object-based multiple inheritance) and [[Smalltalk]]'s [[Squeak]] traits (requiring explicit composition of traits by the programmer). It builds on the research on ''stateful'' and ''freezable'' traits to enable state within traits, which was not allowed in the first definitions.<ref>{{cite conference | url=http://soft.vub.ac.be/Publications/2009/vub-prog-tr-09-04.pdf | title=Adding State and Visibility Control to Traits Using Lexical Nesting | conference=European Conference on Object-Oriented Programming (ECOOP 2009) | first1=Tom | last1=Van Cutsem | first2=Alexandre | last2=Bergel | first3=Stéphane | last3=Ducasse | first4=Wolfgang | last4=De Meuter | pages=220–243 | publisher=Springer-Verlag | isbn=978-3-642-03012-3 | date=2009 | series=Lecture Notes in Computer Science | volume=5653 |doi=10.1007/978-3-642-03013-0_11 |citeseerx=10.1.1.372.1265}}</ref>
*[[C Sharp (programming language)|C#]]: Since version 8.0, C# has support for ''default interface methods'',<ref>{{cite web | url=https://docs.microsoft.com/en-gb/dotnet/csharp/whats-new/csharp-8#default-interface-methods | title=Default interface methods | website=What's new in C# 8.0 | publisher=Microsoft | access-date=November 29, 2019}}</ref> which have some properties of traits.<ref>{{cite web | url=https://www.talkingdotnet.com/default-implementations-in-interfaces-in-c-sharp-8/ | title=Interfaces in C# 8.0 gets a makeover | website=Default Implementation in Interfaces in C# 8.0 | date=9 September 2019 | publisher=Talking Dotnet | access-date=November 29, 2019}}</ref>
* [[C++]]: Used in [[Standard Template Library]] and the [[C++
* [[Curl (programming language)|Curl]]: Abstract classes as mixins permit method implementations and thus constitute traits by another name.{{citation needed|date=January 2016}}
* [[Fortress (programming language)|Fortress]]<ref>{{cite web | url=http://stephane.ducasse.free.fr/Teaching/CoursAnnecy/0506-Master/ForPresentations/Fortress-PLDITutorialSlides9Jun2006.pdf | title=Fortress Programming Language Tutorial | first1=Guy | last1=Steele | first2=Jan-Willem | last2=Maessen | publisher=Sun Microsystems | date=June 11, 2006 | access-date=January 23, 2016}}</ref>
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* [[Java (programming language)|Java]]: Since version 8, Java has support for ''default methods'',<ref>{{cite web | url=https://docs.oracle.com/javase/tutorial/java/IandI/defaultmethods.html | title=Default Methods | website=The Java Tutorials | publisher=Oracle | access-date=January 23, 2016}}</ref> which have some properties of traits.<ref>{{cite journal | url=https://hal.inria.fr/inria-00432538/ | title= FeatherTrait: A Modest Extension of Featherweight Java | first1=Luigi | last1=Liquori | first2=Arnaud | last2=Spiwack | journal= ACM Transactions on Programming Languages and Systems | date=2008 | volume= 30 | issue= 2 | pages= 11:1 |doi=10.1145/1330017.1330022| s2cid= 17231803 | doi-access=free }}</ref><ref>{{cite journal | url=https://hal.inria.fr/inria-00432540/ | title= Extending FeatherTrait Java with Interfaces | first1=Luigi | last1=Liquori | first2=Arnaud | last2=Spiwack | journal= Theoretical Computer Science | date=2008 | volume= 398 | issue= 1–3 | pages= 243–260 |doi=10.1016/j.tcs.2008.01.051| s2cid= 12923128 | doi-access=free }}</ref><ref>{{cite conference | url=https://hal.inria.fr/hal-01026531/en | title=Trait-oriented Programming in Java 8 | first1=Viviana | last1=Bono | first2=Enrico | last2=Mensa | first3=Marco | last3=Naddeo | conference=International Conference on Principles and Practices of Programming on the Java Platform: virtual machines, languages, and tools (PPPJ ’14) | date=September 2014 | conference-url=http://pppj2014.pk.edu.pl/ |pages=181–6 |doi=10.1145/2647508.2647520 |citeseerx=10.1.1.902.161}}</ref><ref>{{cite web | title=Definition of the Trait Pattern in Java | url=http://ageofjava.com/2016/02/definition-of-trait-pattern-in-java.html | archive-url=https://web.archive.org/web/20160804005218/http://ageofjava.com/2016/02/definition-of-trait-pattern-in-java.html | url-status=dead | archive-date=August 4, 2016 | website=Age of Java | date=February 3, 2016 | access-date=February 3, 2016 | first=Emil | last=Forslund}}</ref>
* [[JavaScript]]: Traits can be implemented via functions and delegations<ref>{{cite web | url=http://peterseliger.blogspot.com/2014/04/the-many-talents-of-javascript.html | title=The Many Talents of JavaScript | first=Peter | last=Seliger | date=April 11, 2014 | access-date=January 23, 2015}}</ref> or through libraries that provide traits.<ref>{{cite web | url=https://traitsjs.github.io/traits.js-website/ | title=Traits.js: Traits for JavaScript | access-date=January 23, 2016}}</ref><ref>{{cite journal | url=http://soft.vub.ac.be/Publications/2012/vub-soft-tr-12-19.pdf | title=Robust Trait Composition for Javascript | first1=Tom | last1=Van Cutsem | first2=Mark S. | last2=Miller | journal=Science of Computer Programming| year=2012 | access-date=January 23, 2016}}</ref><ref>{{cite web | url=https://cocktailjs.github.io/ | title=CocktailJS | access-date=January 23, 2016}}</ref>
* [[Julia (programming language)|Julia]]: Several packages implement traits, e.g.,<ref>{{cite web | url=https://github.com/mauro3/SimpleTraits.jl | title=SimpleTraits.jl | author=
* [[Kotlin (programming language)|Kotlin]]: Traits have been called ''interfaces''<ref>{{cite web | url=http://kotlinlang.org/docs/reference/interfaces.html | title=Interfaces | website=Kotlin Reference | publisher=JetBrains | access-date=January 23, 2016}}</ref> since M12.<ref>{{cite web | url=http://blog.jetbrains.com/kotlin/2015/05/kotlin-m12-is-out/ | title=Kotlin M12 is out! | first=Andrey | last=Breslav | website=Kotlin Blog | publisher=JetBrains | date=May 29, 2015 | access-date=January 23, 2016}}</ref>
* [[Lasso (programming language)|Lasso]]<ref>{{cite web | url=http://lassoguide.com/language/traits.html | title=Traits | website=Lasso Language Guide | publisher=LassoSoft | date=January 6, 2014 | access-date=January 23, 2016}}</ref>
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=== Rust ===
A trait in Rust declares a set of methods that a type must implement.<ref>{{Cite web
<syntaxhighlight lang="rust">
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