Trait (computer programming)

Characteristics

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, traits are somewhere 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]

• symmetric sum: an operation that merges two disjoint traits to create a new trait
• override (or asymmetric sum): an operation that forms a new trait by adding methods to an existing trait, possibly overriding some of its methods
• alias: an operation that creates a new trait by adding a new name for an existing method
• exclusion: an operation that forms a new trait by removing a method from an existing trait. (Combining this with the alias operation yields a shallow rename operation).

Traits are composed in the following ways:

• Trait composition is commutative; the ordering of adding traits does not matter. For example, given trait S = A + B, then trait T = B + A is the same as S.
• Conflicting methods are excluded from the composition.
• Nested traits are equivalent to flattened traits; the composition hierarchy does not affect the traits behaviour. For example, given trait S = A + X, where X = B + C, then trait T = A + B + C is the same as S.^[1]

Supported languages

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

• 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.^[6]
• C#: Since version 8.0, C# has support for default interface methods,^[7] which have some properties of traits.^[8]
• C++: Used in Standard Template Library and the C++ standard library to support generic container classes^[9][10] and in the Boost TypeTraits library.^[11]
• Curl: Abstract classes as mixins permit method implementations and thus constitute traits by another name.^{[citation needed]}
• D: Since version 2.003, the __traits language extension^[12] and std.traits module^[13] helper templates provide compile-time traits. Together with other language features (notably templates and mixins), they allow flexible automatic generation of methods based on interfaces and types. D also allows explicit aliasing of member methods and variables, including forwarding to multiple member classes.^[14]
• Fortress^[15]
• Groovy: Since version 2.3^[16]
• Haskell: In Haskell, Traits are known as Type classes.
• Haxe: Since version 2.4.0.^[17] Called Static Extension^[18] in the manual, it uses using keyword
• Java: Since version 8, Java has support for default methods,^[19] which have some properties of traits.^[20][21]
• JavaScript: Traits can be implemented via functions and delegations^[22] or through libraries that provide traits.^[23][24][25]
• Julia: Several packages implement traits, e.g.,^[26]
• Kotlin: Traits have been called interfaces^[27] since M12.^[28]
• Lasso^[29]
• OCaml: Traits can be implemented using a variety of language features: module and module type inclusion, functors and functor types, class and class type inheritance, et cetera.
• Perl: Called roles, they are implemented in Perl libraries such as Moose, Role::Tiny and Role::Basic. Roles are part of the sister language Raku.^[30]
• PHP: Since version 5.4,^[31][32] PHP allows users to specify templates that provide the ability to "inherit" from more than one (trait-)class, as a pseudo multiple inheritance.
• Python: Via a third-party library,^[33][34] or via higher-order mixin classes^[35]
• Racket: Supports traits as a library and uses macros, structures, and first-class classes to implement them.^[36]
• Ruby: Module mixins can be used to implement traits.^[37]
• Rust^[38]
• Scala^[39][40] trait is builtin supported with the key word trait.
• Smalltalk: Traits are implemented in two dialects of Smalltalk, Squeak^[1] and Pharo.^[41]
• Swift: Traits can be implemented with protocol extensions.^[42]

Examples

C#

On C# 8.0, it is possible to define an implementation as a member of an interface.

using System;

namespace CSharp8NewFeatures
{
    interface ILogger
    {
        // Traditional interface methods
        void Log(string message);
        void LogError(Exception exception);

        // Default interface method
        void LogWarning(string message)
        {
            Console.WriteLine(message);
        }        
    }

    class Logger : ILogger
    {
        public void Log(string message)
        {
            Console.WriteLine(message);
        }

        public void LogError(Exception exception)
        {
            Console.WriteLine(exception.ToString());
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            ILogger logger = new Logger();

            logger.LogWarning("Some warning message");
        }
    }
}

PHP

This example uses a trait to enhance other classes:

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

    private function __construct() {} // Must have private default constructor and be aware not to open it in the class

    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)
    {
        // …
    }
}

Rust

A trait in Rust declares a set of methods that a type must implement.^[43] Rust compilers require traits to be explicated, which ensures the safety of generics in Rust.

// type T must have the "Ord" trait
// so that ">" and "<" operations can be done
fn get_max<T: Ord>(a: &[T]) -> Option<&T> {
    let mut result = a.get(0)?;
    for n in a {
        if *n > *result {
            result = &n;
        }
    }
    Some(result)
}

To simplify tedious and repeated implementation of traits like Debug and Ord, the derive macro can be used to request compilers to generate certain implementations automatically.^[44] Derivable traits include: Clone, Copy, Debug, Default, PartialEq, Eq, PartialOrd, Ord and Hash.

See also

• Extension method
• interface (object-oriented programming)
• UFCS

References

External links

• "Traits: Composable Units of Behavior". Software Composition Group. University of Bern.