A mixin is a class that is inherited but not instantiated, allowing for multiple function collections to be consolidated. Mixins solve ambiguity in method calls and are found in scripting languages. They do not override methods in inherited classes and have more functionality than interfaces in languages like Java.
In computer programming, a mixin is a class that is inherited from another class but is not meant to be instantiated. A single class can inherit many mixins, so many function collections can be consolidated easily. While many common programming languages use the mixin methodology, there are many others that don’t, including C++ and Java. In general, mixins are found in scripting languages or those languages that have the ability to create scripts, such as JavaScript and Perl.
Mixins are unique in that they become part of a subclass during compilation. Their methods and constants are subclassed. Another unique feature of mixin classes is that methods can be defined and associated with objects at runtime. Most languages take a different approach, binding and defining methods at compile-time rather than run-time. This runtime binding allows mixins to be very flexible.
A common problem in multiple inheritance is ambiguity in the method call, and mixins can fix this because they don’t follow exactly the same rules as multiple inheritance. For example, multiple inheritance can create a so-called “death diamond,” where a parent class A has subclasses B and C, and a class D inherits from both classes B and C. The problem in this scenario is that the methods that have the same name in classes A, B, and C can be inherited in class D, creating multiple versions of the same method without clear distinctions. Mixins solve this problem by keeping each version of the method distinct, rather than trying to override them as in standard multiple inheritance.
Mixins do not override methods in classes where they are inherited. For example, if a class A defines a method “myMethod” and inherits a mixin B which also defines the method “myMethod”, both of these methods could still be called unambiguously and the implementation of “myMethod” in class A would still be completely accessible. Exactly how these methods would be called depends on the language in which the mixin is implemented.
Some programming languages that don’t use mixins, such as Java, use interfaces to mimic them. Interfaces in these languages do not emulate the full functionality of mixins, because interfaces can only contain constants and method declarations. Mixins, on the other hand, can contain complete method implementations. Those languages that don’t support mixins often have viable alternatives that can be coaxed into doing much the same thing.
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