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Since metaclass objects do not behave differently (you cannot add class methods for a metaclass, so metaclass objects all have the same methods), they are all instances of the same class—the metaclass of the root class (unlike Smalltalk). Thus, the metaclass of the root class is an instance of itself.
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In some languages, such as Python, Ruby or Smalltalk, a class is also an object; thus each class is an instance of a unique metaclass that is built into the language. [4] [35] [36] The Common Lisp Object System (CLOS) provides metaobject protocols (MOPs) to implement those classes and metaclasses. [37]
In Python, everything is an object, even classes. Classes, as objects, have a class, which is known as their metaclass. Python also supports multiple inheritance and mixins. The language supports extensive introspection of types and classes. Types can be read and compared—types are instances of type. The attributes of an object can be ...
Classes can have multiple superclasses, a list of slots (member variables in C++/Java parlance) and a special metaclass. Slots can be allocated by class (all instances of a class share the slot) or by instance. Each slot has a name and the value of a slot can be accessed by that name using the function slot-value. Additionally special generic ...
Python's Guido van Rossum summarizes C3 superclass linearization thus: [11] Basically, the idea behind C3 is that if you write down all of the ordering rules imposed by inheritance relationships in a complex class hierarchy, the algorithm will determine a monotonic ordering of the classes that satisfies all of them.
The Web Ontology Language (OWL) is a family of knowledge representation languages for authoring ontologies.Ontologies are a formal way to describe taxonomies and classification networks, essentially defining the structure of knowledge for various domains: the nouns representing classes of objects and the verbs representing relations between the objects.
Python: Via a third-party library, [34] [35] or via higher-order mixin classes [36] Racket: Supports traits as a library and uses macros, structures, and first-class classes to implement them. [37] Ruby: Module mixins can be used to implement traits. [38] Rust [39] Scala [40] [41] trait is builtin supported with the key word trait.