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Before a class derived from an abstract class can be instantiated, all abstract methods of its parent classes must be implemented by some class in the derivation chain. [25] Most object-oriented programming languages allow the programmer to specify which classes are considered abstract and will not allow these to be instantiated. For example ...
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.
In languages supporting multiple inheritance, such as C++, interfaces are implemented as abstract classes. In languages without explicit support, protocols are often still present as conventions. This is known as duck typing. For example, in Python, any class can implement an __iter__ method and be used as a collection. [3]
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 ...
The bridge pattern is a design pattern used in software engineering that is meant to "decouple an abstraction from its implementation so that the two can vary independently", introduced by the Gang of Four. [1]
Abstract data types Algebraic data types Data is immutable Type classes Garbage collection First appeared Common Lisp: No [1] Simulated with thunks [2] Dynamic [3] Yes [4] Extension [5] No [6]? Yes: 1984 Scheme: No [7] Yes [8] Dynamic [7] Yes [9] Simulated with thunks [10] No [11] No: Yes: 1975 Racket: No: Default in Lazy Racket [12] Dynamic by ...
It should be possible to define a new operation for (some) classes of an object structure without changing the classes. When new operations are needed frequently and the object structure consists of many unrelated classes, it's inflexible to add new subclasses each time a new operation is required because "[..] distributing all these operations across the various node classes leads to a system ...
If the Number type is abstract, it may not even be possible to get your hands on an object whose most-derived type is Number (see abstract data type, abstract class). This particular kind of type hierarchy is known, especially in the context of the Scheme language , as a numerical tower , and usually contains many more types.