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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.
In most class-based object-oriented languages like C++, an object created through inheritance, a "child object", acquires all the properties and behaviors of the "parent object", with the exception of: constructors, destructors, overloaded operators and friend functions of the base class. Inheritance allows programmers to create classes that ...
It is distinct from single inheritance, where an object or class may only inherit from one particular object or class. Multiple inheritance has been a controversial issue for many years, [1] [2] with opponents pointing to its increased complexity and ambiguity in situations such as the "diamond problem", where it may be ambiguous as to which ...
Not all languages support multiple inheritance. For example, Java allows a class to implement multiple interfaces, but only inherit from one class. [22] If multiple inheritance is allowed, the hierarchy is a directed acyclic graph (or DAG for short), otherwise it is a tree. The hierarchy has classes as nodes and inheritance relationships as links.
Composition over inheritance (or composite reuse principle) in object-oriented programming (OOP) is the principle that classes should favor polymorphic behavior and code reuse by their composition (by containing instances of other classes that implement the desired functionality) over inheritance from a base or parent class. [2]
On the other hand, inheritance can be statically type-checked, while delegation generally cannot without generics (although a restricted version of delegation can be statically typesafe [7]). Delegation can be termed "run-time inheritance for specific objects." Here is a pseudocode example in a C#/Java like language:
An extension implementation for earlier versions of Perl 5 named Class::C3 exists on CPAN. [10] 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 ...
Each class is an instance of the metaclass of that class. Metaclasses in turn are also objects, and are all instances of a class named Metaclass. Classes contain method dictionaries that map selectors (the equivalent of function procedure names in other languages) to method objects, objects that are executed to evaluate messages.