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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]
In C#, class methods, indexers, properties and events can all be overridden. Non-virtual or static methods cannot be overridden. The overridden base method must be virtual, abstract, or override. In addition to the modifiers that are used for method overriding, C# allows the hiding of an inherited property or method.
Multiple inheritance where one class can have more than one superclass and inherit features from all parent classes. "Multiple inheritance ... was widely supposed to be very difficult to implement efficiently. For example, in a summary of C++ in his book on Objective C, Brad Cox actually claimed that adding multiple inheritance to C++ was ...
The non-virtual interface pattern (NVI) controls how methods in a base class are overridden. Such methods may be called by clients and overridable methods with core functionality. [1] It is a pattern that is strongly related to the template method pattern. The NVI pattern recognizes the benefits of a non-abstract method invoking the subordinate ...
A notable language in which this is a fairly common paradigm is C++. C# supports return type covariance as of version 9.0. [1] Covariant return types have been (partially) allowed in the Java language since the release of JDK5.0, [2] so the following example wouldn't compile on a previous release:
Nonetheless, the principle is useful in reasoning about the design of class hierarchies. Liskov substitution principle imposes some standard requirements on signatures that have been adopted in newer object-oriented programming languages (usually at the level of classes rather than types; see nominal vs. structural subtyping for the distinction):
A sample UML class and sequence diagram for the Bridge design pattern. [3]In the above Unified Modeling Language class diagram, an abstraction (Abstraction) is not implemented as usual in a single inheritance hierarchy.
The g++ compiler implements the multiple inheritance of the classes B1 and B2 in class D using two virtual method tables, one for each base class. (There are other ways to implement multiple inheritance, but this is the most common.) This leads to the necessity for "pointer fixups", also called thunks, when casting. Consider the following C++ code: