View source for Open–closed principle

{{Short description|Concept in object-oriented programming}}
[[File:Bertrand_Meyer_recent.jpg|thumb|alt=Portrait of Bertrand Meyer|The open-closed principle was introduced by [[Bertrand Meyer]].]]
{{Solid principles}}

In [[object-oriented programming]], the '''open–closed principle''' (OCP) states "''software entities (classes, modules, functions, etc.) should be open for extension, but closed for modification''";<ref name=":0">
{{cite book|last=Meyer|first=Bertrand|title=Object-Oriented Software Construction|publisher=Prentice Hall|year=1988|isbn=0-13-629049-3|authorlink=Bertrand Meyer}}
</ref>
that is, such an entity can allow its behaviour to be extended without modifying its [[source code]].

The name ''open–closed principle'' has been used in two ways. Both ways use generalizations (for instance, [[Inheritance (object-oriented programming)|inheritance]] or delegate functions) to resolve the apparent dilemma, but the goals, techniques, and results are different.

The open–closed principle is one of the five [[SOLID (object-oriented design)|SOLID]] principles of object-oriented design.

==Meyer's open–closed principle==
[[Bertrand Meyer]] is generally credited for having originated the term ''open–closed principle'',<ref name="Martin1996">[[Robert C. Martin]] [http://docs.google.com/a/cleancoder.com/viewer?a=v&pid=explorer&chrome=true&srcid=0BwhCYaYDn8EgN2M5MTkwM2EtNWFkZC00ZTI3LWFjZTUtNTFhZGZiYmUzODc1&hl=en "The Open-Closed Principle", <cite>C++ Report</cite>, January 1996] {{webarchive |url=https://web.archive.org/web/20060822033314/http://www.objectmentor.com/resources/articles/ocp.pdf |date=August 22, 2006}}</ref> which appeared in his 1988 book ''[[Object-Oriented Software Construction]]''.<ref name=":0" />{{rp|23}}

<blockquote>
* A module will be said to be open if it is still available for extension. For example, it should be possible to add fields to the data structures it contains, or new elements to the set of functions it performs.
* A module will be said to be closed if [it] is available for use by other modules. This assumes that the module has been given a well-defined, stable description (the interface in the sense of information hiding).
</blockquote>

At the time Meyer was writing, adding fields or functions to a library inevitably required changes to any programs depending on that library.{{Citation needed|date=May 2020}}  Meyer's proposed solution to this problem relied on the notion of object-oriented [[Inheritance (object-oriented programming)|inheritance]] (specifically [[implementation inheritance]]):<ref name=":0" />{{rp|229}}

<blockquote>
A class is closed, since it may be compiled, stored in a library, [[baseline (configuration management)|baselined]], and used by client classes. But it is also open, since any new class may use it as parent, adding new features. When a descendant class is defined, there is no need to change the original or to disturb its clients.
</blockquote>

==Polymorphic open–closed principle==
During the 1990s, the open–closed principle became popularly redefined to refer to the use of [[abstraction (computer science)|abstracted]] interfaces, where the implementations can be changed and multiple implementations could be created and [[Polymorphism (computer science)|polymorphically]] substituted for each other.

In contrast to Meyer's usage, this definition advocates inheritance from [[abstract base class]]es. Interface specifications can be reused through inheritance but implementation need not be. The existing interface is closed to modifications and new implementations must, at a minimum, implement that interface.

[[Robert C. Martin]]'s 1996 article "The Open-Closed Principle"<ref name="Martin1996" /> was one of the seminal writings to take this approach. In 2001, [[Craig Larman]] related the open–closed principle to the pattern by [[Alistair Cockburn]] called ''Protected Variations'', and to the [[David Parnas]] discussion of ''[[information hiding]]''.<ref name="Larman2001">{{cite journal|first=Craig |last=Larman |title=Protected Variation: The Importance of Being Closed |journal=[[IEEE Software]] |publisher=IEEE |volume=18 |issue=2 |date=May–June 2001 |pages=89–91 |doi=10.1109/52.922731 |url=http://codecourse.sourceforge.net/materials/The-Importance-of-Being-Closed.pdf}}</ref>

==See also==
* [[SOLID (object-oriented design)|SOLID]] &ndash; the "O" in "SOLID" represents the open–closed principle
* [[Robustness principle]]

==References==
<references/>

==External links==
* [http://butunclebob.com/ArticleS.UncleBob.PrinciplesOfOod The Principles of OOD]
* [https://blog.symprise.net/articles/open-closed-principle-concerns-about-change-in-software-design The Open/Closed Principle: Concerns about Change in Software Design]
* [https://medium.com/@wrong.about/the-open-closed-principle-c3dc45419784 The Open-Closed Principle -- and What Hides Behind It]

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[[Category:Object-oriented programming]]
[[Category:Type theory]]
[[Category:Software design]]
[[Category:Programming principles]]
[[Category:Software development philosophies]]