Rule of three (computer programming)
Rule of three is a code refactoring rule of thumb to decide when a replicated piece of code should be replaced by a new procedure. It states that the code can be copied once, but that when the same code is replicated three times, it should be extracted into a new procedure. The rule was introduced by Martin Fowler in Refactoring[1] and attributed to Don Roberts.
Duplication in programming is a bad practice because it makes the code harder to maintain. When the rule encoded in a replicated piece of code changes, whoever maintains the code will have to change it in all places correctly. This process is error-prone and often leads to problems. If the code exists in only one place, then it can be easily changed there. This rule is typically only applied to a small number of lines of code, or even single lines of code. For example, if a program calls a function, and then calls it again when it fails, it is acceptable OK to have two call sites; however, if it is to be called five times before giving up, there should only be one call site inside a loop.
As Charles Petzold puts it, "Three or more? use a for!"[2] [3]
[edit] See also
- Don't repeat yourself (DRY)
[edit] References
- ^ Refactoring: Improving the Design of Existing Code, by Martin Fowler (ISBN: 978-0201485677)
- ^ http://www.charlespetzold.com/etc/DoesVisualStudioRotTheMind.html
- ^ http://msdn.microsoft.com/en-us/site/hh288085
