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 used three times, it should be extracted into a new procedure. The rule was introduced by Martin Fowler in Refactoring and attributed to Don Roberts.
Duplication in programming is a bad practice because it makes the code harder to maintain. However, code refactoring to eliminate duplication also takes time, which might be better spent on other tasks. Triplication has an even higher cost because it makes maintenance harder yet. 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. The rule proposes that the cost of maintenance certainly outweighs the cost of refactoring when there are three copies, and may or may not if there are two copies.
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 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.
- Martin Fowler; Kent Beck; John Brant; William Opdyke; Don Roberts (1999). Refactoring: Improving the Design of Existing Code. Addison-Wesley Professional. ISBN 978-0201485677.
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