What is "code smell"

n. (context computing programming English) Anything in a program's source code that suggests the presence of a design problem.

Code smell, also known as bad smell, in computer programming code, refers to any symptom in the source code of a program that possibly indicates a deeper problem. According to Martin Fowler, "a code smell is a surface indication that usually corresponds to a deeper problem in the system". Another way to look at smells is with respect to principles and quality: "smells are certain structures in the code that indicate violation of fundamental design principles and negatively impact design quality". Code smells are usually not bugs—they are not technically incorrect and do not currently prevent the program from functioning. Instead, they indicate weaknesses in design that may be slowing down development or increasing the risk of bugs or failures in the future. Bad code smells can be an indicator of factors that contribute to technical debt. Robert C. Martin calls a list of code smells a "value system" for software craftsmanship.

Often the deeper problem hinted by a code smell can be uncovered when the code is subjected to a short feedback cycle where it is refactored in small, controlled steps, and the resulting design is examined to see if there are any further code smells that indicate the need of more refactoring. From the point of view of a programmer charged with performing refactoring, code smells are heuristics to indicate when to refactor, and what specific refactoring techniques to use. Thus, a code smell is a driver for refactoring.

A 2015 empirical study of half a million code commits to 200 open source software projects found that

• Most bad smells affecting a piece of code are already present since its creation, rather than being introduced later via evolutionary code changes.
• Although code smells are often introduced while adding new features and enhancing existing ones, refactoring activities can also add bad smells.
• "Newcomers are not necessary [sic] responsible for introducing bad smells [sic], while developers with high workloads and release pressure are more prone to introducing smell instances", indicating a need for increased code inspection efforts in such stressful work situations.

The term appears to have been coined by Kent Beck on WardsWiki in the late 1990s. Usage of the term increased after it was featured in Refactoring: Improving the Design of Existing Code. Code smell is also a term used by agile programmers.

Determining what is and is not a code smell is subjective, and varies by language, developer and development methodology. There are tools, such as Checkstyle, PMD and FindBugs for Java, to automatically check for certain kinds of code smells.