Editing Gotcha (programming)

{{short description|Code that is valid but counter-intuitive}}
In [[computer programming|programming]], a '''gotcha''' is a valid construct in a system, program or [[programming language]] that works as documented but is [[counter-intuitive]] and almost invites mistakes because it is both easy to invoke and unexpected or unreasonable in its outcome.<ref name=jf/>

== Example ==
The classic gotcha in [[C (programming language)|C]]/[[C++]] is the construct
<syntaxhighlight lang="c">
if (a = b) code;
</syntaxhighlight>
It is [[syntactically]] valid: it puts the value of <code>b</code> into <code>a</code> and then executes <code>code</code> if <code>a</code> is non-zero. Sometimes this is even intended. However most commonly it is a typo: the programmer probably meant
<syntaxhighlight lang="c">
if (a == b) code;
</syntaxhighlight>
which executes <code>code</code> if <code>a</code> and <code>b</code> are equal.<ref name=jf>[http://catb.org/jargon/html/G/gotcha.html Gotcha definition at The Jargon File]</ref> Modern [[compiler]]s will usually generate a warning when encountering the former construct (conditional branch on assignment, not comparison), depending on compiler options (e.g., the <code>-Wall</code> option for [[GNU Compiler Collection|gcc]]). To avoid this gotcha, some programming languages such include specific syntax for when this is desired behavior, such as [[Python (programming language)#Expressions|Python's "walrus" operator]] (<code>:=</code>). In languages where this specific syntax does not exist, there is a recommendation<ref>[https://www.securecoding.cert.org/confluence/display/c/VOID+EXP21-C.+Place+constants+on+the+left+of+equality+comparisons "VOID EXP21-C. Place constants on the left of equality comparisons"]</ref> to keep the [[Constant (programming)|constants]] in the left side of the comparison, e.g. <code>42 == x</code> rather than <code>x == 42</code>. This way, using <code>=</code> instead of <code>==</code> will cause a compiler error (see [[Yoda conditions]]). Many kinds of gotchas are not detected by compilers, however.{{cn|date=September 2018}} 

==See also==
* [[Usability]]

==References==
{{Reflist}}

==Further reading==
* {{cite book|author=Stephen C. Dewhurst|title=C++ Gotchas (Avoiding Common Problems in Coding and Design)|publisher=Addison-Wesley|date=2003|isbn=0321125185}}

==External links==
{{wiktionary|gotcha}}
* [http://www.literateprogramming.com/ctraps.pdf C Traps and Pitfalls] by Andrew Koenig
* [https://web.archive.org/web/20160817045649/http://buhoz.net/public/books/lenguajes/c++/tecnicas/C++.Gotchas-.Avoiding.Common.Problems.in.Coding.and.Design.pdf C++ Gotchas] A programmer's guide to avoiding and correcting ninety-nine of the most common, destructive, and interesting C++ design and programming errors, by Stephen C. Dewhurst

[[Category:Computer programming folklore]]
[[Category:Programming language folklore]]
[[Category:Programming language design]]