Gotcha (programming)

In 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.^[1]

Example

The classic gotcha in C/C++ is the construct

if (a = b) code;

It is syntactically valid: it puts the value of b into a and then executes code if a is non-zero. Sometimes this is even intended. However most commonly it is a typo: the programmer probably meant

if (a == b) code;

which executes code if a and b are equal.^[1] Modern compilers will usually generate a warning when encountering the former construct (conditional branch on assignment, not comparison), depending on compiler options (e.g., the -Wall option for gcc). To avoid this gotcha, some programming languages such include specific syntax for when this is desired behavior, such as Python's "walrus" operator (:=). In languages where this specific syntax does not exist, there is a recommendation^[2] to keep the constants in the left side of the comparison, e.g. 42 == x rather than x == 42. This way, using = instead of == will cause a compiler error (see Yoda conditions). Many kinds of gotchas are not detected by compilers, however.^{[citation needed]}

See also

• Usability

References

1. Gotcha definition at The Jargon File
2. "VOID EXP21-C. Place constants on the left of equality comparisons"

Further reading

• Stephen C. Dewhurst (2003). C++ Gotchas (Avoiding Common Problems in Coding and Design). Addison-Wesley. ISBN 0321125185.

External links

• C Traps and Pitfalls by Andrew Koenig
• 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