8.8. Being Defensive About Defensive Programming

Too much defensive programming creates problems of its own. If you check data passed as parameters in every conceivable way in every conceivable place, your program will be fat and slow. What's worse, the additional code needed for defensive programming adds complexity to the software. Code installed for defensive programming is not immune to defects, and you're just as likely to find a defect in defensive-programming code as in any other code more likely, if you write the code casually. Think about where you need to be defensive, and set your defensive-programming priorities accordingly.

Too much of anything is bad, but too much whiskey is just enough.

Mark Twain

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Checklist: Defensive Programming

General

• Does the routine protect itself from bad input data?

• Have you used assertions to document assumptions, including preconditions and postconditions?

• Have assertions been used only to document conditions that should never occur?

• Does the architecture or high-level design specify a specific set of error-handling techniques?

• Does the architecture or high-level design specify whether error handling should favor robustness or correctness?

• Have barricades been created to contain the damaging effect of errors and reduce the amount of code that has to be concerned about error processing?

• Have debugging aids been used in the code?

• Have debugging aids been installed in such a way that they can be activated or deactivated without a great deal of fuss?

• Is the amount of defensive programming code appropriate neither too much nor too little?

• Have you used offensive-programming techniques to make errors difficult to overlook during development?

Exceptions

• Has your project defined a standardized approach to exception handling?

• Have you considered alternatives to using an exception?

• Is the error handled locally rather than throwing a nonlocal exception, if possible?

• Does the code avoid throwing exceptions in constructors and destructors?

• Are all exceptions at the appropriate levels of abstraction for the routines that throw them?

• Does each exception include all relevant exception background information?

• Is the code free of empty catch blocks? (Or if an empty catch block truly is appropriate, is it documented?)

Security Issues

• Does the code that checks for bad input data check for attempted buffer overflows, SQL injection, HTML injection, integer overflows, and other malicious inputs?

• Are all error-return codes checked?

• Are all exceptions caught?

• Do error messages avoid providing information that would help an attacker break into the system?