Design by contract using assertions or exceptions?

exception assert design-by-contract
andreas buykx picture andreas buykx · Sep 22, 2008 · Viewed 24.7k times · Source

When programming by contract a function or method first checks whether its preconditions are fulfilled, before starting to work on its responsibilities, right? The two most prominent ways to do these checks are by assert and by exception.

  1. assert fails only in debug mode. To make sure it is crucial to (unit) test all separate contract preconditions to see whether they actually fail.
  2. exception fails in debug and release mode. This has the benefit that tested debug behavior is identical to release behavior, but it incurs a runtime performance penalty.

Which one do you think is preferable?

See releated question here

Answer

Dima picture Dima · Sep 22, 2008

The rule of thumb is that you should use assertions when you are trying to catch your own errors, and exceptions when trying to catch other people's errors. In other words, you should use exceptions to check the preconditions for the public API functions, and whenever you get any data that are external to your system. You should use asserts for the functions or data that are internal to your system.

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