Design by contract using assertions or exceptions?

Question

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 1

Disabling assert in release builds is like saying "I will never have any issues whatsoever in a release build", which is often not the case. So assert shouldn't be disabled in a release build. But you don't want the release build crashing whenever errors occur either, do you?

So use exceptions and use them well. Use a good, solid exception hierarchy and ensure that you catch and you can put a hook on exception throwing in your debugger to catch it, and in release mode you can compensate for the error rather than a straight-up crash. It's the safer way to go.