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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.
Which one do you think is preferable?
See releated question here
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A: Contract assertions give the programmer a way to deliver precise facts to the toolchain about the expected state of a program at one point.
Assertions should be used to check something that should never happen, while an exception should be used to check something that might happen. For example, a function might divide by 0, so an exception should be used, but an assertion could be used to check that the harddrive suddenly disappears.
Exceptions versus assertions Use assert statements to test for conditions during development that should never be true if all your code is correct. There's no point in handling such an error by using an exception, because the error indicates that something in the code has to be fixed.
The key differences between exceptions and assertions are: Assertions are intended to be used solely as a means of detecting programming errors, aka bugs. By contrast, an exception can indicate other kinds of error or "exceptional" condition; e.g. invalid user input, missing files, heap full and so on.
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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