Swift Assertions
If we dive into a bit more of the detail behind assertions in Swift, we find that the Swift language actually supports five, different types of assertion. These are:
–assert()
–precondition()
–assertionFailure()
–preconditionFailure()
–fatalError()Each of these assertions behave slightly differently and in the majority of cases the optimisation level that the code is compiled with has a direct effect on whether they are evaluated or not.
Update (2017-03-15): Previously: precondition vs. assert in Swift.
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Beware that the precondition function, that targets the Release configuration, *does not* output failure message in the crash logs, unlike fatalError. This prevents libraries that use preconditions to provide a meaningful message whenever they validate their function inputs, and leaves their users naked with blank errors that don't say what's wrong. Today it's better to use fatalError when you write a library. See https://bugs.swift.org/browse/SR-905 for more information.
