In Obj-C, you can use asserts like this:

NSAssert([self foo], @"Something something");

Similarly in Swift:

assert(foo())

In Obj-C, asserts are controlled by ENABLE_NS_ASSERTIONS (Enable Foundation Assertions in Xcode Build Settings) which is default to NO in release builds.

But in Swift, asserts are controlled by the Swift optimization level. With a release build — which defaults SWIFT_OPTIMIZATION_LEVEL (Swift Compiler - Code Generation > Optimization Level) to -O, the assert() doesn't run. It runs with -Onone.

It's not complicated once we realise these are 2 different flags, but it's still a good practice to only run code which has no side effects with an assert to present such errors.

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