Predict the output
What does f() print?
func f() {
defer { print("1") }
defer { print("2") }
print("3")
}
f()Defers behave like a stack.
Choose an answer, then open Show answer below to check it.
Answer: C 3 2 1
Correct. The body runs, then the defers run in reverse order.
defer schedules code to run when the current scope exits, however it exits: return, throw or falling off the end. Multiple defers run in reverse order of declaration. It keeps cleanup next to setup, for example unlocking a lock or resetting a loading flag.
Explain defer.
Say your answer out loud, then check which of these points you covered:
defer runs at the end of its scope, not the function, if declared inside an if or a loop.
You can't return or throw from inside a defer block.
Combine with isLoading = true; defer { isLoading = false }.
Defers run after the return value is computed, so they can't change what was already returned.
Seniors use defer for paired setup and cleanup and know the ordering rule.
Think of your answer first, then open the hint.
Does defer run if the function throws?
Yes.
Does a defer inside a loop run once per iteration?
Yes, at the end of each iteration.
Suggest a fix if something here is wrong or out of date.