CODEWITHSUNDEEP
swift
Boon
Boon

Reputation: 41510

Implicitly unwrapped optional is nil but does not cause runtime exception

I was under the impression that implicitly unwrapped optional will cause a runtime exception when used and it is nil. But the following code works without runtime exception, why?

var str:String?
println(str!)  // Crashes as expected

var str:String! // Implicitly unwrapped
println(str)    // Does not crash, not what I expect - it prints nil

Upvotes: 0

Views: 92

Answers (2)

newacct
newacct

Reputation: 122489

An implicitly-unwrapped optional is only forcibly unwrapped in a situation which expects a non-optional. println() accepts all types, including optionals, and so there is no need to forcibly unwrap it before passing to println(). Since it is not unwrapped, it does not crash.

Upvotes: 0

Antonio
Antonio

Reputation: 72770

It prints the variable as enum (i.e. optional), because internally an optional is enum Optional<T>. More precisely, I presume it uses the debugDescription property, in fact this is what happens:

var str:String?
println(str) // Prints "nil"
str.debugDescription // Prints "nil"

Upvotes: 1

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