![]() You can more safely unwrap with an exclamation point within an if statement because this sound is nominally guaranteed to be non- nil in non-extreme (that is, nonthreaded) situations: if sound != nil Īlthough useful for accessing values, if-let bindings build into code structures colloquially known as pyramids of doom. It’s pretty important to check whether an item is nil before unwrapping it. In your case, a cast from a Dog to an Animal is an upcast, because a Dog is-a Animal. Upcasting is always allowed, but downcasting involves a type check and can throw a ClassCastException. ![]() The nil value is the Swift equivalent of your aunt’s home-crocheted vest with a large friendly moose decoration on it. Upcasting is casting to a supertype, while downcasting is casting to a subtype. If the value is nil, you experience a fatal runtime error-which is not nearly as fun or gratifying for your workday. If the value is wrapped (that “optional” result you just read about), the exclamation point returns the value stored within the wrapped element. This is also the most dangerous approach, and you want to generally avoid it in production code. ![]() Using type casting we check the instance that it. The simplest way to extract an optional value is to append an exclamation point: print("The sound is \(sound!)") // may or may not crash In swift, Type casting is the process in which we can convert the type of one object into another type. There isnt a way to do it in one if statement that will be runtime-safe, but there is a way. The expressive possibilities expanded in Apple’s Swift 2 update to provide greater developer flexibility. The reason your desired formulation isnt working is that youre trying to unwrap two Optionals with a single as.There are two Optionals because both the subscripting of your dictionary and the attempted cast to String return optional values. There are more ways to get at a wrapped optional value than you might expect. Swift offers a variety of mechanisms for unwrapping optionals and recovering the underlying data type stored within them. In the Swift world, it is always Christmas, and there are always presents-or at least variables-to unwrap. You cannot get to that value (in this case, "moo") without unwrapping it. This does have the same truncating effect as using init (truncatingIfNeeded: BinaryInteger). let i: NSNumber 300 let j i as Int8 // j 44. Upcasting and downcasting gives us advantages, like Polymorphism or. As Hamish and OOPer mentioned in the comments, it is now possible to cast an NSNumber directly to Int8. Up-casting is casting to a supertype, while downcasting is casting to a subtype. Wrapping means that any actual content is stored within a logical outer structure. Directly casting NSNumber with as Int8 in Swift 3.0.1 and above. Optional types always return either a wrapped value or nil.
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