cocoa - Downcasting in Swift with as and as?

Question

What's the difference between these two code snippets:

let cell = tableView.dequeueReusableCellWithIdentifier("cellId") as UITableViewCell?
// vs
let cell = tableView.dequeueReusableCellWithIdentifier("cellId") as? UITableViewCell

Isn't the result exactly the same?

Answer 1

In that code there's no difference, in both cases it evaluates to UITableViewCell?

The real difference is:

• in the first case a downcast to UITableViewCell? is expected to always succeed (even if it's nil), so if dequeueReusableCellWithIdentifier returns something that's not an instance of UITableViewCell (or an instance of a class inherited from it), it fails at runtime. The expression returns an optional UITableViewCell?

• in the second case the cast is optional: if the object returned by dequeueReusableCellWithIdentifier is neither an instance of UITableViewCell nor an instance of a subclass, the downcast gracefully evaluates to nil (hence with no runtime error).

Of course dequeueReusableCellWithIdentifier always returns a UITableViewCell, that's why there's no difference in your code. But in other contexts the difference may exist and you have to take care of that to prevent runtime errors