It is an "implicitly unwrapped optional", or IOU.
See:
https://developer.apple.com/library/content/documentation/Swift/Conceptual/Swift_Programming_Language/TheBasics.html
- Daniel
> On Jan 9, 2017, at 4:23 PM, Ethin Probst via swift-users
> <swift-users@swift.org> wrote:
>
> Hello all,
> I was wondering what an exclamation mark in a swift parameter listing
> means. For instance, I used Elements Oxidizer Live to translate some
> C# code into Swift. Reviewing it, I found the following (among
> others):
> @DllImport("ScriptHookV.dll")
> private static __extern func scriptRegister(_ module: System.IntPtr!,
> _ LP_SCRIPT_MAIN: LP_SCRIPT_MAINDelegate!)
> What I'm wondering is what does the '!' in that list of parameters
> mean? I also saw it when writing another program:
> func MainForm_Load(_ sender: System.Object!, _ e: System.EventArgs!) {
> // ...
> }
> I don't recall the swift book teaching you what that means, so could
> someone tell me what those mean?
> --
> Signed,
> Ethin D. Probst
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