[swift-users] Extracting arbitrary types (e.g. UInt16) out of Data

Charles Srstka cocoadev at charlessoft.com
Mon Jun 26 01:11:00 CDT 2017

> On Jun 26, 2017, at 12:00 AM, Roderick Mann via swift-users <swift-users at swift.org> wrote:
> I mean, it's as straightforward as my example. I have a Data of arbitrary size (anywhere from 3 to 29 bytes, let's say). The last two bytes form a UInt16 CRC. I need to get those last two out and compare them against the CRC I compute for the rest of the bytes.
> Having said that, I just used withUnsafeBytes() and grabbed the last two bytes, and assembled them into a UInt16 with shift and or.
> I'd like to be able to do something like value<Double>(at: 3), though.

You kinda can, although it’s a bit more verbose:

let crc = myData[(myData.endIndex - 2)…].withUnsafeBytes { UInt16(littleEndian: $0.pointee) }

If you look at the source, withUnsafeBytes calls withMemoryRebound with the inferred generic type (UInt16 in this case), so I think this should be safe.


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