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

[Philippe Hausler]

> On Jun 26, 2017, at 10:20 AM, Charles Srstka <cocoadev at charlessoft.com> wrote:
> 
> Rats, I was hoping that one of the reasons about being so explicit what we’re going to access and where with bindMemory() and friends would be to take care of these sorts of issues.
> 
> In that case, the simplest way to do it is probably just this:
> 
> let crc = (UInt16(myData[myData.endIndex]) << 8) | UInt16(myData[myData.endIndex - 1])

Yea that should work; and should be really fast as well. My example was just funneling it to memcpy and give you are guard where the bytes were not fully initialized.

> 
> (or the reverse, depending on the endianness of the source data)
> 
> Charles
> 
>> On Jun 26, 2017, at 12:05 PM, Philippe Hausler via swift-users <swift-users at swift.org <mailto:swift-users at swift.org>> wrote:
>> 
>> Data.copyBytes will do that under the hood
>> 
>> var crc: UInt16 = 0
>> let amountCopied = withUnsafeMutablePointer(to: &crc) { data.copyBytes(to: UnsafeMutableBufferPointer(start: $0, count: 1)) }
>> if amountCopied == MemoryLayout<UInt16>.size {
>>    // we have a full crc
>> }
>> 
>> That will probably do what you want; plus it will allow you to do it from a given range of bytes.
>> 
>> 
>>> On Jun 26, 2017, at 9:57 AM, Joe Groff via swift-users <swift-users at swift.org <mailto:swift-users at swift.org>> wrote:
>>> 
>>> 
>>>> On Jun 26, 2017, at 1:55 AM, Daniel Vollmer via swift-users <swift-users at swift.org <mailto:swift-users at swift.org>> wrote:
>>>> 
>>>> Hi Rick,
>>>> 
>>>>> On 26. Jun 2017, at 02:37, Rick Mann via swift-users <swift-users at swift.org <mailto:swift-users at swift.org>> wrote:
>>>> 
>>>> [snip]
>>>> 
>>>>> I'd also like to avoid unnecessary copying of the data. All of it is immutable for the purposes of this problem.
>>>>> 
>>>>> How can I get the UInt16 that starts at byte X in a Data? Same goes for Double or Int32 or whatever.
>>>> 
>>>> I’m not sure what Swift’s stance on this is, but not all platforms allow misaligned memory accesses (such as your attempt to access a UInt16 that lies at an odd memory address).
>>> 
>>> Unaligned memory accesses are not currently allowed by the language semantics, regardless of the underlying ISA. You should use memcpy if you need to load potentially-unaligned values out of raw memory.
>>> 
>>> -Joe
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>> 
> 

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