[swift-users] raw buffer pointer load alignment

[Kelvin Ma]

yikes there’s no less verbose way to do that? and if the type isn’t an
integer there’s no way to avoid the default initialization? Can this be
done with opaques or something?

On Wed, Nov 8, 2017 at 7:04 PM, Johannes Weiß <johannesweiss at apple.com>
wrote:

> Hi Kelvin,
>
> > On 8 Nov 2017, at 4:54 pm, Kelvin Ma via swift-users <
> swift-users at swift.org> wrote:
> >
> > According to the docs, the load(fromByteOffset:as:) method requires the
> instance to be “properly aligned” Does that mean if I have raw data meant
> to be interpreted as
> >
> > 0      1      2      3      4      5      6
> > [ Int8 |    Int16    |    Int16    | Int8 | Int8 ]
> >
> >
> > i can’t just load the Int16 from byte offset 1?
>
> you can't just dereference that pointer as an Int16 (in any language)
> without causing UB but it's not really an issue, just do this (assuming
> `ptr` is the pointer into your data and `index` is the index to where your
> Int16 lives):
>
>
>     var value: Int16 = 0
>     withUnsafeMutableBytes(of: &value) { valuePtr in
>         valuePtr.copyBytes(from: UnsafeRawBufferPointer(start:
> ptr.baseAddress!.advanced(by: index),
>                                                         count:
> MemoryLayout<Int16>.size))
>     }
>
> you can have the whole thing generic too for <T: FixedWidthInteger>
>
>     var value: T = 0
>     withUnsafeMutableBytes(of: &value) { valuePtr in
>         valuePtr.copyBytes(from: UnsafeRawBufferPointer(start:
> ptr.baseAddress!.advanced(by: index),
>                                                         count:
> MemoryLayout<T>.size))
>     }
>
> -- Johannes
>
>
> > _______________________________________________
> > swift-users mailing list
> > swift-users at swift.org
> > https://lists.swift.org/mailman/listinfo/swift-users
>
>
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