[swift-evolution] [Proposal] Introduces endianness specific type

[Jens Persson]

On Sat, Jul 8, 2017 at 6:28 PM, Chris Lattner via swift-evolution <
swift-evolution at swift.org> wrote:

> Hi Susan,
>
> Swift does not currently specify a layout for Swift structs.  You
> shouldn’t be using them for memory mapped i/o or writing to a file, because
> their layout can change.  When ABI stability for fragile structs lands, you
> will be able to count on it, but until then something like this is probably
> a bad idea.
>
> -Chris
>

Does this imply that you should never use Swift structs to eg interact with
Metal?

This seems to be a very common practice. Here is a typical example (from a
Metal tutorial at raywenderlich.com):

struct Vertex {
  var x,y,z: Float     // position data
  var r,g,b,a: Float   // color data

  func floatBuffer() -> [Float] {
    return [x,y,z,r,g,b,a]
  }
}

(
https://www.raywenderlich.com/146416/metal-tutorial-swift-3-part-2-moving-3d
 )

Also, does it imply that we cannot use structs (of only primitive types)
like:

struct RgbaFloatsLinearGamma {
    var r, g, b, a: Float
    …
}
struct BgraBytesSrgbGamma {
    var b, g, r, a: UInt8
}

for manipulating raster image data?

I vaguely remember a swift evo discussion where it was concluded that such
usage was considered OK provided the stored properties of the structs was
only primitive types, but I can't find it now.

Perhaps it could be considered OK at least when the intended platforms are
known to be only iOS devices?

/Jens
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