<html><head><meta http-equiv="Content-Type" content="text/html charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><br class=""><div><blockquote type="cite" class=""><div class="">On Jul 9, 2017, at 12:23 AM, Jens Persson <<a href="mailto:jens@bitcycle.com" class="">jens@bitcycle.com</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div dir="ltr" class=""><br class=""><div class="gmail_extra"><div class="gmail_quote">On Sat, Jul 8, 2017 at 6:28 PM, Chris Lattner via swift-evolution <span dir="ltr" class=""><<a href="mailto:swift-evolution@swift.org" target="_blank" class="">swift-evolution@swift.org</a>></span> wrote:<br class=""><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-style:solid;border-left-color:rgb(204,204,204);padding-left:1ex"><div style="word-wrap:break-word" class="">Hi Susan,<div class=""><br class=""></div><div class="">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.</div><div class=""><br class=""></div><div class="">-Chris</div></div></blockquote></div><br class=""></div><div class="gmail_extra">Does this imply that you should never use Swift structs to eg interact with Metal?</div></div></div></blockquote><div><br class=""></div>No.</div><div><br class=""></div><div><blockquote type="cite" class=""><div class=""><div dir="ltr" class=""><div class="gmail_extra">This seems to be a very common practice. Here is a typical example (from a Metal tutorial at <a href="http://raywenderlich.com/" class="">raywenderlich.com</a>):</div><div class="gmail_extra"><br class=""></div><div class="gmail_extra"><div class="gmail_extra">struct Vertex {</div><div class="gmail_extra"> var x,y,z: Float // position data</div><div class="gmail_extra"> var r,g,b,a: Float // color data</div><div class="gmail_extra"><br class=""></div><div class="gmail_extra"> func floatBuffer() -> [Float] {</div><div class="gmail_extra"> return [x,y,z,r,g,b,a]</div><div class="gmail_extra"> }</div><div class="gmail_extra">}</div></div></div></div></blockquote><div><br class=""></div><div>This doesn’t appear to expose the layout of the struct.</div><blockquote type="cite" class=""><div class=""><div dir="ltr" class=""><div class="gmail_extra">Also, does it imply that we cannot use structs (of only primitive types) like:</div><div class="gmail_extra"><br class=""></div><div class="gmail_extra">struct RgbaFloatsLinearGamma {</div><div class="gmail_extra"> var r, g, b, a: Float</div><div class="gmail_extra"> …</div><div class="gmail_extra">}</div><div class="gmail_extra">struct BgraBytesSrgbGamma {</div><div class="gmail_extra"> var b, g, r, a: UInt8</div><div class="gmail_extra">}</div><div class="gmail_extra"><br class=""></div><div class="gmail_extra">for manipulating raster image data?</div></div></div></blockquote><div><br class=""></div><div>I don’t see why that would be a problem.</div><div><br class=""></div><blockquote type="cite" class=""><div class=""><div dir="ltr" class=""><div class="gmail_extra">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.</div><div class="gmail_extra"><br class=""></div><div class="gmail_extra">Perhaps it could be considered OK at least when the intended platforms are known to be only iOS devices?</div></div></div></blockquote><br class=""></div><div>I think you’re misunderstanding what I’m saying. It isn’t correct to take (e.g.) an unsafepointer to the beginning of a struct, and serialize that out to disk, and expect that the fields are emitted in some order with some specific padding between them. None of the uses above try to do this.</div><div><br class=""></div><div>-Chris</div><div><br class=""></div><br class=""></body></html>