<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=""><div class="">AFAIK, you can legally access the <a href="http://en.cppreference.com/w/c/language/type#Compatible_types" class="">compatible</a> prefix of structure fields in C. (Regardless of whether you have a pointer to a Sample1 or to a Sample2, casting it to either and accessing a or b is guaranteed to work; c or d is UB.)</div><div class=""><br class=""></div><div class="">You can port your second example to Swift as well, by pulling out the common fields in a struct and using an enum as a union for the variable part:</div><div class=""><br class=""></div><div class=""><div class="">struct Sample {</div><div class=""><span class="Apple-tab-span" style="white-space:pre"> </span>enum CD {</div><div class=""><span class="Apple-tab-span" style="white-space:pre"> </span>case float(Float)</div><div class=""><span class="Apple-tab-span" style="white-space:pre"> </span>case double(Double)</div><div class=""><span class="Apple-tab-span" style="white-space:pre"> </span>}</div><div class=""><span class="Apple-tab-span" style="white-space:pre"> </span>var a: Int</div><div class=""><span class="Apple-tab-span" style="white-space:pre"> </span>var b: Int</div><div class=""><span class="Apple-tab-span" style="white-space:pre"> </span>var cd: CD</div><div class="">}</div></div><div class=""><br class=""></div><div class="">Félix</div><br class=""><div><blockquote type="cite" class=""><div class="">Le 22 juil. 2017 à 17:07, Daryle Walker via swift-evolution <<a href="mailto:swift-evolution@swift.org" class="">swift-evolution@swift.org</a>> a écrit :</div><br class="Apple-interchange-newline"><div class=""><meta http-equiv="Content-Type" content="text/html charset=utf-8" class=""><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><div class="">Way back in the macOS pre-X days, I remember some APIs had types like:</div><div class=""><br class=""></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><font face="Monaco" class="">struct Sample1 {</font></div></blockquote><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><font face="Monaco" class="">int a;</font></div></blockquote><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><font face="Monaco" class="">int b;</font></div></blockquote><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><font face="Monaco" class="">float c;</font></div></blockquote></blockquote><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><font face="Monaco" class="">};</font></div><div class=""><font face="Monaco" class=""><br class=""></font></div><div class=""><font face="Monaco" class="">struct Sample2 {</font></div></blockquote><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><font face="Monaco" class="">int a;</font></div></blockquote><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><font face="Monaco" class="">int b;</font></div></blockquote><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><font face="Monaco" class="">double d;</font></div></blockquote></blockquote><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><font face="Monaco" class="">};</font></div></blockquote><div class=""><br class=""></div><div class="">The types are supposed to be related, and it was supposed to be guaranteed that if you type-punned Sample1 to or from Sample2, the common members (“a” and “b” here) would have the same semantics. I guess it could be done like this:</div><div class=""><br class=""></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><font face="Monaco" class="">struct Sample {</font></div></blockquote><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><font face="Monaco" class="">int a;</font></div></blockquote><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><font face="Monaco" class="">int b;</font></div></blockquote><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><font face="Monaco" class="">union {</font></div></blockquote></blockquote><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><font face="Monaco" class="">float c;</font></div></blockquote></blockquote><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><font face="Monaco" class="">double d;</font></div></blockquote></blockquote></blockquote><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><font face="Monaco" class="">}</font></div></blockquote></blockquote><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><font face="Monaco" class="">};</font></div></blockquote><div class=""><br class=""></div><div class="">too.</div><div class=""><br class=""></div><div class="">I wonder if we need something similar in Swift:</div><div class=""><br class=""></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><font face="Monaco" class="">enum Sample {</font></div></blockquote><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><font face="Monaco" class="">@common case withFloat(a: Int, b: Int, c: Float)</font></div></blockquote><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><font face="Monaco" class="">@common case withDouble(a: Int, b: Int, d: Double)</font></div></blockquote></blockquote><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><font face="Monaco" class="">}</font></div></blockquote><div class=""><br class=""></div><div class="">The attribute would ensure that the shared initial members would map to the same internal offset. And, if there were no cases that didn’t have the common members, you can use them as instance level properties:</div><div class=""><br class=""></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><font face="Monaco" class="">var x: Sample = .withFloat(1, 2, 3.0)</font></div><div class=""><font face="Monaco" class="">//…</font></div><div class=""><font face="Monaco" class="">assert(x.a == 1) // Instead of “x.withFloat.a"</font></div></blockquote><div class=""><br class=""></div><div class="">Good idea? Does this already exist?</div><div class=""><br class=""></div><div class="">
<div style="letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><div class="">— </div><div class="">Daryle Walker<br class="">Mac, Internet, and Video Game Junkie<br class="">darylew AT mac DOT com </div></div>
</div>
<br class=""></div>_______________________________________________<br class="">swift-evolution mailing list<br class=""><a href="mailto:swift-evolution@swift.org" class="">swift-evolution@swift.org</a><br class="">https://lists.swift.org/mailman/listinfo/swift-evolution<br class=""></div></blockquote></div><br class=""></body></html>