<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 Dec 30, 2015, at 1:22 PM, Kevin Ballard <<a href="mailto:kevin@sb.org" class="">kevin@sb.org</a>> wrote:</div><br class="Apple-interchange-newline"><div class="">
<title class=""></title>
<div class=""><div class="">A uniquely-owned class that guarantees stack allocation is pretty much the same thing as a move-only value type, isn't it? The only real difference I can think of is classes allow for subclassing.<br class=""></div>
</div></div></blockquote><div><br class=""></div><div>At this point, we’re talking about two unspecified and hypothetical models, so of course they’re both equivalent and completely different :-)</div><div><br class=""></div><div>We should talk about this in more detail later (perhaps next year, perhaps the year after), but I am pretty concerned with saying that unique ownership of classes replaces move-only types. From a programming model perspective (how the programmer thinks about & designs their code) both capabilities are important. You want move-only struct types in various cases and unique ownership of class instances.</div><div><br class=""></div><div>For example, IMO, a uniquely-owned class instance has to be on the heap, because it would have to convert to a multiply owned reference in many cases, and “moving” a class from the stack to the heap is, uh, complicated.</div><div><br class=""></div><div>-Chris</div><div><br class=""></div><div><br class=""></div><br class=""><blockquote type="cite" class=""><div class=""><div class=""><div class=""> </div>
<div class="">-Kevin</div>
<div class=""> </div>
<div class="">On Wed, Dec 30, 2015, at 01:18 PM, Chris Lattner wrote:<br class=""></div>
<blockquote type="cite" class=""><div class=""> </div>
<div class=""><blockquote type="cite" class=""><div class="">On Dec 30, 2015, at 10:31 AM, Joe Groff <<a href="mailto:jgroff@apple.com" class="">jgroff@apple.com</a>> wrote:<br class=""></div>
<div class=""> </div>
<div class=""><div style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant:normal;font-weight:normal;letter-spacing:normal;orphans:auto;text-align:start;text-indent:0px;text-transform:none;white-space:normal;widows:auto;word-spacing:0px;-webkit-text-stroke-width:0px;" class=""><blockquote type="cite" class=""><div class=""><div class=""> </div>
<div class="">On Dec 30, 2015, at 10:26 AM, Chris Lattner <<a href="mailto:clattner@apple.com" class="">clattner@apple.com</a>> wrote:<br class=""></div>
</div>
<div class=""> </div>
<div class=""><div style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant:normal;font-weight:normal;letter-spacing:normal;orphans:auto;text-align:start;text-indent:0px;text-transform:none;white-space:normal;widows:auto;word-spacing:0px;-webkit-text-stroke-width:0px;" class=""><blockquote type="cite" class=""><div class=""><div class=""> </div>
<div class="">On Dec 30, 2015, at 9:53 AM, Joe Groff via swift-evolution <<a href="mailto:swift-evolution@swift.org" class="">swift-evolution@swift.org</a>> wrote:<br class=""></div>
</div>
<div class=""> </div>
<div class=""><div style="word-wrap:break-word;-webkit-line-break:after-white-space;" class=""><div class=""> </div>
<div class=""><blockquote type="cite" class=""><div class="">On Dec 29, 2015, at 8:55 PM, Kevin Ballard via swift-evolution <<a href="mailto:swift-evolution@swift.org" class="">swift-evolution@swift.org</a>> wrote:<br class=""></div>
<div class=""> </div>
<div class=""><div class=""><div class="">An alternative solution is to do what Rust and C++ do, which is to use RAII. Which is to say, instead of introducing a new language construct that's explicitly tied to a scope, you just use a struct to represent the resource that you hold (e.g. a File that represents an open file). Of course, this does require some changes to structs, notably the addition of a deinit. And if structs have a deinit, then they also need to have a way to restrict copies. This is precisely what Rust does; any struct in Rust that implements Drop (the equivalent to deinit) loses the ability to be implicitly copied (a second trait called Clone provides a .clone() method that is the normal way to copy such non-implicitly-copyable structs).<br class=""></div>
</div>
</div>
</blockquote><div class=""> </div>
<div class="">deinit doesn't make sense for value types. <br class=""></div>
</div>
</div>
</div>
</blockquote></div>
<div style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant:normal;font-weight:normal;letter-spacing:normal;orphans:auto;text-align:start;text-indent:0px;text-transform:none;white-space:normal;widows:auto;word-spacing:0px;-webkit-text-stroke-width:0px;" class="">It would if we extended the model for value types to be richer, e.g. to introduce the notion of "move only” structs.<br class=""></div>
</div>
</blockquote></div>
<div style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant:normal;font-weight:normal;letter-spacing:normal;orphans:auto;text-align:start;text-indent:0px;text-transform:none;white-space:normal;widows:auto;word-spacing:0px;-webkit-text-stroke-width:0px;" class="">Perhaps, but I feel like it's a more natural extension of our existing model to support uniquely-owned classes though, which would give you all the same benefits.<br class=""></div>
</div>
</blockquote></div>
<div class="">So long as it guarantees no heap allocation for the class instance, ok.<br class=""></div>
<div class=""> </div>
<div class="">-Chris<br class=""></div>
</blockquote><div class=""> </div>
</div>
</div></blockquote></div><br class=""></body></html>