<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 27 Apr 2017, at 09:41, Rien via swift-users <<a href="mailto:swift-users@swift.org" class="">swift-users@swift.org</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><blockquote type="cite" style="font-family: Helvetica; font-size: 18px; font-style: normal; font-variant-caps: 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-size-adjust: auto; -webkit-text-stroke-width: 0px;" class=""><br class="Apple-interchange-newline">On 27 Apr 2017, at 09:54, Rick Mann <<a href="mailto:rmann@latencyzero.com" class="">rmann@latencyzero.com</a>> wrote:<br class=""><br class=""><blockquote type="cite" class=""><br class="">On Apr 26, 2017, at 23:37 , Rien via swift-users <<a href="mailto:swift-users@swift.org" class="">swift-users@swift.org</a>> wrote:<br class=""><br class="">1) When you obtain a pointer, it can no longer be ensured by the compiler that you won’t write to it.<br class="">2) A ‘let’ variable (constant) allows way more optimizations than a ‘var’. I would not be surprised if the majority of ‘let’ constants never see any memory allocation at all.<br class=""></blockquote><br class="">In my case, it's a field in a struct that's allocated elsewhere, and most certainly exists, and is passed as a parameter, so it's 'let'. I really want to be able to say "I promise I won't write to this", which seems reasonable if the language is willing to give me an unsafe pointer in the first place.<br class=""></blockquote><br style="font-family: Helvetica; font-size: 18px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><span style="font-family: Helvetica; font-size: 18px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; float: none; display: inline !important;" class="">IIRC this is the reason for the mutable and non-mutable version of the pointer. With the non-mutable pointer you cannot write to the pointee.</span><br style="font-family: Helvetica; font-size: 18px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><span style="font-family: Helvetica; font-size: 18px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; float: none; display: inline !important;" class="">(At least not without some additional code)</span><br style="font-family: Helvetica; font-size: 18px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><span style="font-family: Helvetica; font-size: 18px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; float: none; display: inline !important;" class="">But it is another step entirely to then allow a let assignment to be used as the pointee. I assume the Swift team decided to keep things simple and not allow let’s to be used as pointee’s. It could probably be done in theory, but I imagine that to be a nightmare case for compiler writers...</span><br style="font-family: Helvetica; font-size: 18px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""></div></blockquote></div><br class=""><div class="">The let constant may not even be stored in a single place; if it's known to be constant it can be in-lined at the point of use and potentially unpacked and dead code elimination throw away the unused members, for example.</div><div class=""><br class=""></div><div class="">If you want to pass in a let constant into the pointer, you can create a copy of it locally in a local variable and then use that instead. However this will be in the local scope, so the pointer isn't valid after it returns.</div><div class=""><br class=""></div><div class="">Alex</div></body></html>