<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="">Hi Jordan.</div><div class=""><br class=""></div><div class="">Thanks for the lengthy answer.</div><br class=""><div><blockquote type="cite" class=""><div class="">On Mar 1, 2017, at 6:21 PM, Jordan Rose <<a href="mailto:jordan_rose@apple.com" class="">jordan_rose@apple.com</a>> wrote:</div><div class=""><div dir="auto" applecontenteditable="true" class="" style="font-family: Helvetica; font-size: 13px; 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-stroke-width: 0px; word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;"><div class=""><div class=""><br class=""></div><div class="">Hey, Kenny. The const vs non-const part is important, since both the implicit conversion and cString(using:) are allowed to return a pointer to the internal data being used by the String, and modifying that would be breaking the rules (and could potentially cause a crash). In the case of 'fieldSep', it's unlikely that anyone is going to mutate the contents of the string; it was probably just the original author (you?) not bothering to be const-correct. If you control this struct, a better fix would be to use 'const char *' for the field.</div></div></div></div></blockquote><div><br class=""></div><div>This is the PostgreSQL client library, so I don’t really want to change it. (Although the source is available. Maybe I should submit a patch…)</div><br class=""><blockquote type="cite" class=""><div class=""><div dir="auto" applecontenteditable="true" class="" style="font-family: Helvetica; font-size: 13px; 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-stroke-width: 0px; word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;"><div class=""><div class="">That said, that's not the main isuse. The implicit conversion from String to UnsafePointer<CChar> is only valid when the string is used as a function argument, because the conversion might need to allocate temporary storage. In that case, it’s important to know when it’s safe to<span class="Apple-converted-space"> </span><i class="">deallocate</i> that storage. For a function call, that’s when the call returns, but for storing into a struct field it’s completely unbounded. So there’s no implicit conversion there.</div><div class=""><br class=""></div><div class="">If you’re willing to limit your use of the pointer value to a single block of code, you can use <a href="https://developer.apple.com/reference/swift/string/1538904-withcstring" class="">withCString</a>:</div><div class=""><br class=""></div></div></div><blockquote class="" style="font-family: Helvetica; font-size: 13px; 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-stroke-width: 0px; margin: 0px 0px 0px 40px; border: none; padding: 0px;"><div dir="auto" applecontenteditable="true" class="" style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;"><div class=""><div class="">myString.withCString {</div><div class=""> var opt :PQprintOpt = PQprintOpt()</div><div class=""> opt.fieldSep = UnsafeMutablePointer(mutating: $0)</div><div class=""> // use 'opt'</div><div class="">}</div></div></div></blockquote></div></blockquote><div><br class=""></div><div>Unfortunately, this is not always an option, since there are multiple char * files in PQprintOpt. Or could I just nest .withCString calls? Looks ugly, but might work, I guess.</div><br class=""><blockquote type="cite" class=""><div class=""><span style="font-family: Helvetica; font-size: 13px; 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-stroke-width: 0px; float: none; display: inline !important;" class="">Note that it is illegal to persist the pointer value beyond the execution of withCString, because it might point to a temporary buffer.</span><br class="" style="font-family: Helvetica; font-size: 13px; 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-stroke-width: 0px;"><div dir="auto" applecontenteditable="true" class="" style="font-family: Helvetica; font-size: 13px; 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-stroke-width: 0px; word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;"><div class=""><div class=""><br class=""></div><div class="">Alternately, if you’re willing to call free() later, you can use the C function strdup:</div><div class=""><br class=""></div></div></div><blockquote class="" style="font-family: Helvetica; font-size: 13px; 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-stroke-width: 0px; margin: 0px 0px 0px 40px; border: none; padding: 0px;"><div dir="auto" applecontenteditable="true" class="" style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;"><div class=""><div class="">opt.fieldSep = strdup(myString)</div></div></div></blockquote><blockquote class="" style="font-family: Helvetica; font-size: 13px; 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-stroke-width: 0px; margin: 0px 0px 0px 40px; border: none; padding: 0px;">// use ‘opt’</blockquote><blockquote class="" style="font-family: Helvetica; font-size: 13px; 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-stroke-width: 0px; margin: 0px 0px 0px 40px; border: none; padding: 0px;">free(opt.fieldSep)</blockquote><br class="" style="font-family: Helvetica; font-size: 13px; 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-stroke-width: 0px;"><div dir="auto" applecontenteditable="true" class="" style="font-family: Helvetica; font-size: 13px; 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-stroke-width: 0px; word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;"><div class=""><div class=""><br class=""></div><div class="">Of course, all of this is overkill for a string<span class="Apple-converted-space"> </span><i class="">literal.</i> Perhaps when Swift gets conditional conformances, we could consider making UnsafePointer<CChar> conform to ExpressibleByStringLiteral. Meanwhile, you can use the type designed specifically for this purpose, <a href="https://developer.apple.com/reference/swift/staticstring" class="">StaticString</a>:</div><div class=""><br class=""></div></div></div><blockquote class="" style="font-family: Helvetica; font-size: 13px; 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-stroke-width: 0px; margin: 0px 0px 0px 40px; border: none; padding: 0px;"><div dir="auto" applecontenteditable="true" class="" style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;"><div class=""><div class="">let separator: StaticString = “|”</div><div class="">opt.fieldSep = UnsafeMutablePointer(mutating: separator.utf8start)</div></div></div></blockquote><div class="" style="font-family: Helvetica; font-size: 13px; 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-stroke-width: 0px;"><br class=""></div><span style="font-family: Helvetica; font-size: 13px; 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-stroke-width: 0px; float: none; display: inline !important;" class="">Note that you<span class="Apple-converted-space"> </span></span><i class="" style="font-family: Helvetica; font-size: 13px; 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-stroke-width: 0px;">still</i><span style="font-family: Helvetica; font-size: 13px; 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-stroke-width: 0px; float: none; display: inline !important;" class=""> have to do the init(mutating:) workaround for the fact that you’re not allowed to mutate this string.</span></div></blockquote><div><br class=""></div><div>Ah - that's </div><br class=""><blockquote type="cite" class=""><div class=""><div dir="auto" applecontenteditable="true" class="" style="font-family: Helvetica; font-size: 13px; 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-stroke-width: 0px; word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;"><div class=""><blockquote type="cite" class=""><div class=""><div class="" style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;"><div class=""><span class="" style="font-variant-ligatures: no-common-ligatures;"><div class=""><br class=""></div><div class="">Also, why is the conversion to Swift an IUO? NULL is a totally valid value for fieldSep.</div></span></div></div></div></blockquote><br class=""></div><div class="">You're welcome to mark that field as _Nullable on the C side, but without annotations Swift makes very few assumptions about pointers imported from C.</div></div></div></blockquote><div><br class=""></div><div>If it’s “making very few assumptions”, I would think that, for safety’s sake, functions that return a pointer would always be optional, forcing the user to deal with any possible null pointer returns.</div><br class=""><blockquote type="cite" class=""><div class=""><div dir="auto" applecontenteditable="true" class="" style="font-family: Helvetica; font-size: 13px; 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-stroke-width: 0px; word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;"><div class="">Hope that helps!</div></div></div></blockquote><div><br class=""></div><div>Definitely!</div><br class="">-Kenny</div><div><br class=""></div><div><br class=""></div><div><br class=""></div><br class=""></body></html>