<html><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;" class=""><br class=""><div><br class=""><blockquote type="cite" class=""><div class="">On 2019 Apr 17, at 11:06, Louis Dionne <<a href="mailto:ldionne@apple.com" class="">ldionne@apple.com</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><br class="Apple-interchange-newline"><br class="" style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; 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; text-decoration: none;"><blockquote type="cite" class="" style="font-family: Helvetica; font-size: 12px; 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; text-decoration: none;"><div class="">On Apr 17, 2019, at 13:38, JF Bastien <<a href="mailto:jfbastien@apple.com" class="">jfbastien@apple.com</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><br class="Apple-interchange-newline"><br class="" style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; 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; text-decoration: none;"><blockquote type="cite" class="" style="font-family: Helvetica; font-size: 12px; 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; text-decoration: none;"><div class="">On Apr 17, 2019, at 10:12 AM, Louis Dionne via libcxx-dev <<a href="mailto:libcxx-dev@lists.llvm.org" class="">libcxx-dev@lists.llvm.org</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div class="" style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;"><br class=""><div class=""><br class=""><blockquote type="cite" class=""><div class="">On Apr 17, 2019, at 12:39, Duncan P. N. Exon Smith <<a href="mailto:dexonsmith@apple.com" class="">dexonsmith@apple.com</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div class="" style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;">I'm curious about how people feel this should/could behave with -fno-exceptions. IIRC, Libc++ calls the copy constructor (following the standard), but -fno-exceptions isn't really standard, and from a user perspective that doesn't make sense. `noexcept` is essentially meaningless with -fno-exceptions.<br class=""></div></div></blockquote><div class=""><br class=""></div><div class="">I personally feel like it would be surprising if -fno-exceptions caused the move constructor to be called instead of the copy constructor. My mental model is that -fno-exceptions only disables exceptions as a language feature, but doesn't really instruct libraries to provide a different API (even though the difference in this case is very small). Instead, I feel like it would make more sense if "everything" was implicitly marked noexcept when -fno-exceptions is used (because that's really the case). The library would behave optimally with -fno-exceptions without having to do anything special.</div></div></div></div></blockquote><div class="" style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; 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; text-decoration: none;"><br class=""></div><div class="" style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; 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; text-decoration: none;">IIUC what you’re saying: the library should just use the noexcept operator to do its business, and it seem like -fno-exceptions should make noexcept always return true? That would be pretty consistent (more than teaching the library about -fno-exceptions for move / copy).</div></div></blockquote><div style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; 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; text-decoration: none;" class=""><br class=""></div><div style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; 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; text-decoration: none;" class="">Yes, that's what I mean. I agree with Eric that moving instead of copying provides value, but I think the right way to do it is not to special case libc++, but instead to have all code bases become better under -fno-exceptions by making noexcept do the right thing.</div></div></blockquote><div><br class=""></div><div>Yeah, that makes sense to me too.</div><div><br class=""></div><div>+Richard, I could have sworn you and I chatted quickly about this back in 2016 by my mail-search-fu is failing me. What are your thoughts on having -fno-exceptions affect the value of `noexcept`?</div><br class=""><blockquote type="cite" class=""><div class=""><blockquote type="cite" class="" style="font-family: Helvetica; font-size: 12px; 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; text-decoration: none;"><div class=""><div class="" style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; 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; text-decoration: none;"><br class=""></div><br class="" style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; 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; text-decoration: none;"><blockquote type="cite" class="" style="font-family: Helvetica; font-size: 12px; 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; text-decoration: none;"><div class=""><div class="" style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;"><div class=""><div class="">Louis</div><br class=""><blockquote type="cite" class=""><div class=""><div class="" style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;"><div class=""><br class=""></div><div class="">Thoughts?</div><div class=""><br class=""><blockquote type="cite" class=""><div class="">On Apr 16, 2019, at 23:19, Eric Fiselier via libcxx-dev <<a href="mailto:libcxx-dev@lists.llvm.org" class="">libcxx-dev@lists.llvm.org</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div dir="ltr" class=""><div dir="ltr" class=""><br class=""></div><br class=""><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Tue, Apr 16, 2019 at 2:25 PM Michael Narigon via libcxx-dev <<a href="mailto:libcxx-dev@lists.llvm.org" class="">libcxx-dev@lists.llvm.org</a>> wrote:<br class=""></div><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;">std:vector is using the element copy constructor on existing elements during its resizing operation in reserve(). I would expect it to use the move constructor for efficiency. Is this intended behavior?<br class=""></blockquote><div class=""><br class=""></div><div class="">Yes, this behavior is mandated by the standard. The copy constructor is used when the move constructor is not noexcept. From cppreference's `std::move_if_noexcept` documentation:<br class=""><br class=""></div><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;"><span class="gmail-t-lc" style="font-size: 12.8px; font-family: DejaVuSansMono, "DejaVu Sans Mono", courier, monospace;"><a href="https://en.cppreference.com/w/cpp/container/vector/resize" title="cpp/container/vector/resize" class="" style="color: rgb(11, 0, 128); background-image: none;">std::vector::resize</a></span><span class="" style="font-family: DejaVuSans, "DejaVu Sans", arial, sans-serif; font-size: 12.8px;">, which may have to allocate new storage and then move or copy elements from old storage to new storage.<br class=""></span><span class="" style="font-family: DejaVuSans, "DejaVu Sans", arial, sans-serif; font-size: 12.8px;">If an exception occurs during this operation, </span><span class="gmail-t-lc" style="font-size: 12.8px; font-family: DejaVuSansMono, "DejaVu Sans Mono", courier, monospace;"><a href="https://en.cppreference.com/w/cpp/container/vector/resize" title="cpp/container/vector/resize" class="" style="color: rgb(11, 0, 128); background-image: none;">std::vector::resize</a></span><span class="" style="font-family: DejaVuSans, "DejaVu Sans", arial, sans-serif; font-size: 12.8px;"> undoes everything it did to this point, which is only possible if<br class=""></span><code class="" style="font-size: 12.8px; font-family: DejaVuSansMono, "DejaVu Sans Mono", courier, monospace;">std::move_if_noexcept</code><span class="" style="font-family: DejaVuSans, "DejaVu Sans", arial, sans-serif; font-size: 12.8px;"> was used to decide whether to use move construction or copy construction. (unless copy constructor is</span> </blockquote><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;"><span class="" style="font-family: DejaVuSans, "DejaVu Sans", arial, sans-serif; font-size: 12.8px;">not available, in which case move constructor is used either way and the strong exception guarantee may be waived)</span> </blockquote><div class=""> <a href="https://en.cppreference.com/w/cpp/utility/move_if_noexcept" class="">https://en.cppreference.com/w/cpp/utility/move_if_noexcept</a></div><div class=""><br class=""></div><div class="">If you add `noexcept` to your move constructor, vector should call the move constructor.</div><div class=""><br class=""></div><div class=""><br class=""></div><div class=""> </div><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;"><br class="">This is with the libc++ delivered with Xcode 10.1.<br class=""><br class="">Attached is a sample program displaying the behavior.<br class="">_______________________________________________<br class="">libcxx-dev mailing list<br class=""><a href="mailto:libcxx-dev@lists.llvm.org" target="_blank" class="">libcxx-dev@lists.llvm.org</a><br class=""><a href="https://lists.llvm.org/cgi-bin/mailman/listinfo/libcxx-dev" rel="noreferrer" target="_blank" class="">https://lists.llvm.org/cgi-bin/mailman/listinfo/libcxx-dev</a><br class=""></blockquote></div></div>_______________________________________________<br class="">libcxx-dev mailing list<br class=""><a href="mailto:libcxx-dev@lists.llvm.org" class="">libcxx-dev@lists.llvm.org</a><br class=""><a href="https://lists.llvm.org/cgi-bin/mailman/listinfo/libcxx-dev" class="">https://lists.llvm.org/cgi-bin/mailman/listinfo/libcxx-dev</a><br class=""></div></blockquote></div><br class=""></div></div></blockquote></div><br class=""></div>_______________________________________________<br class="">libcxx-dev mailing list<br class=""><a href="mailto:libcxx-dev@lists.llvm.org" class="">libcxx-dev@lists.llvm.org</a><br class=""><a href="https://lists.llvm.org/cgi-bin/mailman/listinfo/libcxx-dev" class="">https://lists.llvm.org/cgi-bin/mailman/listinfo/libcxx-dev</a></div></blockquote></div></blockquote></div></blockquote></div><br class=""></body></html>