<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=""><div class=""><blockquote type="cite" class=""><div dir="ltr" class=""><div class="gmail_quote"><div class="">Can you explain what your specialization does differently? And why it's needed?</div></div></div></blockquote></div><div class=""><br class=""></div>I have a class which is a C++ interface for an object defined in a C library. Internally, it is a pointer which can never be null (if C interface returns null, the class constructor throws).<div class=""><div class="">I want to use this fact to optimise optional's storage. That would allow shrinking optional's size to eight bytes. Since we store a lot of optionals of this class, this change would reduce memory usage by about five percent (I've done some testing).</div><div class="">I really don't want to invent a new optional specifically for this case. I also don't want to allow storing null pointers inside of the original class because this would require an assert before almost any usage of the class.</div><div class=""><br class=""></div><div class=""><blockquote type="cite" class=""><div dir="ltr" class=""><div class="gmail_quote"><div class="">I think the standard is being too permissive here.</div></div></div></blockquote></div><div class=""><br class=""></div><div class="">Maybe it is. But again, if I can provide a more efficient specialization which would utilise some knowledge about the wrapped type, why shouldn't I?</div><div class=""><br class=""></div><div class=""><blockquote type="cite" class=""><div dir="ltr" class=""><div class="gmail_quote"><div class="">`<variant>` is a good example.</div></div></div></blockquote></div><div class=""><br class=""></div><div class="">By the way, providing custom specializations for variadic templates is forbidden =)</div><div class=""><br class=""><div class="">
<div style="caret-color: rgb(0, 0, 0); color: rgb(0, 0, 0); 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;">Kind regards, Vladimir Goncharov</div>
</div>
<div><br class=""><blockquote type="cite" class=""><div class="">On 7 Mar 2019, at 08:39, Eric Fiselier <<a href="mailto:eric@efcs.ca" class="">eric@efcs.ca</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 Wed, Mar 6, 2019 at 5:41 PM Владимир Гончаров 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:1px solid rgb(204,204,204);padding-left:1ex"><div style="overflow-wrap: break-word;" class=""><div style="overflow-wrap: break-word;" dir="auto" class="">Hello!<div class=""><br class=""></div><div class="">I'm implementing a specialization for std::optional for my program-defined type. </div></div></div></blockquote><div class=""><br class=""></div><div class="">Can you explain what your specialization does differently? And why it's needed?</div><div class=""> </div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div style="overflow-wrap: break-word;" class=""><div style="overflow-wrap: break-word;" dir="auto" class=""><div class="">As I understand <font color="#0069d9" class=""><u class="">[namespace.std]</u></font>, nothing forbids me from doing that.</div></div></div></blockquote><div class=""><br class=""></div><div class="">I agree that you've interpreted the standard correctly. But I think the standard is being too permissive here.</div><div class="">Implementing a complex component may necessitate knowledge of the internals. `<variant>` is a good example.</div><div class=""></div><div class=""><br class=""></div><div class="">I would like to know what other library maintainers think.</div><div class=""> </div><div class="">/Eric</div></div></div>
</div></blockquote></div><br class=""></div></div></body></html>