> On Dec 14, 2018, at 1:54 PM, Saam Barati <sbar...@apple.com> wrote: > > > >> On Dec 14, 2018, at 1:37 PM, Chris Dumez <cdu...@apple.com >> <mailto:cdu...@apple.com>> wrote: >> >> Hi, >> >> I have now been caught twice by std::optional’s move constructor. It turns >> out that it leaves the std::optional being moved-out *engaged*, it merely >> moves its value. >> >> For example, testOptional.cpp: >> #include <iostream> >> #include <optional> >> >> int main() >> { >> std::optional<int> a = 1; >> std::optional<int> b = std::move(a); >> std::cout << "a is engaged? " << !!a << std::endl; >> std::cout << "b is engaged? " << !!b << std::endl; >> return 0; >> } >> >> $ clang++ testOptional.cpp -o testOptional -std=c++17 >> $ ./testOptional >> a is engaged? 1 >> b is engaged? 1 >> >> I would have expected: >> a is engaged? 0 >> b is engaged? 1 > I would have expected this too. > >> >> This impacts the standard std::optional implementation on my machine as well >> as the local copy in WebKit’s wtf/Optional.h. >> >> As far as I know, our convention in WebKit so far for our types has been >> that types getting moved-out are left in a valid “empty” state. > I believe it's UB to use an object after it has been moved. I'm wrong here. Apparently objects are left in a "valid but unspecified state".
https://stackoverflow.com/questions/32346143/undefined-behavior-with-stdmove <https://stackoverflow.com/questions/32346143/undefined-behavior-with-stdmove> - Saam > > - Saam > >> As such, I find that std::optional’s move constructor behavior is >> error-prone. >> >> I’d like to know how do other feel about this behavior? If enough people >> agree this is error-prone, would we consider having our >> own optional type in WTF which resets the engaged flag (and never allow the >> std::optional)? >> >> Thanks, >> -- >> Chris Dumez >> >> >> >> >> _______________________________________________ >> webkit-dev mailing list >> webkit-dev@lists.webkit.org <mailto:webkit-dev@lists.webkit.org> >> https://lists.webkit.org/mailman/listinfo/webkit-dev > > _______________________________________________ > webkit-dev mailing list > webkit-dev@lists.webkit.org <mailto:webkit-dev@lists.webkit.org> > https://lists.webkit.org/mailman/listinfo/webkit-dev > <https://lists.webkit.org/mailman/listinfo/webkit-dev>
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