Filed: <https://bugs.webkit.org/show_bug.cgi?id=192728>
Dave On Dec 14, 2018, at 2:18 PM, Adrian Perez de Castro <ape...@igalia.com> wrote: > Hello, > > On Fri, 14 Dec 2018 14:02:39 -0800, Saam Barati <sbar...@apple.com> wrote: > >>> On Dec 14, 2018, at 1:59 PM, Chris Dumez <cdu...@apple.com> wrote: >>> >>>> On Dec 14, 2018, at 1:56 PM, Saam Barati <sbar...@apple.com >>>> <mailto:sbar...@apple.com>> wrote: >>>> >>>>> On Dec 14, 2018, at 1:54 PM, Saam Barati <sbar...@apple.com >>>>> <mailto: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 is also what I would have expected. > >>>>>> 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> >>> I believe in WebKit, however, we’ve made sure our types are left in a valid >>> “empty” state, thus my proposal for our own optional type that would be >>> less error-prone / more convenient to use. >> I think your proposal is reasonable. I agree it's less error prone. >> >> I think if we make this change, we should pull optional out of std and put >> it in WTF, since we're now implementing behavior we will rely on being >> specified. > > I am also in favor of having an implementation in WTF that empties the > optional after moving the value out from it. > > Cheers, > > > -Adrián > _______________________________________________ > webkit-dev mailing list > webkit-dev@lists.webkit.org > https://lists.webkit.org/mailman/listinfo/webkit-dev _______________________________________________ webkit-dev mailing list webkit-dev@lists.webkit.org https://lists.webkit.org/mailman/listinfo/webkit-dev
