It's true that WTFMove or std::move doesn't do anything if the moved variable is not used because WTFMove / std::move is just a type cast.
However, that behavior is orthogonal from the issue that calling WTFMove / std::move on std::optional, and the returned value is assigned to another std::optional, the original std::optional will be left a bad state. I completely disagree with your assessment that this calls for the use of std::exchange. On Mon, Dec 17, 2018 at 3:55 PM Alex Christensen <achristen...@apple.com> wrote: > Let me give a concrete example on why, even with our nice-to-use WTF > types, the state of a C++ object is undefined after being moved from: > > #include <wtf/RefCounted.h> > #include <wtf/RefPtr.h> > #include <iostream> > > class Test : public RefCounted<Test> { }; > > void useParameter(RefPtr<Test>&& param) > { > RefPtr<Test> usedParam = WTFMove(param); > } > > void dontUseParameter(RefPtr<Test>&&) { } > > int main() { > RefPtr<Test> a = adoptRef(new Test); > RefPtr<Test> b = adoptRef(new Test); > std::cout << "a null? " << !a << std::endl; > std::cout << "b null? " << !b << std::endl; > useParameter(WTFMove(a)); > dontUseParameter(WTFMove(b)); > std::cout << "a null? " << !a << std::endl; > std::cout << "b null? " << !b << std::endl; > return 0; > } > > // clang++ test.cpp -I Source/WTF -L WebKitBuild/Debug -l WTF -framework > Foundation -L /usr/lib -l icucore --std=c++17 && ./a.out > > // a null? 0 > > > // b null? 0 > > > // a null? 1 > > > // b null? 0 > > > > > As you can see, the internals of callee dontUseParameter (which could be > in a different translation unit) affects the state of the local variable b > in this function. This is one of the reasons why the state of a moved-from > variable is intentionally undefined, and we can’t fix that by using our own > std::optional replacement. If we care about the state of a moved-from > object, that is what std::exchange is for. I think we should do something > to track and prevent the use of moved-from values instead of introducing > our own std::optional replacement. > > On Dec 17, 2018, at 2:47 PM, Ryosuke Niwa <rn...@webkit.org> wrote: > > Yeah, it seems like making std::optional more in line with our own > convention provides more merits than downsides here. People are using > WTFMove as if it's some sort of a swap operation in our codebase, and as > Maciej pointed out, having rules where people have to think carefully as to > when & when not to use WTFMove seems more troublesome than the proposed > fix, which would mean this work for optional. > > - R. Niwa > > On Mon, Dec 17, 2018 at 2:24 PM Geoffrey Garen <gga...@apple.com> wrote: > >> I don’t understand the claim about “undefined behavior” here. As Maciej >> pointed out, these are our libraries. We are free to define their behaviors. >> >> In general, “undefined behavior” is an unwanted feature of programming >> languages and libraries, which we accept begrudgingly simply because there >> are practical limits to what we can define. This acceptance is not a >> mandate to carry forward undefined-ness as a badge of honor. In any case >> where it would be practical to define a behavior, that defined behavior >> would be preferable to undefined behavior. >> >> I agree that the behavior of move constructors in the standard library is >> undefined. The proposal here, as I understand it, is to (a) define the >> behaviors move constructors in WebKit and (b) avoid std::optional and use >> an optional class with well-defined behavior instead. >> >> Because I do not ❤️ security updates, I do ❤️ defined behavior, and so I >> ❤️ this proposal. >> >> Geoff >> >> On Dec 17, 2018, at 12:50 PM, Alex Christensen <achristen...@apple.com> >> wrote: >> >> This one and the many others like it are fragile, relying on undefined >> behavior, and should be replaced by std::exchange. Such a change was made >> in https://trac.webkit.org/changeset/198755/webkit and we probably need >> many more like that, but we are getting away with relying on undefined >> behavior which works for us in most places. >> >> On Dec 17, 2018, at 11:24 AM, Chris Dumez <cdu...@apple.com> wrote: >> >> >> >> On Dec 17, 2018, at 11:10 AM, Chris Dumez <cdu...@apple.com> wrote: >> >> >> >> On Dec 17, 2018, at 10:27 AM, Alex Christensen <achristen...@apple.com> >> wrote: >> >> On Dec 14, 2018, at 1:37 PM, Chris Dumez <cdu...@apple.com> wrote: >> >> >> 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. >> >> This is not necessarily true. When we move out of an object to pass into >> a function parameter, for example, the state of the moved-from object >> depends on the behavior of the callee. If the callee function uses the >> object, we often have behavior that leaves the object in an “empty” state >> of some kind, but we are definitely relying on fragile undefined behavior >> when we do so because changing the callee to not use the parameter changes >> the state of the caller. We should never assume that WTFMove or std::move >> leaves the object in an empty state. That is always a bug that needs to be >> replaced by std::exchange. >> >> >> Feel like we’re taking about different things. I am talking about move >> constructors (and assignment operators), which have a well defined behavior >> in WebKit. And it seems you are talking about WTFMove(), which despite the >> name does not “move” anything, it is merely a cast. >> In the case you’re talking about the caller does NOT call the move >> constructor, it merely does a cast so I do not think your comment >> invalidates my statement. Note that in my patch, I was nearly WTFMove()ing >> the data member and assigning it to a local variable right away, calling >> the move constructor. >> >> >> Also note that may of us already rely on our move constructors’ behavior, >> just search for WTFMove(m_responseCompletionHandler) in: >> https://trac.webkit.org/changeset/236463/webkit >> >> >> _______________________________________________ >> 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 >> > >
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