[webkit-dev] Watch out for std::optional's move constructor

[Adrian Perez de Castro]

Hello,

On Fri, 14 Dec 2018 14:02:39 -0800, Saam Barati <sbarati at apple.com> wrote:
 
> > On Dec 14, 2018, at 1:59 PM, Chris Dumez <cdumez at apple.com> wrote:
> > 
> >> On Dec 14, 2018, at 1:56 PM, Saam Barati <sbarati at apple.com <mailto:sbarati at apple.com>> wrote:
> >> 
> >>> On Dec 14, 2018, at 1:54 PM, Saam Barati <sbarati at apple.com <mailto:sbarati at apple.com>> wrote:
> >>> 
> >>>> On Dec 14, 2018, at 1:37 PM, Chris Dumez <cdumez at apple.com <mailto:cdumez at 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
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