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

[David Kilzer]

Filed:  <https://bugs.webkit.org/show_bug.cgi?id=192728>

Dave


On Dec 14, 2018, at 2:18 PM, Adrian Perez de Castro <aperez at igalia.com> wrote:

> 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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