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Mon, 17 Dec 2018 16:03:43 -0800

> On 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.

Yes, this is a good example of case where std::exchange should be used or where 
the parameter should be a RefPtr<Test> and not a RefPtr<Test>&&. But as I 
mentioned earlier, the issue here is that the caller of WTFMove() is not 
actually calling any move constructor (it is merely doing a cast) and therefore 
cannot rely on the effects or a move constructor.
I do not think that this example is a good reason why we would not want 
optional to have a more predictable move constructor.

>> On Dec 17, 2018, at 2:47 PM, Ryosuke Niwa <rn...@webkit.org 
>> <mailto: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 
>> <mailto: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 
>>> <mailto: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 
>>> <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 
>>>> <mailto:cdu...@apple.com>> wrote:
>>>>> On Dec 17, 2018, at 11:10 AM, Chris Dumez <cdu...@apple.com 
>>>>> <mailto:cdu...@apple.com>> wrote:
>>>>>> On Dec 17, 2018, at 10:27 AM, Alex Christensen <achristen...@apple.com 
>>>>>> <mailto:achristen...@apple.com>> wrote:
>>>>>>>>>> On Dec 14, 2018, at 1:37 PM, Chris Dumez <cdu...@apple.com 
>>>>>>>>>> <mailto: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 
>>>> <https://trac.webkit.org/changeset/236463/webkit>
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