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Mon, 17 Dec 2018 15:58:12 -0800

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 
> <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 
>>> <free online bettingmailto: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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