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Mon, 17 Dec 2018 14:27:19 -0800

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 
> free online bettinghttps://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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