DIP 36: Rvalue References

[Manu]

On 24 April 2013 00:24, Andrei Alexandrescu
<SeeWebsiteForEmail at erdani.org>wrote:

>
>> The very point of this DIP is to not create
>> syntax-driven features, instead better define existing ones that make
>> sense on their own so they can be used for same purpose.
>>
>
> It's a new feature, no two ways about it. It overlaps ref and auto ref
> without any palpable benefit and defines yet another way to achieve the
> same thing as auto ref. On this ground alone the proposal has a large
> problem.


How does it overlap ref? It simply justifies the argument with an extra
constraint and isn't tied to 'ref' at all, it's just useful in conjunction.

I can't agree that it overlaps auto-ref at all. They're fundamentally
different concepts. auto-ref is a template concept; it selects the ref-ness
based on the received arg. 'auto ref', ie, 'automatic ref-ness'. It makes
no sense on a non-template situation.
I'm still completely amazed that the very reason this DIP makes perfect
sense to me(/us) is the same reason you have a problem with it.

 2. The proposal is sketchy and does not give many details, such as the
>>> lifetime of temporaries bound to scope ref objects.
>>>
>>
>> It can't because lifetime of temporaries is not defined in D at all and
>> suck stuff needs to be consistent. It is not really different from a
>> lifetime of struct literal temporary used for pass-by-value.
>>
>
> A proposal aimed at binding rvalues to references must address lifetime of
> temporaries as a central concern.


It's not an r-value, it's a standard stack-allocated temporary. It's
lifetime is identical to any other local.
The reason it's not detailed in the proposal is because it adds no such new
feature, and makes no changes. The lifetime of a local is well understood.


 3. The relationship with auto ref is insufficiently described, e.g.
>>> there should be clarification on why auto ref cannot be improved to
>>> fulfill the desired role.
>>>
>>
>> auto ref is a template-world entity. If by "improved" you mean
>> "completely reworked" than sure, I can add this rationale. Will do today.
>>
>
> I think we should focus on http://d.puremagic.com/issues/**
> show_bug.cgi?id=9238 <http://d.puremagic.com/issues/show_bug.cgi?id=9238>and on making ref safe.


I don't believe it's possible to make ref safe. Can you suggest any vision
for this?
It's unsafe by definition... you are passing a pointer of unknown origin to
a function that could do anything with that pointer.
Hence 'scope ref', which appropriately restricts what the callee is able to
do with it.

 4. Above all this is a new language feature and again we want to
>>> resort to adding new feature only if it is clear that the existing
>>> features are insufficient and cannot be made sufficient. In particular
>>> we are much more inclined to impart real, demonstrable safety to "ref"
>>> and to make "auto ref" work as a reference that can bind to rvalues as
>>> well as lvalues.
>>>
>>
>> Can't agree. "scope" is defined (but not implemented, a pity). Creating
>> temporaries already exists for some cases. No really new language
>> feature is added. No special syntax created. "scope ref" is still
>> "scope" and "ref" with all semantics this imposes, rvalue references are
>> just a side-effect. Contrary to this, "auto ref" IS a new feature
>> created specifically for syntax hack. That is a very problem you are
>> speaking about - introducing new concepts instead of making use of ones
>> that exist for ages.
>>
>
> You are of course to disagree, but that would make you wrong.


Explain why? Proclaiming he is simply wrong when he presents sound reason
isn't helpful.
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