DIP 36: Rvalue References

Andrei Alexandrescu SeeWebsiteForEmail at erdani.org
Tue Apr 23 07:24:08 PDT 2013


On 4/23/13 4:08 AM, Dicebot wrote:
> You consider "scope" a new language feature?

Yes.

> And consider "auto ref"
> improvable solution.

Yes.

> 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.

>> 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.

>> 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 and on making ref safe.

>> 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.


Andrei


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