DIP 36: Rvalue References

[Andrei Alexandrescu]

On 4/23/13 12:04 PM, Manu wrote:
> On 24 April 2013 00:24, Andrei Alexandrescu
> <SeeWebsiteForEmail at erdani.org <mailto: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.

The best setup would be:

1. To take lvalues by reference, write "ref".

2. To take lvalues and rvalues by reference, write "auto ref".

Everything else is superfluous and puts the burden of justification on 
the proposer. With DIP36, the setup would be:

1. To take lvalues by reference, write "ref".

2. To take lvalues and rvalues by reference:

2.1. Is it a template? Then write "auto ref".

2.2. Is it a non-template? Then write "scope ref".

> 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.

I don't know how to respond to this. To me is it painfully obvious DIP 
36 is poor language design and fails to solve a variety of issues, such 
as clarifying lifetime of temporaries, safety, and returning ref from 
functions.

>             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.

Currently rvalues are destroyed immediately after the call they are 
passed into. DIP 36 would need to change that, but fails to specify it.

>             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?

http://d.puremagic.com/issues/show_bug.cgi?id=9238

> 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.

Our intent is to make "ref" always scoped and reserve non-scoped uses to 
pointers. We consider this good language design: we have unrestricted 
pointers for code that doesn't care much about safety, and we have "ref" 
which is almost as powerful but sacrifices a teeny bit of that power for 
the sake of guaranteed safety. Safety is guaranteed by making sure "ref" 
is always scoped (references can be passed down but never escape their 
bound value).

>             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.

I assumed it was obvious from what I'd already written. The claim has 
been made that "scope ref" is not a new language feature. I don't know 
how to reply to that other than, well, it is.


Andrei