DIP 36: Rvalue References

Tue Apr 23 11:29:07 PDT 2013

On 24 April 2013 03:33, Andrei Alexandrescu
<SeeWebsiteForEmail at erdani.org>wrote:

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

That's not a good setup at all. It still doesn't make sense.
There's nothing 'automatic' about it, I've specified ref, it is ref,
there's no other choice.
And it relies on a major breaking change to ref, which restricts the
functionality of ref by default.

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".

Stop talking about r-values, rather, consider safety of passing
temporaries. This problem has nothing to do with r-values, this is equally
unsafe:
  void f(ref int x);
  int x;
  f(x);

It's precisely the same problem, and should be fixed with the same solution.

I would rewrite your list as such:

1. 'un-safely' pass a value (may not be a local), write 'ref'
2. safely pass a value (may be a local), write 'scope ref'
3. if you are concerned with templates do you:
3.a. want unsafe auto-ref, type 'auto ref' (only non-locals would generate
'ref')
3.b. want safe auto-ref, type 'scope auto ref'

The automatic selection of ref for templates is fairly unrelated to this
issue.

 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.
>

Lifetime of temporaries is the most basic of principles. A local lives the
life of the function in which it is defined.
Safety is the whole point, and intrinsic to the proposal; safety by
explicit specification, thus the programmer retains the option.

If a function receives 'scope ref', and wants to return it, I think it
should also return 'scope ref' (I guess this wasn't defined in the DIP),
otherwise it would be considered an escape.

             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.
>

Again, I think it was presumed (I can't conceive any other approach), and
certainly Kenji read it that way, because his code appears to do just that.

             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>
>>
>>     <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<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).
>

That's a massive breaking change...

[response in other thread, we need to stop repeating in 2 threads I think]
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.puremagic.com/pipermail/digitalmars-d/attachments/20130424/66fcc9c4/attachment-0001.html>