rvalue references

[Manu]

On 24 April 2013 03:15, Andrei Alexandrescu
<SeeWebsiteForEmail at erdani.org>wrote:

> On 4/23/13 11:27 AM, Manu wrote:
>
>> On 24 April 2013 00:30, Andrei Alexandrescu
>> <SeeWebsiteForEmail at erdani.org <mailto:SeeWebsiteForEmail@**erdani.org<SeeWebsiteForEmail at erdani.org>
>> >>
>>
>> wrote:
>>
>>     On 4/23/13 10:05 AM, Manu wrote:
>>
>>         I can't see the fault in DIP36's reasoning. It just makes sense.
>>         Why is
>>         everyone so against it? I'm yet to understand a reason...
>>
>>
>>     1. It defines a new language feature instead of improving the
>>     existing ones. At this point in the development of the language, our
>>     preference should be putting the existing features in good order.
>>
>>
>> I see it in exactly the opposite way.
>> This does put an existing feature in good order, ie, scope, which is
>> defined but barely implemented, and might as well have been invented for
>> this purpose as far as I can tell from what little information is
>> available about it.
>>
>
> "scope" is a keyword, not a language feature. In case you are referring to
> scope variables, the feature "scope ref" has little to do with it.
>

How so? 'scope' simply promises that a variable may not escape its scope,
no?
I think it's important to recognise it as 'scope' + 'ref', the 2 don't have
any special meaning when put together, just the logical compound, which
allows for a safe situation for temporaries that wasn't previously
available.

     2. The proposal is sketchy and does not give many details, such as
>>     the lifetime of temporaries bound to scope ref objects.
>>
>>
>> Is that the only detail missing?
>>
>
> Many details are missing. This is not a simple problem.
>

So what are some others?

 An r-value passed this way produces a
>> temp, which is a stack variable. It's life is identical to any other
>>
>> stack variable, ie, it lives for the life of the function where it
>> appears.
>>
>
> That's a possibility, but it's a departure from current semantics and is
> not mentioned in the DIP.
>

I think it's presumed in the DIP, and it's certainly how Kenji implemented
it.
What 'current' semantic is it a departure from? The one where passing a
literal produces a compile error? Certainly, that's the point.

 auto-ref on the other hand IS a new feature (in this context), and it
>> also makes no sense if you ask me. It's a template concept which is not
>> applicable here.
>>
>
> It is a feature that has been implemented and works, just not in all cases.
>

This isn't a 'case'. It's a separate issue.
Safely passing a temp to a ref function arg, and whether a template
argument is automatically determined to be ref or not are barely related
problems.
I still can't see how auto-ref has any business in this context.

     In particular we are much more inclined to impart real, demonstrable
>>     safety to "ref"
>>
>>
>> ref is unsafe by definition.
>>
>
> We want to aim at making ref safe, thus making it useful as restricted
> pass-down pointers. For full possibilities, one should use pointers.


Okay, I'm good with that too, but how is that intended to work?
If the intent is to make ref escaping disallowed by default, that is a
major breaking change...
Can we start talking about virtual-by-default again while we're at it?

 I don't believe this is possible without
>> some further justification.
>>
>
> The justification is that unsafe uses of ref are few and uninteresting
> (they can be replaced with pointers). It would be very powerful to be able
> to guarantee that safe code can use ref.
>

Again, this sounds like a major breaking change.
Why is scope-ref inferior? It's more informative, and offers more
flexibility (ie, the option of ref with or without scope)

 DIP36 however creates a situation where it's known that passing a temp
>> is actually safe.
>>
>>     and to make "auto ref" work as a reference that can bind to rvalues
>>     as well as lvalues.
>>
>>
>> What does it mean to make a reference bind to r-values aswell as
>> l-values? Lots of people keep saying this too, but it doesn't really
>> make sense to me either.
>>
>
> I don't understand the question as the answer is in it.
>
>
>  No reference can bind to r-values, r-values can not be addressed.
>>
>
> But auto ref and scope ref do bind to r-values.
>
>
>  It's
>> really a temp copy of said r-value that we're dealing with, which is an
>> l-value, ie, a local with a lifetime that's unsuitable for passing by
>> non-scope-ref.
>> scope-ref would promise that it won't escape the callee, and thus is
>> safe to pass a temp.
>>
>
> Our aim is to have ref make that promise.
>
>
>  ref is fundamentally broken in D right now. DIP36 creates a situation
>> where it could be fixed.
>>
>
> A new feature is not a fix.


If scope is a new feature, then the keyword shouldn't compile and pretend
that it does stuff.
It's an incomplete/unimplemented feature, not a new one.
People are aware of it, they can write code that presumes it's present and
working. It compiles successfully.

 I would personally take DIP36 one step further,
>> and ban all local's from being passed to non-scope ref.
>> Yes, a breaking change, but you could argue that any code that passes a
>> stack variable to any ref arg is already broken. But this can be
>> addressed in a future DIP.
>>
>>
>> ...perhaps I'm missing something fundamental in DIP36, or about 'auto
>> ref'?
>> I can't understand why there seem to be 2 polarised parties on this
>> issue, which appear to see the problem completely differently, and can't
>> visualise the counter perspective at all.
>>
>
> DIP36 should be closed. We must focus on making ref safe and on making
> auto ref work with non-templates.


I'm fine with that, but it sounds like a massive breaking change.
However upon the presumption of this new goal, I don't see the relevance of
auto-ref anymore? Why continue to bring it up?
If ref is safe, nothing else is needed.
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