The liabilities of binding rvalues to ref

[Manu]

On 10 May 2013 05:39, Rob T <alanb at ucora.com> wrote:

> On Thursday, 9 May 2013 at 19:26:37 UTC, Jonathan M Davis wrote:
>
>> On Thursday, May 09, 2013 19:45:16 Peter Alexander wrote:
>>
>>> It seems that 'auto ref' would be suitable, provided we can find
>>> a way for it to work with normal functions (in a sensible way,
>>> not like templates).
>>>
>>
>> That's trivial enough. All you have to do is lower code like
>>
>> auto foo(auto ref int i) {...}
>>
>> foo(5);
>>
>> to something like
>>
>> auto foo(ref int i) {...}
>>
>> auto __temp = 5;
>> foo(__temp);
>>
>> And temporaries end up on the stack anyway, so you wouldn't really even
>> have
>> to lower it to quite like that, but that's what would be happening
>> conceptually. It's also what would happen if plain ref accepted rvalues.
>> It's
>> just that we avoid certain classes of issues by having something distinct
>> from
>> plain ref for accepting rvalues by ref. The implementation itself is
>> straightforward. If anything, I think that argument comes down primarily
>> to
>> two things:
>>
>> 1. Should plain ref accept rvlaues?
>>
>> 2. If plain ref shouldn't accept rvalues, then what attribute do we use to
>> accept rvalues by ref?
>>
>> And given that the whole point of adding auto ref to the language was to
>> solve
>> exactly this problem, I think that it makes perfect sense to just use auto
>> ref.
>>
>> - Jonathan M Davis
>>
>
> So, if I understand correctly, auto ref for templates will end up doing
> exactly the same thing as auto ref for non-template functions? That would
> be perfect, otherwise it'll be terribly confusing.
>

I don't think this is entirely true, auto ref is a template concept, that
is, "automatic ref-ness", it selects the ref-ness of the argument
automatically, at compile time, just like auto applied everywhere else
(selects a type for instance, at compile time). This concept doesn't make
any sense applied to a non-template. It *IS* a ref as specified by the
programmer, there's nothing 'automatic' about it.

So to say it will do 'exactly the same thing' is a misunderstanding. I
argue that 'auto ref' as applied to non-templates will only create
confusion, it effectively re-enforces the type of confusion that you have
just shown.

This is the reasoning for the argument behind scope ref, which to my mind
actually makes good sound sense, and should lead people to a proper
understanding of what you are actually doing.
Considering the key argument against 'scope ref' is that people don't want
to require more attributes to make use of it, I don't see how 'auto ref'
satisfies this argument either.
Thus, I am quite happy happy with 'ref', it can be made safe, satisfies the
argument above, and this seems like a very good start that we might
actually all agree on.
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