Const ref and rvalues again...

[Timon Gehr]

On 10/19/2012 03:26 AM, Malte Skarupke wrote:
> On Friday, 19 October 2012 at 00:03:49 UTC, Timon Gehr wrote:
>>
>> Const is different in D and in C++. Relating const and rvalues is
>> arbitrary and does not make a lot of sense.
>>
>> Regarding 'in ref'/'scope ref': What should 'scope' apply to in
>>
>> void foo(scope ref int* x);
>
> Not sure what you mean with "relating." I'm not making any claims about
> there being a relationship between rvalues and constness.
>

You do it again right away:

> This is about finding a way that you can define a function which safely
> accepts lvalues and rvalues without having to make a copy. If we specify
> the argument as "ref in", then we can safely pass for example the number
                   ^~~~~~~~
                     const
> 5 to it.
   ^
rvalue

> And this would never break existing code, so that something
> like swap(5, 4) would never be possible code.
>

That does not break existing code except code that checks validity of
code, but every language change does that.

> For the example that you gave you'd be unable to store the address of x.
> So doing
>
> int** storage;
> void foo(scope ref int * x)
> {
>      storage = &x;
> }
>
> would be illegal.
>

Then how to specify that the value of x cannot be escaped?
I'm in favour of doing it the other way round and disallow escaping of
ref parameters without an unsafe cast.

> @jerro: the same thing: I'm not trying to fix the problem that you
> mention. I'm trying to define a function which can safely accept rvalues
> and lvalues without having to make a copy.