Call site 'ref'

[Timon Gehr]

On 01/16/2012 12:39 AM, Peter Alexander wrote:
> On 15/01/12 10:36 PM, Timon Gehr wrote:
>> On 01/15/2012 11:41 PM, Peter Alexander wrote:
>>> On 15/01/12 10:10 PM, Timon Gehr wrote:
>>>> Another thing that would have to be discussed: what happens to const
>>>> ref
>>>> parameters? It is very reasonable that someone will decide to change
>>>> calling conventions from by value to by const ref or the other way
>>>> round
>>>> after profiling. It is very convenient that such a change is
>>>> syntactically transparent to the caller and this should stay.
>>>
>>> Actually, that would be very dangerous. const ref does not have the
>>> privilege of being able to bind to rvalues in D like it does in C++.
>>>
>>> void foo(const ref int x) {...}
>>>
>>> foo(1); // this is legal in C++, but illegal in D
>>>
>>
>> Who on earth wants to change an int parameter from by value to by const
>> ref? struct literals can bind to const ref parameters so there is
>> absolutely no issue.
>
> Are you sure about struct literals binding to const ref? I can't find
> anything in the language spec or TDPL that supports that claim.
>
> In fact, TDPL says:
>
> "If a function expects a ref, it accepts only "real" data, not
> temporaries; anything that's not an lvalue is rejected during compilation"
>

Well, struct literals are lvalues, at least in DMD.

> Unfortunately it doesn't say anything explicitly about struct literals
> or const ref, but that would lead me to believe that they fall under the
> same rule.
>
> DMD allows struct literals to bind to const ref, but then again it also
> allows them to bind to ref, so I'm not sure how reliable that is.
>
> This should probably be clarified. It's kind of important.

I agree, but banning it would be an arbitrary and pointless restriction.