'Auto can only be used for template function arguments' what?

Artur Skawina art.08.09 at gmail.com
Wed Jun 27 09:04:04 PDT 2012

On 06/27/12 17:38, Jonathan M Davis wrote:
> On Wednesday, June 27, 2012 13:33:10 Timon Gehr wrote:
>> On 06/27/2012 01:12 PM, kenji hara wrote:
>>> 2012/6/27 Timon Gehr<timon.gehr at gmx.ch>:
>>>> On 06/27/2012 07:27 AM, kenji hara wrote:
>>>>> 2012/6/27 Jonathan M Davis<jmdavisProg at gmx.com>:
>>>>>> On Wednesday, June 27, 2012 07:02:28 Mehrdad wrote:
>>>>>>> On Wednesday, 27 June 2012 at 04:26:08 UTC, Jonathan M Davis
>>>>>>> wrote:
>>>>>>>> And how would that work? auto ref duplicates the function. If
>>>>>>>> you pass it an lvalue, then it generates a ref version.
>>>>>>> Two solutions:
>>>>>>> - Turn 'auto ref' into 'ref', but simply have the compiler
>>>>>>> generate a copy for the caller if he wants to pass by value?
>>>>>> That at least sounds like it would work. Walter may have a reason why
>>>>>> it
>>>>>> doesn't though, since he's the one that said that he didn't think that
>>>>>> it
>>>>>> was
>>>>>> possible. Maybe he just didn't think of it, or maybe it causes some
>>>>>> other
>>>>>> problem that I can't think of.
>>>>> After considering about 'auto ref', I was concluded that is an
>>>>> inconsistency of current language spec and we cannot fix it correctly.
>>>>> http://d.puremagic.com/issues/show_bug.cgi?id=8204
>>>>> Therefore, I have created a thread to suggest new 'auto ref' recently.
>>>>> http://forum.dlang.org/thread/CAFDvkcvf6G8Mc01Tds6ydXqCZbfp1q-a-oeFVk6BG
>>>>> EtwCiUAqg at mail.gmail.com
>>>>> Kenji Hara
>>>> How does it deal with this case?
>>>> auto ref id(auto ref arg){ return arg; }
>>> In this case, arg might bind rvalue allocated in the caller of id, so
>>> returning it with ref should be rejected statically.
>>> But it is yet not implemented in pull#1019.
>>> Kenji Hara
>> This is a quite severe limitation compared to how auto ref works today.
>> As the return ref-ness is inferred and therefore the source code must
>> be available, wouldn't it be a better option to decide
>> lvalue/rvalue-ness at the call site? The compiler would just have to
>> examine the first return statement in the function body.
> Considering that if you said that a parameter is auto ref, you're basically 
> saying that it's being passed by value except that you'd like the compiler to 
> avoid the copy if it can, I don't see why it's restrictive at all to make 

That's not what auto-ref does. We already had this discussion. auto-ref lets
you eg wrap other ref-taking functions; it matters for things like templates
and variadics, where ParameterTypeTuple can't be used. You can also check if
an argument was passed by reference with traits(isRef) and handle it differently.


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