[variadic templates]feature request: n-th element of expansion

[Larry Evans]

On 11/19/09 17:23, Jason Merrill wrote:
> On 11/17/2009 09:36 AM, Larry Evans wrote:
>> Could g++ provide this feature? How hard would it be to implement.
> 
> It probably wouldn't be difficult to implement, but I'd want someone to 
> champion the extension with the C++ committee as well.  Have you asked 
> Doug Gregor what he thinks?  

Yes:

> Hi Doug,
> 
> Your post:
> 
>> http://groups.google.com/group/comp.std.c++/msg/40705c1e2a6f78f8
> 
> contains:
> 
>> > 3) A guaranteed non-recursive way to access elements of parameter
>> > packs
>> >  template<int N, class ... V> struct get_type
>> >  {
>> >     typedef V@N type;  // or implementation_defined<N,V...>::type -
>> > guaranteed linear
>> >  };
>> >  template<int N, class ... V> get_type<N,V...>::type get(V...v) {
>> >     return ::implementation_defined<N>(v...);
>> >  }
>>
>> This is probably the most-requested feature for variadic templates,
>> and it never it made it because we never found a good, unambiguous
>> syntax.
>>
> Could you elaborate on why it's hard to find some unambiguous syntax?
> For example, what would be wrong with the syntax:
> 
>>  get-nth-expansion-element:
>>     expansion-pattern '...[' constant-expression ']' 
> shown in the following post to gmane.comp.gcc.devel:
> 
>> http://thread.gmane.org/gmane.comp.gcc.devel/110252
> 
> TIA.
> 
> -regards,
> Larry
> 
> 
> 


> I assume that omitting this functionality 
> was deliberate.

As noted in my quote of Doug's post to comp.std.c++, there
*might* be something ambiguous about:

    expansion-pattern '...[' constant-expression ']'

I'm awaiting Doug's reply.

> 
> Jason
>