Re: [boost] [type_at] Variadic template utility

Subject: Re: [boost] [type_at] Variadic template utility
From: tymofey (tymofey_at_[hidden])
Date: 2011-05-26 07:41:57

>Larry Evans <cppljevans_at_[hidden]>
> On 05/26/11 04:47, tymofey wrote:
> >> Robert Jones <robertgbjones_at_[hidden]>
> >> On Thu, May 26, 2011 at 8:34 AM, tymofey <tymofey_at_[hidden]> wrote:
> >>
> >>> While experimenting with varidic templates of the upcoming standard i
> >>> discovered that quite often one would need to extract a type from parameter
> >>> pack at specified position. 
> [snip]
> >> Forgive me if I'm missing the point, but isn't this essentially Boost.MPL's
> >> 'at' metafunction?
> >>
> >> - Rob.
> > 
> > It is, but without the need to drag in mpl::vector with the limitations it imposes.
> > 
> Could you please describe those limitations, other than the one for
> maximum arity, which, IIUC, is modifiable by redefining the macro:
> 
>   BOOST_MPL_LIMIT_VECTOR_SIZE
> 
> ?  Also, IIUC, one reason for using the macro, and the preprocessor,
> is to speed compile time by reducing the number of template
> instantiations. Thus, maybe one advantage of mpl::at vs your type_at
> is compile time. You might check that somehow.
> 
> -regards,
> Larry

The size of the vector is the only limitation i'm know of. And I wasn't aware that one can change it thus making my proposal pretty usless. Thanks for the info.