Re: [boost] A C++ puzzler

From: Matus Chochlik (chochlik_at_[hidden])
Date: 2008-04-12 06:41:19

Hey,

my implementation is in the attachment. It's rather ugly ;) so it's probably
not what You had in mind, but AFAIK it works.

the common_type<...> template is declared only for three args, but with a
little help from boost/preprocessor it could be
generalized to N params. Unfortunatelly i don't have the time to play with
that right now :-P.

Nice puzzler though ;)

Best,
On Sat, Apr 12, 2008 at 12:15 PM, JOAQUIN M. LOPEZ MUÑOZ <joaquin_at_[hidden]>
wrote:

>  ________________________________________
> De: boost-bounces_at_[hidden] [boost-bounces_at_[hidden]] En
> nombre de Eric Niebler [eric_at_[hidden]]
> Enviado el: sábado, 12 de abril de 2008 9:28
> Para: Boost mailing list
> Asunto: [boost] A C++ puzzler
>
> > I recently had a C++ problem and found an answer that tickled my brain.
> > In the spirit of Car Talk on NPR, I thought I'd share it in the form of
> > a puzzler.
> [...]
>
> Hello Eric, I am not sure if this is what you have in mind, since from
> your description
> is not clear how the input types are represented: I've assumed they're
> just the
> args of a class template with a fixed number of template parameters:
>
>  struct default_t{};
>
>  template<typename T>
>  struct holder_base{typedef T type;};
>
>  template<>
>  struct holder_base<default_t>{};
>
>  template<typename T,int>
>  struct holder:virtual holder_base<T>{};
>
>  template<typename T1,typename T2,typename T3,typename T4,typename T5>
>  struct common_type_base:
>    holder<T1,1>,holder<T2,2>,holder<T3,3>,holder<T4,4>,holder<T5,5>{};
>
>  template<typename T1,typename T2,typename T3,typename T4,typename T5>
>  struct common_type:common_type_base<T1,T2,T3,T4,T5>
>  {
>    typedef typename common_type_base<T1,T2,T3,T4,T5>::type type;
>  };
>
>  template<>
>  struct common_type<default_t,default_t,default_t,default_t,default_t>
>  {
>    typedef default_t type;
>  };
>
> Best,
>
> Joaquín M López Muñoz
> Telefónica, Investigación y Desarrollo
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-- 
________________
::matus_chochlik