Re: [boost] [fusion] transform into vector

From: shunsuke (pstade.mb_at_[hidden])
Date: 2007-11-22 20:35:09

Joel de Guzman wrote:
> Ok, tell you what: I understand the confusion. If you or anyone else
> can find a way to do value_at that passes the exact underlying element
> (undecorated) type, through the transform function, then, I'd gladly
> rethink this matter. Here's a possibility:
> 
> struct identity
> {
>      template <class FunCall>
>      struct result;
> 
>      template <class Fun, class T>
>      struct result<Fun(T&)>
>      {
>          typedef T& type;
>      };
> 
>      template <class T>
>      struct value
>      {
>          typedef T& type;
>      };
> 
>      template <class T>
>      T& operator()(T& v) const
>      {
>          return v;
>      }
> };

I think this is not a good solution.

If we are following Boost.Iterator's way,
"value concept" should be placed in not FunctionObject but in transform_view.
Because boost::transform_iterator can't always determine
its `value_type` or `reference` from FunctionObject,
you can pass explicitly `value_type` and `reference` type to boost::transform_iterator.

Say,

template<class Seq, class F, class ResultOfDeref = use_default, class ValueOf = use_default>
struct transform_view;

seems better.

Regards,

-- 
Shunsuke Sogame