Re: [boost] [Proto] Computing type in expression

Subject: Re: [boost] [Proto] Computing type in expression
From: joel falcou (joel.falcou_at_[hidden])
Date: 2010-06-15 09:42:39

Eric Niebler wrote:
> Don't feel stupid; there are contexts in which you actually *can* use
> wrap in type computations within wrap itself, so it's easy to get confused.
>
>  template<class Expr>
>  struct wrap
>    : proto::extends<Expr, wrap<Expr> >
>  {
>    template<struct Sig>
>    struct result
>    {
>      typedef typename some_fun<wrap>::type type; //OK!
>    };
>  };
>
>   
I ended with that anyway as the result_type of a NT² expression depends 
on how you access them
as we support floating point indexing that perform linear interpolation, 
ranges and more. So in the
end , the geenral value_type was a bad idea whiel result_of compliance 
was OK.

> The difference is that by the time result gets instantiated somewhere
> else in your program, wrap will be complete. This will work fine until
> you try to instantiate result from within wrap itself (e.g. to compute
> the return type of a nullary operator() overload). Then it blows up with
> very mysterious errors. It's tricky
I saw that :€