Re: [boost] [fusion] transform into vector

From: Joel de Guzman (joel_at_[hidden])
Date: 2007-11-22 19:33:20

Joel de Guzman wrote:
> shunsuke wrote:

>> I might probably understand this problem.
>>
>> transform_view, in order to implement `deref` etc, uses `result_of`
>> in the standard manner, where `result<F(int)> means that rvalue is passed.
> 
> Correction: result<F(int&)> which means lvalue is passed.
> The lvalue is correctly propagated through the transform.
> 
>> But, as_vector applied to transform_view uses `result_of` in a strange manner
>> through value_of.
>>
>> A FunctionObject for transform_view requires two result_of implementations;
>> It seems impossible.
> 
> I provided an example, didn't I? Actually, there's another way to do
> it -- take a look at how fusion zip does it.

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;
     }
};

That would add another requirement to the PolymorphicFunctionObject
concept though and is not compatible with prior schemes.

Regards,
-- 
Joel de Guzman
http://www.boost-consulting.com
http://spirit.sf.net