Re: [boost] [review] Fusion review begins:extension.htm:why iterator param Struct?

From: dan marsden (danmarsden_at_[hidden])
Date: 2006-05-06 18:11:58

Larry Evans wrote:
>In extension.htm, there's a section:
>
>   Designing a suitable iterator
>
>which has code:
>
>template<typename Struct, int Pos>
>struct example_struct_iterator
>     : iterator_base<example_struct_iterator<Struct, Pos> >
>{
>     BOOST_STATIC_ASSERT(Pos >=0 && Pos < 3);
>     typedef Struct struct_type;
>     typedef mpl::int_<Pos> index;
>     typedef example_struct_iterator_tag ftag;
>     typedef random_access_traversal_tag category;
>
>     example_struct_iterator(Struct& str)
>         : struct_(str) {}
>
>     Struct& struct_;
>};
>
>However, there's no declaration of example_struct_iterator_tag.  I
>assume it can just be declared (no need to define it) like
>the example_sequence_tag occuring just below the section titled:

There's going to be a couple of changes in this example. You are correct
though that a forward declaration of example_struct_iterator_tag is
needed, if only to make the docs clearer.

>
>    Enabling Tag Dispatching
>
>Actually, I think that's another typo,  IOW instead of
>example_sequence_tag, it should be example_tag.  But that
>makes me wonder if you didn't meand that
>example_struct_iterator_tag should also be example_tag.

The correct structure should be that the tag_of the example_struct
should be example_sequence_tag, and the tag_of the iterator
should be example_struct_iterator_tag. (There should be 2 distinct
tags). This should be tidied up and consistent in the next version of
the docs.

>Also, since the above code is all for a specific structure, i.e.
>example_struct, why is there any need for including a Struct
>template parameter.  Why not just:
>
>   template<int Pos>
>   struct example_struct_iterator
>       : iterator_base<example_struct_iterator<example_struct, Pos> >
>   {
>       BOOST_STATIC_ASSERT(Pos >=0 && Pos < 3);
>       typedef example_struct struct_type;
>   ...
>   };

The Struct parameter allows us to capture whether the iterator points into
an example_struct or a const example_struct. Some of the rest of the example
code needs to make this distinction.

Thanks for the feedback
Cheers
Dan