Re: [Boost-users] [Fusion] Output of adapted ADT

Subject: Re: [Boost-users] [Fusion] Output of adapted ADT
From: Joel de Guzman (joel_at_[hidden])
Date: 2011-04-10 19:53:36

On 4/9/2011 3:39 PM, TONGARI wrote:
> 2011/4/9 Joel de Guzman <joel_at_[hidden] <mailto:joel_at_[hidden]>>
>
>     Adapted classes are different beasts. Adapted classes may not
>     expose their attributes by reference due to abstraction and
>     information hiding. It's not uncommon for a class not to actually
>     have an actual member variable. The best we can assume is that a
>     class has a getter and setter member functions. When you access
>     its member (e.g. using at_c or dereferencing an iterator), here's
>     what happens:
>
>     1) If the adapted class is immutable, it returns the class' adapted
>        get member function for the member.
>
>        E.g. T get_mem() const;
>
>
> Really? Right after seeing your words I almost came up with a workaround...
> However, it turns out that it's either me misunderstanding you or it misbehaving.
>
> For example,
>      const foo f("foo"); // immutable, right?
>      boost::fusion::front(f); // we still get a proxy, why?

You'll have to forgive me. Christophe just pointed out off-list that
this is no longer the behavior for the sake of consistency. Here's
the relevant discussion:

Stefan Strasser proposed changes to the adapt class/adt interface:

,---- [(http://lists.boost.org/Archives/boost/2010/08/169996.php)]
|Also very good points. I think the proxy should behave similar to Boost
|ref, as it should expose an implicit conversion to T. This alone will
|satisfy many code that expects T. As for exposing the proxy as the
|value type of the sequence, you are right. However, what we can
|probably do is to be consistent with handling const and non-const and
|always return a proxy so there is no surprise when code works one way
|and not the other way.
`----

Pardon the noise. I think the changes all make sense.

> Do you have any suggestion for using adapted class with Spirit that the OP wants to do?

Well, given the current API, you can write:

   std::cout << boost::fusion::front(f).get() << std::endl;

See:

   [http://tinyurl.com/6aq97lj]
 
http://www.boost.org/doc/libs/1_46_1/libs/fusion/doc/html/fusion/notes.html#fusion.notes.adt_attribute_proxy

Regards,
-- 
Joel de Guzman
http://www.boostpro.com
http://boost-spirit.com