[Boost-users] Re: iterator_adaptor: disguise iterator as const_iterator?

From: David Abrahams (dave_at_[hidden])
Date: 2004-12-02 13:30:19

Martin Wartens wrote:
 > Hi,
 >
 > I came up with a solution that seems to work. It is as simple as 
adding "const"
 > to the Value template parameter of the iterator_adapter.

Yes, I could (and should) have told you that would work.  But it's not
a good answer.  The library really ought to do the right thing for
this sort of iterator.

 > Then I can have a non- const iterator that always returns const when
 > dereferenced. Unfortunately, the iterator_adapter class has a public
 > member function "base" that returns the underlying iterator.  Since
 > I want to prevent public access to the underlying operator, I had to
 > tweak iterator_adapter and make this function protected.

The right answer for that case is not to advertise the function to
your clients.  If they don't know it's there, they can't call it.

We could consider making it a free function in the library, but I
don't see how that could be any more "hidden."  Your FriendHelper is
just as publicly accessible as the base() member function is today.

You can also make it private in your derived class, or hide it with
another member named "base."  That would make it inaccessible unless
your clients actually cast your iterator to its base class.

 > Then I want to give access to the base function only to the
 > container adaptor (a map adaptor in my example). Providing the
 > friend access in a generic way is a little bit awkward, as it has to
 > be done through a helper class. Some sample code for this is given
 > below. I have not yet fully explored this solution, I hope I won't
 > face any nasty surprises.

--
Dave Abrahams
Boost Consulting
http://www.boost-consulting.com