[Boost-users] Re: Converting from const to non-const with iterator_adaptor

6 Jan 2004

      That's just what I needed!  It makes much more sense now that you have
explained that iterator_adaptor adapts a base class, rather than an
iterator, as originally documented.  The concept is amazingly powerful, and
now that I've cleared the first hurdle, I'm much more inclined to explore
other uses.

I've adapted your example to implement begin() and end() methods, with a
little test, and have attached the code.  I hope you can include such an
example with the library.

Thanks again for your help.

Keith MacDonald

"David Abrahams"  wrote in message
news:uwu85i0my.fsf@boost-consulting.com...
...
"Keith MacDonald"  writes:
...
You have proposed a very elegant solution, but I would defy anyone who
does
not understand the library inside out to arrive at that solution. The
documentation (iterator/doc/index.html#iterator-facade-and-adaptor)
states:
We're working on the docs.  That document is the standards proposal,
but the Boost (user-level) docs aren't complete yet.
...
"It is common to define a new iterator which behaves like another
iterator,
but which modifies some aspect of its behavior. For that purpose, the
library supplies the iterator_adaptor class template, which is specially
designed to take advantage of as much of the underlying iterator's
behavior
as possible."
In this case, I can't see what the underlying iterator is, so don't
understand how iteration can proceed.
I quote:
The iterator_adaptor class template adapts some Base 3 type to
  create a new iterator. Instantiations of iterator_adaptor are
  derived from a corresponding instantiation of iterator_facade and
  implement the core behaviors in terms of the Base type. In essence,
  iterator_adaptor merely forwards all operations to an instance of
  the Base type, which it stores as a member.
[3] The term "Base" here does not refer to a base class and is not
  meant to imply the use of derivation. We have followed the lead of
  the standard library, which provides a base() function to access the
  underlying iterator object of a reverse_iterator adaptor.
The user of iterator_adaptor creates a class derived from an
  instantiation of iterator_adaptor and then selectively redefines
  some of the core member functions described in the table above. The
  Base type need not meet the full requirements for an iterator. It
  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  need only support the operations used by the core interface
  functions of iterator_adaptor that have not been redefined in the
  user's derived class.
...
For example, if you add the statement
"++x;" to your example, it fails to compile.
Umm, sorry.  I whipped that one off without too much testing. I've
enclosed a fixed one.
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...
...
To return to my point about the purpose of a software library, my
approach is generally to ask if it will allow me to implement code
more quickly and reliably than without it.  I got to the point of
thinking, I've now got to implement a straightforward iterator,
sigh, so will the boost library help me?  I didn't expect to have to
develop a deep understanding of template metaprogramming and the
curiously recurring template pattern, before I could move on.
Why would you need to develop either of those?  You just follow the
formula.
...
Hence my plea for an example of a plain vanilla iterator, by which
I meant something similar to those used for the std library
containers.
I'm sorry, I need you to be more specific.  Is the example of your
node iterator good enough?
-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com
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