Boost logo

Boost Users :

From: David Abrahams (dave_at_[hidden])
Date: 2004-01-04 13:34:58

"Keith MacDonald" <boost_at_[hidden]> writes:

> Hi,
> I'm stuck trying to use iterator_adaptor from the CVS head. I simply want
> to derive a non-const iterator from a const one derived from
> iterator_facade, as shown in the code below.

Answer: don't do that. Making iterators by derivation from other
iterators is almost always an error (and in your case also: for
example the return type of operator++ is wrong on the derived
iterator). Make a new constant iterator type with iterator_facade.
If you do it right, it will interoperate properly with your mutable

> When I compile it with VC7.1,
> the line marked "/* *ERROR* */" generates one of those impenetratable error
> messages that starts with:
> cannot convert from 'const Node' to
> 'boost::iterator_facade<Derived,Value,CategoryOrTraversal,Reference,Differen
> ce>::reference'

Compile with GCC; you'll get better error messages. The type
is almost certainly 'Node&', which can't be bound to a 'const Node'.

> If I delete that line, so the default is used, dereferencing a non-const
> iterator returns a const Node. This seems such a simple requirement that
> there must be an easy way to do it. What am I doing wrong?

Looks like a job for iterator_adaptor:

  // untested
  template <class V>
  struct iter
    : boost::iterator_adaptor<iter<V>, V*, V, boost::bidirectional_traversal_tag>
         boost::iterator_adaptor<iter<V>, V*, V, boost::bidirectional_traversal_tag>

      iter() {}

      template <class V2>
      iter(iter<V2> x, boost::enable_if_convertible<V2*,V*>::type* = 0)
        : super(x.base())

     friend class iterator;
     friend class boost::iterator_core_access;

    void increment()
     { base() = base()->next(); }

    void decrement()
     { base() = base()->prev(); }

  typedef iter<Node> iterator;
  typedef iter<Node const> const_iterator;


Dave Abrahams
Boost Consulting

Boost-users list run by williamkempf at, kalb at, bjorn.karlsson at, gregod at, wekempf at