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

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
iterator.

> 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
'boost::iterator_facade<Derived,Value,CategoryOrTraversal,Reference,Difference>::reference'
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>
  {
      typedef 
         boost::iterator_adaptor<iter<V>, V*, V, boost::bidirectional_traversal_tag> 
      super;

      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;

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

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

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

HTH,

-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com