Re: [Boost-users] filter_iterator and bind

From: David Abrahams (dave_at_[hidden])
Date: 2006-02-22 10:35:07

Nico Galoppo <nico_at_[hidden]> writes:

> Thanks, that works like a charm. Unfortunately, dereferencing the created 
> iterator gives more trouble:
>
>    boost::function<bool (shared_ptr<node_type const>)> is_constraint
>      = boost::bind(&node_type::IsConstraint, _1);
>    typedef boost::filter_iterator<boost::function<bool (shared_ptr<node_type 
> const>)>, articulated_model_type::node_df_traversal_iterator> joint_node_iterator;
>
>    joint_node_iterator jointnode
>      = boost::make_filter_iterator(is_constraint, ab.begin(), ab.end());
>
>    std::cout << jointnode->Model();   ///< this line fails to compile
>
> There seems to be a problem with const-ness (see below).

It's not a constness problem, exactly.  The problem is that the result
of 

   *ab.begin()

is an rvalue (i.e. returned by value, not by reference).  Yet the
filter_iterator thinks it is iterating over lvalues: its reference
type is Hybrid::Graph<node_traits>::const_node_ptr&, and the compiler
is complaining that you can't initialize such a reference with a
non-lvalue.  It's hard to come up with a complete analysis with the
little information I have here, but I believe

  Hybrid::Graph<Hybrid::ArticulatedModelNodeTraits<MyTypes> >::node_df_traversal_iterator 

is misreporting its reference type or its category.  An iterator whose
operator* returns by value cannot be a forward iterator; it can only
be an input iterator.

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