On Thu, Feb 23, 2012 at 1:57 AM, Rob Desbois <rob.desbois@gmail.com> wrote:

I have the following iterator_adaptor type:

template< class NODE >
struct node_iterator: public boost::iterator_adaptor<
node_iterator<NODE>, // Derived
NODE*, // Base
typename NODE::value_type // Value
>
{
typename NODE::value_type dereference() const {
return this->base_reference()->data();
}
};

// and later:
typedef detail::node_iterator<node_type> iterator;
typedef detail::node_iterator<const node_type> const_iterator;

This throws up the issue that in const_iterator, Value is not const,
producing the following error:
/usr/include/boost/iterator/iterator_facade.hpp:517:32: error:
invalid initialization of non-const reference of type
‘container::detail::node_iterator<const container::detail::node<long
unsigned int> >::reference {aka long unsigned int&}’ from an rvalue of
type ‘container::detail::node<long unsigned int>::value_type {aka long
unsigned int}’

I could do some type traits magic with is_same, is_const and add_const
to add the constness to Value if present on NODE, but is this the
right way to do it?

To reiterate Robert Jones' reply, yes, AFAIK, a const iterator should pass a const-qualified value_type to iterator_facade/iterator_adaptor, as implied by the constant node_iterator example in the docs:

http://www.boost.org/doc/libs/1_48_0/libs/iterator/doc/iterator_facade.html#a-constant-node-iterator

On a related note, you should specify your reference type you pass to iterator_adaptor explicitly as the return type of your dereference member function. Otherwise I think you'll end up returning a reference to a temporary within operator*. I believe that's *actually* what the quoted error is trying to tell you.

HTH,

- Jeff