On Thu, Feb 23, 2012 at 1:57 AM, Rob Desbois
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
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
::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#... 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