Re: [Boost-users] iterator_adaptor: making dereference of adapted tree node* return node value

On 23 February 2012 14:39, Robert Jones wrote:

I favoured the type traits magic approach, and publish the resulting value type as the value type of the iterator...

typedef typename if_::type value_type;

Thanks for the input Rob - see below for the solution I settled on. On 23 February 2012 18:58, Jeffrey Lee Hellrung, Jr. wrote:

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#...

Actually iterator_facade/iterator_adaptor's value type in the tutorials is the _node_ type, not the node's value_type.

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.

Thanks - I've updated to use node_iterator::iterator_adaptor_::reference as the return type from dereference(). The docs show that this defaults to Value&. The error message still stands though, since const node_type::value_type is still not const. I settled on implementing the boost::mpl::if_ method as shown by Rob, which turned out to be a viable solution to the problem I faced, but then after some more thinking and coding realised that I didn't actually want the mutable iterator type to enable modification of the data in the referenced node. Since this is an ordered tree, modifying the data of a node is clearly a Bad Thing! Feeling somewhat silly that I didn't recognise this before leaping down the rabbit hole...what I now have is that Value is *always* const: template< class NODE > node_iterator: public boost::iterator_adaptor< node_iterator<NODE>, // Derived NODE*, // Base const typename NODE::value_type // Value

The reference type is then const typename NODE::value_type& as expected and everything works fine. It was only after discovering that std::set and friends' iterators can be the same type as the const_iterator that this finally clicked! Thanks for the guidance, nice to know I wasn't going completely off my rocker with the mpl approach :-D --rob