Angus Leeming writes:

Consider a container such as std::vector > my_container;

I want to be able to iterate over the ints of this container so, following the tutorial to the iterator_adaptor docs, came up with

template <typename BaseIterator> class first_iter : public boost::iterator_adaptor< first_iter<BaseIterator>, BaseIterator, boost::use_default, boost::use_default, typename BaseIterator::value_type::first_type & > { // snip body };

Why struggle? boost::transform_iterator should give you just what you need: struct select1st { template <class P> P::first_type& operator()(P& pair) { return pair.first; } template <class P> P::first_type const& operator()(P const& pair) { return pair.first; } }; namespace boost { template <class T> struct result_of { typedef P::first_type& type; }; template <class T> struct result_of { typedef P::first_type const& type; }; } transform_iterator

This works fine so long as BaseIterator::value_type is not const, but falls over when it is because I've defined the 'super_t::reference' type to be mutable.

I guess that I need a piece of metafunction magic, but my naive implementation fails

typename boost::mpl::apply_if< boost::is_const<typename BaseIterator::value_type> , typename BaseIterator::value_type::first_type const & , typename BaseIterator::value_type::first_type & >::type

Try plain mpl::if_. apply_if invokes one of two nullary metafunctions, but a (const) int& is not a metafunction.

trial.C:38: error: no type named `type' in `struct boost::mpl::apply_if< boost::is_const >, const int&, int&>'

I must admit I'm a bit baffled. There plainly is a 'type' in the apply_if class template.

Nope: template struct apply_if : typename if::type {};

Any pointers? I attach the code as I have it.

While I'm at it, is there not something in the MPL that will return the appropriate const-qualified reference for me?

What do you mean?

None of the header file names in boost/mpl jump out and grab me, but I may well be looking in the wrong place.

-- Dave Abrahams Boost Consulting http://www.boost-consulting.com