indirect_iterator for map<T, shared_ptr<U>>

Question

I'm trying to implement an iterator of map<T, shared_ptr<U>> that "removes" shared_ptr, i.e. dereferences to pair<T, U&>. I need this to adapt my wrapper over map to range-based for loop.

boost::indirect_iterator seems like something designed for such a need. I tried this:

typedef std::map<int, std::shared_ptr<int>> M;
typedef boost::indirect_iterator<M::iterator, std::pair<int, int>> It;

This doesn't work because of indirect_iterator requirement:

The expression *v, where v is an object of iterator_traits::value_type, shall be valid expression and convertible to reference.

Is it possible to use boost::indirect_iterator for this task? If no, can I do this with boost::iterator_adaptor?

Answer 1

Thanks to @Xeo comment, the solution is to use Boost.Range. What I managed to get from VC10

#include <boost/range/adaptor/map.hpp>
#include <boost/range/adaptor/indirected.hpp>

#if defined(_MSC_VER) && _MSC_VER <= 1600 // VC10
    template< class T >
    typename std::add_rvalue_reference<T>::type declval();
#endif

class Wrapper
{
private:
    typedef std::map<T, std::shared_ptr<U>> Container;

public:
    typedef decltype(declval<Container&>() | boost::adaptors::map_values | boost::adaptors::indirected) IteratorRange;
    typedef decltype(declval<Container&>() | boost::adaptors::map_values | boost::adaptors::indirected) ConstIteratorRange;
    typedef IteratorRange::iterator Iterator;
    typedef ConstIteratorRange::const_iterator ConstIterator;

    Iterator begin()
    {
        return iteratorRange().begin();
    }

    // by analogy
    ConstIterator begin() const;
    ConstIterator cbegin() const;
    Iterator end();
    ConstIterator end() const;
    ConstIterator cend() const;

private:
    IteratorRange iteratorRange()
    {
        return container | boost::adaptors::map_values | boost::adaptors::indirected;
    }

    ConstIteratorRange iteratorRange() const;

private:
    Container container;
};