gistfile1.txt

class feature_kv_iterator :
        public boost::iterator_facade<feature_kv_iterator,
                                      boost::tuple<std::string , value> const,
                                      boost::forward_traversal_tag>
{
public:
    typedef boost::tuple<std::string,value> value_type;

    feature_kv_iterator (feature_impl const& f, bool begin = false)
        : f_(f),
          itr_( begin ? f_.ctx_->begin() : f_.ctx_->end())  
    {}
    
private:
    friend class boost::iterator_core_access;
    
    inline void increment()
    {
        ++itr_;
    }

    inline bool equal( feature_kv_iterator const& other) const
    {
        return ( itr_ == other.itr_);
    }
    
    inline value_type const& dereference() const
    {
        boost::get<0>(kv_) = itr_->first;
        boost::optional<mapnik::value const&> val = f_.get_optional(itr_->second);
        if (val) boost::get<1>(kv_) = *val;
        else boost::get<1>(kv_) = value_null();
        return kv_;
    }

    
    feature_impl const& f_;
    std::map<std::string,std::size_t>::const_iterator itr_;
    mutable value_type kv_;

};