How to forward declare boost::ptree::iterator

Question

I'd like to use boost ptree in my project but since ptree.hpp causes about another 1000 header files to be included this increases compile times drastically (e.g. from 1s to 7s) and as it's needed in over 20 different cpp files this is not acceptable (pre-compiled headers don't improve things much). So I'm thinking of encapsulating the boost ptree in my own class, something like

// myptree.h
#include <boost/property_tree/ptree_fwd.hpp>

class myptree {
   private:
      boost::property_tree::ptree *m_tree;

   public:
      ...
     // adding new (single value) members to the the tree
     void put(const std::string&, double);
     void put(const std::string&, int);
     void put(const std::string&, const std::string&);

     // returning (single value) members of the tree
     double get_double(const std::string&) const;
     int get_int(const std::string&) const;
     std::string get_str(const std::string&) const;

     // working with subtrees
     void push_back(const std::string&, const myptree&);
     myptree get_child(const std::string&) const;

     // import/export
     void read_from_json(const std::string&);
     void write_to_json(const std::string&) const;

};

However, I'm failing to implement an iterator in a nice way. Ideally I'd like to have a boost::property_tree::ptree::iterator as a private member variable which then could be iterated over m_tree using my own member functions but as I understand from How do I forward declare an inner class? this is generally not possible. Any elegant ways of implementing an iterator within this class?