STL like iterator over a collection of hash maps - c++

Question

I'm looking for a way to create a forward iterator which allows to iterate over a collection of hash maps.
An exemplary class which holds several maps looks like follows. (I'm using boost for unordered_map and shared_ptr, but C++11 would provide those classes as well).
In my particular application I'm using this construct to represent sparse hierarchical 2D grid locations; i.e. KeyT is a 2D location, ValueT is an integral counter and the different levels represent different resolutions of the grid.
template <typename KeyT, typename ValueT>
class MapCollection
{
public:
// type-definitions for the map and a shared pointer to the map
typedef boost::unordered_map<KeyT, ValueT> Map;
typedef boost::shared_ptr<Map> MapPtr;
// Constructor for class
MapCollection (int num_levels)
{
levels_.reserve (num_levels);
for (int i = 0; i < num_levels; ++i)
levels_.push_back (MapPtr (new Map()));
}
// adds a key-value pair to the map on the given level
void addValue (const KeyT &key, const ValueT &value)
{
int level = getLevelForKey (key);
(*levels_[level])[key] = value;
}
// TODO define const_iterator for this class
// TODO define member function begin(), returning levels_.front()->begin()
// TODO define member function end(), returning levels_.back()->end()
private:
// return the hierarchy level for the given key
int getLevelForKey (const KeyT &key) { return /* ... */ };
// collection of maps
std::vector<MapPtr> levels_;
};
Within an application I would now like to be able to iterate over all entries of all maps, similarly to what is possible if one just iterates over a single map, i.e.
int main (int argc, char *argv[])
{
int num_levels = 5;
MapCollection maps (num_levels);
// fill maps
maps.addValue ( /* ... */ )
// iterator over all entries
MapCollection::const_iterator iter = maps.begin();
for (; iter != maps.end(); ++iter)
{
std::cout << "Key: " << iter->first << " | Value: " << iter->second << std::endl;
}
return EXIT_SUCCESS;
}
Obviously it would be possible to iterator over the different levels and for each level over it's map, but I would like to hide the creation of different levels for the user.
What is the correct way to define an (const_)iterator for the class MapCollection?
Thanks for your help!

Answer 1

You could use boost iterator facade, that will help you with the task of implementing custom iterators. The idea would be:
1-maintain a reference to the vector containing the maps (levels_).
2-a iterator indicating what element of the previous vector the custom iterator is iterating (of type std::vector<MapPtr>::iterator or std::vector<MapPtr>::const_iterator) or a index with the same info (to retrieve the end of level_[index]).
3-a iterator with current element of the iteration of the previous iterator (the actual element of the iteration).
Some sample code:
#include <boost/iterator/iterator_facade.hpp>
namespace impl
{
template <class Value>
class iterator
: public boost::iterator_facade<
config_iterator<Value>
, Value
, boost::forward_traversal_tag
>
{
public:
config_iterator() {...}
explicit config_iterator(parameters) { /*your specific contructor*/ }
private:
template <class OtherValue>
config_iterator(config_iterator<OtherValue> const& other);
friend class boost::iterator_core_access;
template <class> friend class config_iterator;
template <class OtherValue>
bool equal(config_iterator<OtherValue> const& other) const { } // Verify is two iterators are equals (used to verify if it == end)
void increment() {} // Logic for incrementing the iterator
Value& dereference() const {} // Returning the actual value
// members
};
}
typedef impl::iterator<type> iterator;
This is a template file for very simple iterator (forward), read the Iterator Help for more info, the same can be implementing overloading the right operators (++ post a pre increment, *, ->, etc...), by boost provide a ways of defining the minimum necessary to implement the rest.