Question

I want to create a map, that uses iterators as key-type and integers as values, like in the following example:

#include <list>
#include <unordered_map>

int main(int argc, char* argv[])
{
  typedef std::list<int> ListType;
  typedef std::unordered_multimap<ListType::iterator, unsigned int> MapType;

  ListType _list;
  MapType _map;

  _list.push_back(100);
  _map.insert(std::make_pair(_list.begin(), 10));

  return 0;
}

Unfortunately, this makes the compiler aborting with the error C2440: 'conversion' : cannot convert from 'const std::_List_iterator<_Mylist>' to 'size_t'. Is there something i can do to achieve this anyway?

Answer 1

The error means you have to provide a hash function for the particular iterator type. You can pass a hashing function as third template parameter to std::unordered_map.

unordered_map also requires an equality comparator for its keys, however std::list’s iterator already has that, so you do not need to provide your own. For example:

#include <list>
#include <unordered_map>
#include <cstddef>

typedef std::list<int> ListType;
typedef std::list<int>::iterator ListIterator;
// a very poor hashing functor
struct MyHash {
  size_t operator()(const ListIterator&) const {
    // provide a useful implementation here!
    size_t hash_ = .... ;
    return hash_;
    //return 0; // compiles, but is useless
  }
};

typedef std::unordered_multimap<ListIterator, unsigned int, MyHash> MapType;

int main() {

  ListType list_;
  MapType map_;

  list_.push_back(100);
  map_.insert(std::make_pair(list_.begin(), 10));

  return 0;
}