std::map of iterators to itself - c++

Question

My goal is to map elements of a type to other elements of the same type. Suppose they are size_t for simplicity.
std::map<size_t, size_t> myMapping;
This would do it, but if I want to follow a bunch of such links (they are all the same map), each step is a log(n) lookup.
size_t k = /*whatever*/;
myMapping[myMapping[myMapping[k]]]; //3 * log(n)
I want to make use of the fact that map iterators remain valid and have a map that maps size_t to iterators into itself.
typedef /*myMapTemplate*/::iterator map_iter;
std::map<size_t, map_iter> myMapping;
size_t k = /*whatever*/
map_iter entryPoint = myMapping.find(k);
entryPoint->second->second->first; //log(n) + 2 constant time operations
How would I write this type?
I know copying would keep iterators to old map and plan to take care of this myself.

Answer 1

I understand your question that you want map: key->map<key,>::iterator
So, here it is, a struct with map iterator as value:
template <
template <class K, class V, class C, class A> class mapImpl,
class K,
class V,
class C=std::less<K>,
class A=std::allocator<std::pair<const K, V> >
>
class value_with_iterator {
public:
typedef typename mapImpl<const K,value_with_iterator,C,A>::iterator value_type;
value_type value;
};
Map defined with using struct above:
typedef std::map<size_t, value_with_iterator <std::map, size_t, size_t> > map_size_t_to_itself;
Some insert method - to link key with itself:
map_size_t_to_itself::iterator insert(map_size_t_to_itself& mapRef, size_t value)
{
map_size_t_to_itself::value_type v(value, map_size_t_to_itself::mapped_type());
std::pair<map_size_t_to_itself::iterator, bool> res = mapRef.insert(v);
if (res.second)
res.first->second.value = res.first;
return res.first;
}
And simple test:
int main() {
map_size_t_to_itself mapObj;
map_size_t_to_itself::iterator i1 = insert(mapObj, 1);
map_size_t_to_itself::iterator i2 = insert(mapObj, 1);
map_size_t_to_itself::iterator i3 = insert(mapObj, 2);
std::cout << i1->first << ": " << i1->second.value->first << std::endl;
std::cout << i2->first << ": " << i2->second.value->first << std::endl;
std::cout << i3->first << ": " << i3->second.value->first << std::endl;
}
with OUTPUT:
1: 1
1: 1
2: 2
Full link: http://ideone.com/gnEhw

Answer 2

If I understood your problem correctly, I think I would keep my elements in a vector and use a vector of indices into the first vector for the kind of indirection you want. If you also need ordered access you can always throw in a map to the elements of the first vector.