My main issue is to use as few memory as possible, because there will have a lot of paths in the set. Any idea?
How many is a lot? Do you know the number before hand?
At least 1e7 paths. And the path size (i.e, vector<edge_descriptor>.size()) is about 10 elements.
If the order really doesn't matter and you don't need the associative properties of a hash table, I'd stick with a list. for forward_list (for less memory, but no reverse iteration).
I do need the associative property. Basically, I will draw a lot of paths and I'd like to know the number of *different* paths I've drawn.
That does seem like a fairly large number :) In that case unordered_map should work fine (it's basically the same as hash_map). you might want to pre-seed it with a large initial (prime) number of buckets to avoid a lot of rehashing. You can use the Boost.Functional/Hash library to create a hash function for paths. That library has some support for computing combined hash keys. There may even be an overload for vector, but I'm not sure. Andrew Sutton andrew.n.sutton@gmail.com