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From: Douglas Gregor (doug.gregor_at_[hidden])
Date: 2005-08-08 14:14:41

On Aug 8, 2005, at 12:14 PM, Wolfram Koska wrote:
> I was trying to create hash_maps with vertex_descriptors and
> edge_descriptors.
> The first one was easy with vertex_descriptors being just void*'s, and
> there
> is even an example in the boost detail namespace (although it's not
> activated using gcc):
> template <>
> struct hash< void* >
> {
> std::size_t
> operator()(void* v) const { return (std::size_t)v; }
> }
> But the edge_descriptors give me some serious headache. How could I
> create a
> hash function for them? Is using the source and target a good idea in
> graphs
> where there could easily be more than one edge between two vertices?
> Or is
> there a great method I cannot come up with?

Using source/target in the hash function works so long as the hash on
them is symmetric, because for undirected and bidirectional graphs you
can have the same edge e represented as (u, v) or (v, u), depending on
where you get it. Other than that restriction, you should be fine using
source/target: it's okay to have two edges hash to the same thing, so
long as your equality function compares the edge descriptors directly
instead of using source/target.


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