Question

First off, I would like to make a few points I believe to be true. Please can these be verified?

• A hash map stores strings by
  converting them into an integer
  somehow.
• std::map is not a hash map, and if I’m using strings, I should consider using a hash map for memory issues?
• String compares are not good to rely on.

If std::map is not a hash map and I should not be relying on string compares (basically, I have a map with strings as keys…I was told to look up using hash maps instead?), is there a hash map in the C++ STL? If not, how about Boost?

Secondly, Is a hash map worth it for [originally] an std::map< std::string, non-POD GameState >?

I think my point is getting across…I plan to have a store of different game states I can look up and register into a factory.
If any more info is needed, please ask.

Thanks for your time.

Answer 1

I don’t believe most of your points are correct.

• there is no hash map in the current standard. C++0x introduces unordered_map, who’s implementation will be a hash table and your compiler probably already supports it.

• std::map is implemented as a balanced tree, not a hash table. There are no “memory issues” when using either map type with strings, either as keys or data.

• strings are not stored as numbers in either case – an unordered_map will use a hashing function to derive a numeric key from the string, but this is not stored.

• my experience is that unordered_map is about twice the speed of map – they have basically the same interface, so you can try both with your own data – whenever you are interested in performance you should always perform tests your self with your own real data, rather than depending on the experience of others. Both map types will be somewhat sensitive to the length of the string key.

Assuming you have some class A, that you want to access via a string key, the maps would be declared as:

map <string, A> amap;
unordered_map <string, A> umap;