Question

I’ve read somewhere that std::map is, with current compilers, still the most efficient associative container we have in the STL, even with std::unsorted_map that –from what I read somewhere, I’m not sure where– becomes more efficient on find() only if there is a lot of entries, like more than 40k.

So now I’m not really sure anymore because I always assumed that a hash map is more efficient at least in case of string keys.

So to be short:

If I have to choose an associative container with unknown entry count and with std::string as keys, what would be (at least in theory) the more efficient (on speed) choice for finding?

Answer 1

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The problem with strings as keys is that comparing them is very slow (think difference in the last character of a 1000-character string). The advantage of an unordered_map with a string key comes at least in part from the fact that only the fixed-width hash values have to be compared, so in practice the unordered map may well be a lot faster.

The hash implementation may choose, for example, to use only a fixed number of spread-out digits to compute the hash value and thus end up putting some near-identical strings in the same bucket, so it’s a trade-off. You can probably concoct a set of key values for which both containers would perform very poorly, but for a “random” or “typical” collection of strings, my bet is on the hash container.