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Asked: 2026-05-25T09:32:05+00:00

As far as I understand, Hashmaps are preferable to standard maps because you can

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As far as I understand, Hashmaps are preferable to standard maps because you can find elements in close to O(1) time. This is done by using a hash or the key as an array lookup. We then resolve any collisions and pull out the value.

This works great for lookup, but if our array-space into which we do the hash lookup is sparsely populated, how does the hashmap/unorderedmap efficiently iterate all the elements in our hashmap without exhaustively going through our array-space?

Edit: yet Boost, SGI and C++11 hashmaps/unordered maps have iterators, so how do they work?

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  1. Editorial Team

    Unless there’s a parallel structure (for example a linked list as in a LinkedHashMap) it can’t: iteration will need to check each bucket for content.

    So if your buckets are very sparsely populated, this can become a factor. That’s one of the reasons why you don’t want to choose a bucket count that is too high (the bigger one obviously being wasted memory).

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