vector or map, which one to use?

Question

I\'ve heard many people say that if the number of expected elements in the container is relatively small, it is better to use std::vector instead of std::

Answer 1

Maps are usually implemented as binary search trees, and walking a binary tree always comes with a little overhead (performing comparisons, walking links, etc.) Vectors are basically just arrays. For very small amounts of data, maybe 8 or 12 elements, sometimes it's faster just to do a linear search over an array than to walk a binary search tree.

You can run some timings yourself to see where the break-even point is -- time a search over four elements, then eight, then sixteen, and so on to find the sweet spot for your particular implementation of the STL.

Maps do tend to have a bunch of small allocations all over the heap, whereas vectors are contiguous so the cache-hit rate of vectors can sometimes be a little better in cases where you're iterating over all the elements from front to back.