I see that creating an empty unordered_map sets the bucket count to default ( in my case it's 11). After which if i call unordered_map.reserve(n) where the n is quite big it rehashes immediately. So is it a good idea to pass a big number (possibly n as in reserve to the constructor of unordered_map so as to save a rehash. Correct me if i am wrong in my understanding.
Sure... if you know how many elements you'll have, it's good to let the constructor know too. The constructor wouldn't have an argument for it if it wasn't potentially useful. It's just a hint though, your suggested minimum, but I'd imagine all standard libraries would honour it, possibly after rounding it up to some power-of-two or prime depending on other implementation/design decisions they've made. Balancing the improved speed of creation/population, the risk/cost is a maintenance one: if you let your "hint" get out of sync with your actual needs, you may end up wasting memory or losing performance (e.g. having too low a load factor may mean fewer cache page hits).