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Discussion
You don't always need a full sort ; you usually need less, and rarely you need more. In general order from cheapest to most expensive, your standard sorting algorithm options are: partition, stable_partition, nth_element, partial_sort (with its variant partial_sort_copy ), sort , and stable_sort . Use the least expensive one that does the work you actually need; using a more powerful one is wasteful.partition, stable_partition , and nth_element run in linear time, which is nice.nth_element, partial_sort, sort , and stable_sort require random-access iterators. You can't use them if you have only bidirectional iterators (e.g., list<T>::iterator s). If you need these algorithms but you don't have random-access iterators, consider using the index container idiom: Create a container that does support random-access iterators (e.g., a vector ) of iterators into the range you have, and then use the more powerful algorithm on that using a dereferencing version of your predicate (one that dereferences the iterators before doing its usual comparison).Use the stable_… versions only if you need to preserve the relative ordering of equal elements. Note that partial_sort and nth_element aren't stable (meaning that they don't keep equivalent elements in the same relative order they were in before the sort), and they have no standardized stable versions. If you otherwise want these algorithms but need stability, you probably want stable_sort .Of course, don't use any sorting algorithm at all if you don't have to: If you are using a standard associative container (set /multiset or map /multimap ) or the priority_queue adapter, and only need one sort order, the elements in the container stay sorted all the time.
