Does std::sort check if a vector is already sorted?

Question

I believe that the C++ standard for std::sort does not guarantee O(n) performance on a list that\'s already sorted. But still, I\'m wondering whether to your knowledge any i

Answer 1

I suggest you read this comparison of sorting algorithms, it is very well done and informative, it compares a number of sorting algorithms with each other and with GCC's implementation of std::sort. You will notice, in the charts on the given link, that the performance of std::sort for "almost sorted" and "almost reverse" are linear in the number of elements to sort, that is, O(n). So, no guarantee, but you can easily expect that an almost sorted list will be sorted in almost linear-time. But, of course, it does not do a is_sorted check, and even if it will sort a sorted array in linear-time, it won't be as fast as doing a is_sorted check and skipping the sorting altogether. It is your decision to determine if it is better to check before sorting or not.