Basic CS fail. Most translation units have maybe 100 header files or fewer. While a hash map might be O(1) (O(n^2) worst case) and a linked list O(n^2), the ov

Basic CS fail. Most translation units have maybe 100 header files or fewer. While a hash map might be O(1) (O(n^2) worst case) and a linked list O(n^2), the overhead of every insertion into a hash map is significantly greater than into a linked list. If you have such a tiny n, a linked list is almost always more time- and space-efficient. Switching to a more inappropriate data structure is more likely to negatively impact the performance of your toolchain. Of course the best answer is to make the change and collect measurements before and after using a spectrum of data sets, then compare. Don't let me stop you. But remember for that small a number of items bubble sort is usually the fastest, most efficient algorithm. Also remember that looking up a header name at preprocessor time, no matter how slow, will likely not show up on your profile graph when you turn on the optimizer.