Keeping an unordered list of small objects with frequent insertions and removals

Question

Suppose I have a list of small objects that I iterate through (say, in a loop) with frequent insertions and removals. However, the sequential order that I iterate through the list does not matter. Instead of using std::list to store the elements, I was thinking about using std::vector in the following way (for constant time removals):

Insertion: use push_back to insert at the end of the array.

Removal: let's say I want to remove an element at position k from a vector of size n. Then, I copy the content of the nth (or (n-1)st, depending on how you see it) element to the kth element and use pop_back. Given that the elements are small, the copy operation shouldn't be costly.

This is to take advantage of contiguous memory and not having to dynamically allocate memory for every insertion. Is there a downside for this approach? I also noticed that C++11 has unordered_set, but I think this may be overkill for what I'm trying to do.

I apologize if this idea sounds blatantly obvious.

Answer 1

Your idea is the basic approach to keep an array efficient. If the order really doesn't matter for you, I think it's the ideal approach. You might want to encapsulate it in a class (a wrapper around std::vector) so that you can employ it in multiple places without code duplication, test it separately and generally follow the "single responsibility" principle.

If you have access to C++11 features, you won't even have to copy the n-th element - you can move it instead, making this feasible even for heavier objects.