Question

Let me explain my problem together with the background, so it would be easier to understand why I’m asking for this specific type of thing. I’m developing an instant messenger. Most of the architecture is outlined by my teacher, however implementation detail may vary. There is an “Engine” class, EventManager, which registers clients. To identify them and to easily remove them, I use a map (with client-id’s) or a set with pointers. So far, so good. But then this EventManager uses poll() (or select(), but that’s nowhere as comfortable to use as poll(), as you have to rebuild the array each time, which is slow and not-so-nice, I guess, and I can restrict myself to UNIX environment, if you ask) in its main loop. Which needs an array of struct pollfd. Now every time a new client comes or goes, this array needs to be rebuilt. Either I use a dynamic array by hand and allocate memory every time (baaaaaad), or I use a vector, which would handle new client’s struct pollfd insertion pretty well at the end of the container, or a deque, which would insert and remove anywhere pretty well. Now my two questions are:

1. If I choose vector, will it automatically shrink and move elements in the middle of itself instead of full reallocation? and
2. That would anyway copy a lot, if it’s in the beginning, so I’d like to use deque. Does that have an array interface (like you would do with vector – &myVector[0]) or is it strictly non-contiguous?

Answer 1

1. If you remove something from the middle of a vector it will move all the following elements one position towards the beginning. It will not reallocate. You don’t have to consider reallocations at all because they are amortized to give O(1) time per insertion.

2. deque is not much better than vector. Removing from the beginning or end is efficient. Not from the middle. If you remove from anywhere, then it will hopefully be twice as fast a vector, but not faster. Since it’s a more complicated structure it’ll probably be even slower. deque doesn’t guarantee continuous storage, so although indexing is allowed and done in O(1) time, you still can’t reliably convert it to a pointer.

Anyway it smells like premature optimization. Use vector. Since the order of clients is not significant, you can speed up the erasure of clients by swapping the element that you want to remove with the last element in the vector and calling pop_back() after that.