A pool allocator is an O(1) allocator that is capable of allocating and freeing. It is able to do both in constant time because it maintains a linked list of all the free elements in the pool. When an element is requested the first element of linked list is returned and the list is updated. When an element is freed, it becomes the first element. The catch with this strategy however, is that all of the allocations must be of the same size. This would not be an allocator to use when dealing with heterogenous data, but when doing homogenous data, such as game entities, this allocator is great. It allows you to allocate and free as many times as you want, without worrying about fragmentation or slow allocators. Just make sure you don't allocate more than the pool can provide.