std::queue >::size() is slow in O(n)?

Question

I experienced unexpected performance behavior of my code which uses a queue. I realized that performance degraded when more elements were in the queue. It turned out that us

Answer 1

You're your queue to use a list container, instead of the deque default:

typedef std::queue > QueueType;

You shouldn't be surprised that size takes O(n), since that's a perfectly valid implementation of the list container pre C++11. The previous version of the standard did not guarantee the complexity of the size member function for lists.

If you change your code to:

typedef std::queue > QueueType;

You will see the behavior you want (O(1) size complexity).