Dequeuing

Dequeuing is the action of removing an element from a queue. It is the counterpart to enqueue. In a typical first-in, first-out queue, the element removed is the one that has been in the queue the longest—the front element. Dequeuing may return the value removed or raise an error if the queue is empty.

In queue implementations, the data structure must update internal pointers or indices. Mechanics differ by structure: linked-list queues use a head pointer; array-based queues use a head index and a count; circular buffers wrap around to the beginning.

In a double-ended queue, or deque, elements can be removed from either end; the term dequeuing can refer to removing from the front or back, depending on context, but in standard queue usage, dequeuing targets the front.

Complexity: typical dequeuing operation is O(1) time; memory overhead is O(1) aside from the storage of the element.

In concurrent contexts, dequeuing may require synchronization, locking, or atomic operations. Blocking queues wait if the queue is empty; non-blocking variants return a value indicating emptiness. Concurrency controls are important to prevent race conditions and ensure thread safety.

Common applications include message queues, task schedulers, and job queues in software systems. Underflow, attempting to dequeue from an empty queue, is a typical error condition that may trigger exceptions, empty-value returns, or blocking behavior depending on the implementation.