Error code Queue serve(; Post: If the Queue is not empty, the front of the Queue has been removed otherwise an error code of underflow is returned Error code Queue: retrieve Queue entry &x)const Post: If the Queue is not empty the front of the Queue has been recorded as x. otherwise an Error code of underflow is returnedError_code Queue :: serve( ); Post: If the Queue is not empty, the front of the Queue has been removed. Otherwise an Error code of underflow is returned. Error_code Queue::retrieve(Queue entry &x) const; Post: If the Queue is not empty, the front of the Queue has been recorded as x. Otherwise an Error code of underflow is returned