7.1 The C++ Queue Class

The Queue class is derived from a Connector base class. It provides a base class used by particular types of (derived) queue classes, as well as a call-back function to implement blocking (see next section). The following definitions are provided in queue.h:

        class Queue : public Connector {
         public:
                virtual void enque(Packet*) = 0;
                virtual Packet* deque() = 0;
                void recv(Packet*, Handler*);
                void resume();
                int blocked();
                void unblock();
                void block();
         protected:
                Queue();
                int command(int argc, const char*const* argv);
                int qlim_;         /* maximum allowed pkts in queue /
                int blocked_;
                int unblock_on_resume_; /* unblock q on idle? /
                QueueHandler qh_;
        };

The enque and deque functions are pure virtual, indicating the Queue class is to be used as a base class; particular queues are derived from Queue and implement these two functions as necessary. Particular queues do not, in general, override the recv function because it invokes the the particular enque and deque.

The Queue class does not contain much internal state. Often these are special monitoring objectsChapterchap:trace. The qlim_ member is constructed to dictate a bound on the maximum queue occupancy, but this is not enforced by the Queue class itself; it must be used by the particular queue subclasses if they need this value. The blocked_ member is a boolean indicating whether the queue is able to send a packet immediately to its downstream neighbor. When a queue is blocked, it is able to enqueue packets but not send them.