ansaurus

Question

Cross Reference and Passing the This Pointer between Classes [NS2/C++]

Answer 1

+1 A:

class OLSR : public Agent

You class OLSR is derived from some class 'Agent' (which I don't know what it is). I assume that is not one of those classes derived from 'Handle' (since it is not shown in the diagram).

Since 'OLSR' is not derived from 'Handle', dynamic_cast of 'Handle' to 'OLSR' fails. You can do dynamic_cast from polymorphic Base to Derived only and not to unrelated classes.

Chubsdad 2010-09-29 09:32:31

I am sorry, my mistake. Class 'Agnet' has been added to the diagram. I will try to modify my program, thanks very much for your tip.

Edward 2010-09-29 10:35:07

Thank you very much for you help and valuable advice. I have found where the problem is, please see my own posted-answer.

Edward 2010-09-30 07:31:46

Answer 2

+1 A:

The code below works with my compiler. It outputs "20", which was the value I gave to the member OLSR::ra_addr_. A few unstated assumptions I had to add to get things compiling:

OLSR or some parent defines recv() so that it's not abstract.
Class Handler has at least one virtual function (otherwise using Handler* with dynamic_cast would be ill-formed, and your compiler should complain).
You call OLSR::send_pkt at some point. I assume you have checked for its debug output line. (But maybe it's being called with a different PriQueue object?)
Ignore Packet::get(). That's just there to give me a pointer so I can call functions matching your signatures.

If you can't figure out why your code isn't working, always try this: Make a copy of all your code, and then remove things a bit at a time until you either pinpoint the problem or get a simple example that you can post in entirety and ask why it doesn't do what you expect.

#include <iostream>

class Packet {
public:
    static Packet* get() { return &dummy_; }
private:
    static Packet dummy_;
};
Packet Packet::dummy_;

class Handler {
public:
    virtual ~Handler() {}
};

class NsObject : public Handler {
public:
    virtual void recv(Packet*, Handler* callback = 0) = 0;
};

class Connector : public NsObject {};

class Queue : public Connector {
public:
    virtual void recv(Packet*, Handler*) {}
};

class DropTail : public Queue {};

class OLSR;

class PriQueue : public DropTail {
public:
    inline PriQueue() : DropTail(), olsr_callback(NULL) {}
    void recv(Packet* p, Handler* h);
    Packet* deque();
private:
    OLSR* olsr_callback;
};

class Agent : public Connector {};

class OLSR : public Agent {
public:
    explicit OLSR(int ra_addr) : Agent(), ra_addr_(ra_addr) {}
    inline int ra_addr() { return ra_addr_; }
    void send_pkt(PriQueue* q);
    Packet* add_rr_ack(Packet* p, PriQueue*);
    virtual void recv(Packet*, Handler*) {}
private:
    int ra_addr_;
};

void PriQueue::recv(Packet* p, Handler* h) {
    olsr_callback = dynamic_cast<OLSR*>(h);
}

Packet* PriQueue::deque() {
    return olsr_callback->add_rr_ack(Packet::get(), this);
}

void OLSR::send_pkt(PriQueue* q) {
    q->recv( Packet::get(), this );
}

Packet* OLSR::add_rr_ack(Packet* p, PriQueue*) {
    std::cout << ra_addr() << std::endl;
    return p;
}

int main() {
    PriQueue q;
    OLSR olsr(20);
    olsr.send_pkt(&q);
    q.deque();

    return 0;
}

aschepler 2010-09-29 17:19:01

Thank you very much for your help and notable effort. I have found where the problem is, please see my own posted-answer.

Edward 2010-09-30 07:32:51

Answer 3

A:

Thank you all for your help, Chubsdad and aschepler. I have found where the problem is.

Normally, a packet is scheduled as an event on the simulation timeline using the following statement:

Scheduler::instance().schedule(target_,p,0.0);

where p is the packet which is casted to a event; '0.0' is the delay time of the event, in this case it is zero; and the key parameter, 'target_' is the handler which will process the event.

Here is part of the class NsObject and its implementation:

   //----------- object.h ----------//
    class NsObject ： public TclObject, public Handler {
    public:
       NsObject();
       virtual ~NsObject();
       virtual void recv(Packet*, Handler* callback = 0) = 0;
    protected:
       void handle(Event*);
    }

   //------------ object.cc -----------//
    void NsObject::handle(Event* e)
   {
       recv((Packet*)e);   // In my case, this will call the recv(Packet*,Handler*) of PriQueue class.
   }

and here is the implementation of class Handler:

   class Handler {
   public:
       virtual ~Handler() {}
       virtual void Handler(Event* event) = 0;
   }

Based on my previous understanding about NS2, I tried to use

   target_->recv(p,h);

to avoid event scheduling, and directly call the recv(Packet*, Handler*) function of PriQueue, which turned out to be wrong.

The control will still enter the NsObject::handle(),despite of using target_->recv(p,h), as the NsObject::handle() function takes only a Event* type parameter, the Handler* parameter will always be lost. That is how the olsr_callback variable turned out to be always NULL. (This has been verified in my debugging process.)

So the next step would be make a few adjustments to NsObject, even if I still don't fully understand how it ends up entering function NsObject::recv() when using target_->recv(p,h). :)

Thanks again for your help.

Shu

Edward 2010-09-30 07:29:59

ansaurus

tags:

views:

answers:

Cross Reference and Passing the This Pointer between Classes [NS2/C++]

related questions