MessageLoop with orphaned pointer

[Youngki Chang]

Hi chromium-dev team,

I have a following class:

class Adapter {

 public:

  Adapter() : ALLOW_THIS_IN_INITIALIZER_LIST(weak_ptr_factory_(this))  {}

  void Start() {

    Poll();

  }

 private:

  friend class base::RefCounted<Adapter>;

  ~Adapter() {}

  void Poll() {

    // Polling task.

    MessageLoop::current()->PostDelayedTask(

      FROM_HERE,

      base::Bind(&Adapter::Poll, weak_ptr_factory_.GetWeakPtr()),

    base::TimeDelta::FromMilliseconds(500));

  }

  base::WeakPtrFactory<Adapter> weak_ptr_factory_;

};

which basically performs polling task once Start() is called. Let's say Poll() is run and Adapter::Poll of the adapter instance is queued into the current message loop, and then the adapter instance is destroyed by having no more references to it. Does message loop crash due to orphaned pointer, or does message loop handle it gracefully by checking the orphaned pointer and not running it? I made a test to verify this and it seems the message loop handles it gracefully, but I'd like to hear from the expert.

Thanks,

Youngki

[Daniel Cheng]

base::Bind/base::Callback understand base::WeakPtr. If the weak pointer is invalidated, the task will not be executed. See http://dev.chromium.org/developers/coding-style/important-abstractions-and-data-structures#TOC-base::Callback-and-base::Bind- and http://www.chromium.org/developers/design-documents/threading#Scoped_factories for more information.

Daniel

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[Mattias Nissler]

Or check the source code: http://code.google.com/searchframe#OAMlx_jo-ck/src/base/bind_internal.h.pump&type=cs&l=316

[Youngki Chang]

Thanks for the pointers..! :)

[Fred Akalin]

It looks like your class is ref-counted yet vends weak pointers.  It should not be ref-counted.

[Youngki Chang]

On Mon, Nov 12, 2012 at 12:31 PM, Fred Akalin <aka...@chromium.org> wrote:

It looks like your class is ref-counted yet vends weak pointers.  It should not be ref-counted.

Could you explain why? I thought weak pointers are not counted as references.

[Fred Akalin]

I don't know what it means for weak pointers to "count as references".  But generally, objects are ref-counted only if they're used from multiple threads.  If you have a single thread, it doesn't have to be ref-counted; weak pointers already solve the problem of callbacks outliving the object itself (since the callback will consult the weak pointer to see if it's still alive).

[Paweł Hajdan, Jr.]

1. src/base/timer.h may be very helpful for your use case.

2. What is your use case? What are you polling for? Can this be solved using events and not polling?

Paweł

--

[Youngki Chang]

On Mon, Nov 12, 2012 at 4:43 PM, Paweł Hajdan, Jr. <phajd...@chromium.org> wrote:

1. src/base/timer.h may be very helpful for your use case.

Thanks for the pointer..!

 

2. What is your use case? What are you polling for? Can this be solved using events and not polling?

This could be off the point of this thread, but I'd like to listen to Bluetooth adapter state changes in Windows. There is a way to get the bluetooth adapter state info, but there is no notification API for bluetooth state change. So far I looked into Windows SDK plus internal framework such as network_change_notifier, ObjectWatcher, etc, and I couldn't find an API I could use in my use case. Hence we decided to go with polling.