Re: OnDocumentComplete

Goran <>
Wed, 5 May 2010 03:05:16 -0700 (PDT)
On May 4, 4:34 pm, Joseph M. Newcomer <> wrote:

The HWND in this case is less critical, because if you manage to destr=

oy the HWND without

deleting the CWnd *, any methods you invoke on the HWND (e.g., PostMes=

sage) will

assert-fail. In a release version, the data posted will result, typ=

ically, in a storage

leak, because the receiving window isn't there. Serious, but not re=

ally fatal (until you

run out of heap!)

Yes, of course. One should never ever allow a following sequence of
pointer ownership:

1. thread
2. "PostMessage" (e.g. pointer passed to it through WPARAM)
3. window/thread that receives the message

Of course, this is the "agent" pattern which is implemented more formally=

 in VS 2010.


... because PostMessage does not guarantee that message will be
delivered, even if HWND is correct and stable throughout. Letting
PostMessage "own" a heap pointer is a resource leak, end of.

Actually, PostMessage *will* deliver the message to the queue. It is i=

mportant that the

queue be processed. The sending pattern should really resemble

        Thing * t = new Thing;
        if(!PostMessage(USER_DEFINED_MSG, (WPARAM)t)
            { /* failed to enqueue */
            with failure in useful=


             delete t;
                     ...additional recovery
            } /* failed to enqueue */

Yeah, I use this approach, too. The only problem I know with this is
that PostMessage can still fail even if HWND was ok. E.g. message
queue saturation ; that could easily count as programming error. But
it might fail for some other reason. What do I know about failure
modes of PostMessage, and even if I did, they might change in the
future, so avoiding that window of uncertain object ownership (while
message with the object in W/LPARAM is in queue) is still a good idea.

If the window is destroyed before the messages are dequeued, this is esse=

ntially a

programming error. I handle this by using PostMessage to post the actu=

al termination

request, so it cannot be seen until alll messages have been dequeued; and=

 this termination

request is not posted until I receive a notification that the thread has =

terminated, so it

cannot be simultaneously posting messages that would come after my termin=

ation request.


But if you pass the HWND, the common failure mode is

class CMyView : public CView {
            static UINT handler(LPVOID p);


AfxBeginThread(handler, this);

/*static */ UINT CMyView::handler(LPVOID p)
     CMyView * view = (CMyView *)p;

     // this works correctly

But I've seen people who are told "You must pass the HWND because I on=

ce read a document

that said passing CWnd * is a Bad Idea" do the following

AfxBeginThread(handler, (LPVOID)m_hWnd);

/* static */ UINT CMyView::handler(LPVOID p)
    CMyVIew * view = (CMyView*)CWnd::FromHandle((HWND)p);


OK, here's a Quickie Quiz: What's wrong with the above code? And wh=

y is it a Really

Really Bad Idea?

If thread is only posts messages before view's HWND gets destroyed,
there is no problem.

See my above comment. I guarantee that both the CWnd* and the HWND hav=

e a lifetime that

exceeds the client threads.
****>If not, this might be hit by HWND reuse. Blind cast is completely

wrong (it presumes that it knows the type of the underlying CWnd) and
playing with HWND achieves strictly nothing.

The blind cast converts what is actually a CWnd* to a CMyView*, so it is =

really in serious

violation of sanity. THis is because the FromHandle looks in the threa=

d-local handle map,

doesn't find the CWnd* associated with the HWND, and synthesizes a new CW=

nd* (so now there

are TWO instances of a CWnd*-derived class wrapping the same HWND!)

It also illustrates why blind casting is not a Really Good Idea!

Ah. I misunderstood, I thought that "Handler" code is supposed to be
running in the UI thread. (Except PostMessage, I avoid touching CWnd
stuff completely in the worker thread, so my mind is biased). Yes, if
this cast is in a thread that did not create/Attach the CWnd, this
really is a horrible idea.

But I see that my code snippets weren't clear, I'll try to be more
precise (but still: compiled with head-compiler and debugged with head-

class CInput { /*whatever*/ };
typedef auto_ptr<CInput> AInput; // A == "auto ptr to..."
class COutput { /*whatever*/ };
typedef auto_ptr<COutput> AOutput;

class CSharedObject /*: public noncopyable? good idea*/
  CSharedObject(HWND wnd, ...) : m_hWnd(hWnd) {}
  AInput GetInput() const {/*whatever*/}
  AOutput GetOutput() const {/*whatever*/}
  void SetInput(AInput&) {/*whatever*/}
  void SetOutput(AOutput&) {/*whatever*/}
  HWND GetHWnd() const { return hWnd; }
  HWND m_hWnd;
  // whetever else
typedef shared_ptr<CSharedObject> SPSharedObject; // SP == "shared
pointer to..."
typedef weak_ptr<CSharedObject> WPSharedObject; // WP == "weak pointer

extern void PassToWorkerThread(const SPSharedObject& PSharedObject);

class CMyWnd : public CSomeMFCWnd
  SPSharedObject m_PSharedObject;

  void OnCreate(...) // Could be some other "window startup" function
    if (-1==__super::OnCreate(...))
      return -1;

    // In production, following two lines need try {} catch(whatever)
{...; return -1; }
    m_PSharedObject = SPSharedObject(new CSharedObject(m_hWnd, ...));

    return 0;

  void OnDestroy()

  LRESULT OnMessageFromThread()
  void Display(const COutput& ) { whatever }

  // MESSAGE_MAP etc...

WPSharedObject g_PWeak;
CCriticalSection g_CritSect;
void PassToWorkerThread(const SPSharedObject& PSharedObject)
  CSingleLock L(&g_CritSect, TRUE);
  g_PWeak = WPSharedObject(PSharedObject);
SPSharedObject GetSharedObjectInThread()
  CSingleLock L(&g_CritSect, TRUE);
  return g_PWeak.lock();

AOutput WorkWorkWork(const CInput& input)
  // work long and hard...
  return SomeAOutput;

UINT MyThreadProc(LPVOID p)
{ // in production, this needs "global" try {} catch(whatever) {}
    SPSharedObject PSharedObject(GetSharedObjectInThread());
    if (PSharedObject)


        ::PostMessage(PSharedObject->GetHWnd(), 0, 0);

The above is the simplest possible form for the thread func. But
there, shared object stays alive while doing WorkWorkWork (because
there's PSharedObject that holds a reference to shared object). That
might be a bad idea if SharedObject holds important resources, so the
following form is IMO preferable:

UINT MyThreadProc(LPVOID p)
{ // still needs "global" try {} catch(whatever) {}
    AInput input;
      SPSharedObject PSharedObject(GetSharedObjectInThread());
      if (PSharedObject1)
        input = PSharedObject.GetInput();
    } // shared object ref held by PSharedObject released here
    if (input.get())
    { // There was a "live" CSharedObject, process it
      AOutput = WorkWorkWork(*input);
      SPSharedObject PSharedObject(GetSharedObjectInThread());
      if (PSharedObject)
        ::PostMessage(PSharedObject->GetHWnd(), 0, 0); // return value
willfully ignored
    } // shared object ref held by PSharedObject released here

Some +/- obvious nitbits: in practice, thread needs to check
"ContinueRunning" more often, meaning that WorkWorkWork needs to do it
(best practice: don't check a flag, have a function that checks it and
throws "termination" exception, and have exception-safe code in the
thread; make that exception invisible to everybody else except the
thread function and the "termination check" function). Also,
WorkWorkWork might want to call GetSharedObjectInThread() periodically
to see whether it should abort because window was destroyed (it might
signal abortion by returning NULL AOutput, and that should be checked
to avoid useless PostMessage).

I hope this is more clear, code-wise. So, what do you say?


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