Passing Pointer to Pointer in SendMessage

Hello all,

I'm attempting to pass the address of a pointer as the LPARAM argument in SendMessage.
I need to have this argument return the (this) pointer of the object receiving the message.
I have the following (doesn't work)

class CClassA
{
     CCommon *pCommon;
}

CClassA::GetObjectPointer()
{
     SendMessage( m_hDlgToSendTo, UM_XXX, xxx, (LPARAM) &pCommon);
}

In the dialog DLL I have:

LRESULT CClassBDlg::OnWorkspaceModelMessage(WPARAM wParam, LPARAM lParam)
{
     . . .
     CCommon * pCommon = *((CCommon **) lParam);
     . . .
     pInterface = (CCommon *) this;
     lParam = (LPARAM) pInterface;
      . . .
}

What is the correct way to do this?

Thank you!
trishm
trishmAsked:
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jkrCommented:
You are passing the address of the pointer. Just use

class CClassA
{
     CCommon *pCommon;
}

CClassA::GetObjectPointer()
{
     SendMessage( m_hDlgToSendTo, UM_XXX, xxx, (LPARAM) pCommon);
}

LRESULT CClassBDlg::OnWorkspaceModelMessage(WPARAM wParam, LPARAM lParam)
{
     . . .
     CCommon * pCommon = ((CCommon **) lParam;
     . . .
     pInterface = (CCommon *) this;
     lParam = (LPARAM) pInterface;
      . . .
}

which should work fine (been there, done that).
0

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trishmAuthor Commented:
jkr:

Sorry, that didn't work. After the call to SendMessage
     pInterface = 0xcdcdcdcd

When I pass:
     pCommon
instead of
     &pCommon
to SendMessage

the receiving function gets a bad lParam value:
     lParam = -89830280

0
jkrCommented:
Sorry, that should be

   CCommon * pCommon = (CCommon *) lParam;

Is 'pCommon' initialized correctly when you call 'SendMessage()'?
0
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peetmCommented:
The thing you're sending this to is in the same process right?
0
trishmAuthor Commented:
jkr:

I initially caught the typo and changed it to your corrected version before I responded.
I did, however, add initialization of pCommon to NULL before SendMessage.

It didn't help. The only different is that the receiving function now receives
     lParam = 0 instead of lParam = -838473984
So, now
     CCommon * pCommon = (CCommon *) lParam;
assigns 0 to pCommon

After SendMessage
     pCommon = 0
I need a valid pointer to call a function in pCommon
     pCommon->DoSomething()





0
trishmAuthor Commented:
peetm:

No, they are in different DLLs.
0
jkrCommented:
Different DLLs, but same process?
0
trishmAuthor Commented:
Yes, same process.

In a previous attempt, I  successfully retrieved the pointer I need with:
     pCommon = (CCommon *) SendMessage(m_hDlgToSendTo, xxxx, xxxx, NULL);
and returning the object pointer
     return (LRESULT) this

This gave me what I needed but I was instructed to pass a pointer to the pointer instead:
     SendMessage( ... ... ..., (LPARAM) &pCommon)

0
trishmAuthor Commented:
The objective is to get the pointer to "this" via an argument to SendMessage()
0
trishmAuthor Commented:
The following code gave me what I needed:

SendMessage(m_hDlgToSendTo, xxx, xxx, (LPARAM) &m_pCommon);

In the receiving dialog:

CCommon ** ppCommon = (CCommon **) lParam;

*ppCommon = this;
lParam = (LPARAM) ppCommon;

This returns a pointer to "this" in the LPARAM argument.

Thank you for your comments.
trishm


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