Re: Why does this cause "data abort" ?
Lisa Pearlson wrote:
Hi imagine construction like below:
class CSomeWnd
{
public:
int m_hWnd;
virtual LRESULT SomeWndProc(HWND hWnd, MSG msg,
WPARAM wParam, LPARAM lParam);
}
How does that which you are trying look like? The reason I'm asking is that
this obviously lacks a semicolon so it probably is not the code that is
compiled and executed, making it hard to guess what's going on.
One other thing here: the member m_hWnd should probably be a HWND. Further,
I guess that the hWnd passed to SomeWndProc is the same as the one stored
in the member, right? In that case, I'd suggest dropping either of them.
Further, I'd drop the 'C' in front of the classname. Using such things to
separate different class libraries is obsolete in the presence of real C++
namespaces. Also, it makes it look like somehow related to the MFC.
CSomeWnd* g_pSomeWnd = new CSomeWnd();
g_pSomeWnd->CreateWindow( ... );
...
LRESULT GlobalWndProc(HWND hWnd, MSG msg, WPARAM wParam, LPARAM lParam)
{
return g_pSomeWnd->SomeWndProc(hWnd,msg,lParam,wParam);
}
Again, what does the code look like? In the first line, you seem to
initialise a local pointer named g_pSomeWnd but in the function you use the
global one.
Just for your info: most class libraries I have seen simply maintain a map
between the HWND and a pointer to the window class. Of course, using
GetWindowLong() is another possible way to achieve that.
Since I can not register window class with a non-static windows procedure,
I can not use a non-static member functions.
So above the global static GlobalWndProc 'relays' the message to
non-static SomeWndProc, so that SomeWndProc can access member variables
easily.
However, the above construction causes data abort exceptions. I don't
understand why, but it's resolved by something like this:
CSomeWnd* g_pSomeWnd = new CSomeWnd();
g_pSomeWnd->CreateWindow( ... );
SetWindowLong(g_pSomeWnd->GetSafeHwnd(), GWL_USERDATA, (LONG) this);
LRESULT GlobalWndProc(HWND hWnd, MSG msg, WPARAM wParam, LPARAM lParam)
{
CSomeWnd* pWnd = (CSomeWnd*) GetWindowLong(hWnd, GWL_USERDATA);
return pWnd->SomeWndProc(hWnd,msg,lParam,wParam);
}
Can someone explain why this is?
Using GetWindowLong()/SetWindowLong(), you simply retrieve/attach a value of
type LONG from a window handle. This value can also be a pointer (though
you should use GetWindowLongPtr() then, see MSDN) to the wrapper class that
handles the windowmessage.
Note: use reinterpret_cast to convert between a pointer and a LONG.
Preferably forget about the fact that C-style casts exist at all.
Uli