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subclassing and inheritance

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I think my flu is getting the best of me. I'm having a very simple problem and some trouble wrapping my head around it. Consider (casting and all such omitted):

typedef void (*callback)(class BaseA *);

class BaseA {
vector<callback> cblist;

HWND handle;

void manage() { cblist[0](this); } //call only the first one for simplicity
void registercallback(callback cb) { cblist.push_back(cb); }

class BaseB : public BaseA {
int _int;

BaseB() { _int = 15; registercallback(BaseB::localproc); }

static void localproc(BaseA * a)
BaseB* b = (BaseB*)a;
printf(b->_int); //address of BaseB::_int is invalid

class C : public BaseB {
handle = CreateWindow();
SetWindowLong(handle, GWL_WNDPROC, C::WndProc);
SetWindowLong(handle, GWL_USERDATA, this); //<- hic!

static LRESULT WndProc(HWND hwnd, UINT iEvent, WPARAM wparam, LPARAM lparam)
C* c = (C*)GetWindowLong(hWnd, GWL_USERDATA);
return 0;

Notably changing the line

SetWindowLong(handle, GWL_USERDATA, this);

produces different results. Eg changing it to

SetWindowLong(handle, GWL_USERDATA, (BaseA*)this);

causes the class' address to not line up when inspecting *this* in BaseB::localproc(). I've never encountered thisbefore . Is this normal?

Normally I don't get WinAPI or C++ brainfarts, but this seems to be one such case. To recap - how do I gain access to BaseB's members in a static member function of BaseB called via a proxy in BaseA and triggered from inherited class C?

Edit: yes, in real life I'm also taking care of WM_CREATE and the window's user data not being set yet.

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