Problem Passing WndProc as Parameter

HRESULT App::Initialize(const AppParameters& ap)
{
	HRESULT hr = window.RegisterWindowClass(ap.wndStyle, App::StaticWndProc, ap.wndIcon, ap.wndCursor, NULL, ap.wndName);
	if(FAILED(hr))
		return hr;
        ...
}

LRESULT App::WndProc(HWND wnd, UINT msg, WPARAM wparam, LPARAM lparam)
{
	if(msg == WM_DESTROY ||
		msg == WM_CLOSE ||
		msg == WM_QUIT)
	{
		mainLoopDone = true;
		return 0;
	}

	return (DefWindowProc(wnd, msg, wparam, lparam));
}

LRESULT CALLBACK App::StaticWndProc(HWND wnd, UINT msg, WPARAM wparam, LPARAM lparam)
{
	App* parent;
	if(msg == WM_NCCREATE)
	{
		parent = (App*)((LPCREATESTRUCT)lparam)->lpCreateParams;
		SetWindowLongPtr(wnd, GWL_USERDATA, (LONG_PTR)parent);
	}
	else
	{
		parent = (App*)GetWindowLongPtr(wnd, GWL_USERDATA);
		if(!parent)
			return (DefWindowProc(wnd, msg, wparam, lparam));
	}

	return parent->WndProc(wnd, msg, wparam, lparam);
}

HRESULT Window::RegisterWindowClass(UINT style, WNDPROC wndproc, HICON icon, HCURSOR cursor, HBRUSH background, LPCSTR className)
{
	windowClass.cbClsExtra = 0;
	windowClass.cbWndExtra = 0;
	windowClass.hbrBackground = background;
	windowClass.hCursor = cursor;
	windowClass.hIcon = icon;
	windowClass.hInstance = GetModuleHandle(NULL);
	windowClass.lpfnWndProc = wndproc;
	windowClass.lpszClassName = className;
	windowClass.lpszMenuName = NULL;
	windowClass.style = style;
	
	HRESULT hr = RegisterClass(&windowClass);
        ...
}