Add TempHWNDSetter RAII helper to wxMSW and use it

No real changes, just replace pairs of SetHWND(hwnd)/SetHWND(0) calls
with the use of TempHWNDSetter.

include/wx/msw/private.h

@@ -374,6 +374,29 @@ inline RECT wxGetClientRect(HWND hwnd)
 // small helper classes
 // ---------------------------------------------------------------------------
 
+// Temporarily assign the given HWND to the window in ctor and unset it back to
+// the original value (usually 0) in dtor.
+class TempHWNDSetter
+{
+public:
+    TempHWNDSetter(wxWindow* win, WXHWND hWnd)
+        : m_win(win), m_hWndOrig(m_win->GetHWND())
+    {
+        m_win->SetHWND(hWnd);
+    }
+
+    ~TempHWNDSetter()
+    {
+        m_win->SetHWND(m_hWndOrig);
+    }
+
+private:
+    wxWindow* const m_win;
+    WXHWND const m_hWndOrig;
+
+    wxDECLARE_NO_COPY_CLASS(TempHWNDSetter);
+};
+
 // create an instance of this class and use it as the HDC for screen, will
 // automatically release the DC going out of scope
 class ScreenHDC

src/msw/colordlg.cpp

@@ -255,7 +255,7 @@ void wxColourDialog::DoGetClientSize(int *width, int *height) const
 void wxColourDialog::MSWOnInitDone(WXHWND hDlg)
 {
     // set HWND so that our DoMoveWindow() works correctly
-    SetHWND(hDlg);
+    TempHWNDSetter set(this, hDlg);
 
     if ( m_centreDir )
     {
@@ -272,9 +272,6 @@ void wxColourDialog::MSWOnInitDone(WXHWND hDlg)
     {
         SetPosition(GetPosition());
     }
-
-    // we shouldn't destroy hDlg, so disassociate from it
-    SetHWND(NULL);
 }
 
 #endif // wxUSE_COLOURDLG

src/msw/filedlg.cpp

@@ -312,7 +312,7 @@ void wxFileDialog::MSWOnInitDone(WXHWND hDlg)
     HWND hFileDlg = ::GetParent((HWND)hDlg);
 
     // set HWND so that our DoMoveWindow() works correctly
-    SetHWND((WXHWND)hFileDlg);
+    TempHWNDSetter set(this, (WXHWND)hFileDlg);
 
     if ( m_centreDir )
     {
@@ -333,9 +333,6 @@ void wxFileDialog::MSWOnInitDone(WXHWND hDlg)
     // Call selection change handler so that update handler will be
     // called once with no selection.
     MSWOnSelChange(hDlg);
-
-    // we shouldn't destroy this HWND
-    SetHWND(NULL);
 }
 
 void wxFileDialog::MSWOnSelChange(WXHWND hDlg)
@@ -402,11 +399,9 @@ static bool ShowCommFileDialog(OPENFILENAME *of, long style)
 
 void wxFileDialog::MSWOnInitDialogHook(WXHWND hwnd)
 {
-   SetHWND(hwnd);
+    TempHWNDSetter set(this, hwnd);
 
-   CreateExtraControl();
-
-   SetHWND(NULL);
+    CreateExtraControl();
 }
 
 int wxFileDialog::ShowModal()

src/msw/msgdlg.cpp

@@ -136,7 +136,7 @@ wxMessageDialog::HookFunction(int code, WXWPARAM wParam, WXLPARAM lParam)
         wnd->m_hook = NULL;
         HookMap().erase(tid);
 
-        wnd->SetHWND((HWND)wParam);
+        TempHWNDSetter set(wnd, (WXHWND)wParam);
 
         // replace the static text with an edit control if the message box is
         // too big to fit the display
@@ -151,9 +151,6 @@ wxMessageDialog::HookFunction(int code, WXWPARAM wParam, WXLPARAM lParam)
         if ( wnd->GetMessageDialogStyle() & wxCENTER )
             wnd->Center(); // center on parent
         //else: default behaviour, center on screen
-
-        // there seems to be no reason to leave it set
-        wnd->SetHWND(NULL);
     }
 
     return rc;