BOOL GetUpdateRect( LPRECT lpRect, BOOL bErase = FALSE );
lpRect
Points to a CRect or RECT structure that is to receive the client coordinates of the update enclosing the update region.
bErase
Specifies whether the background in the update region is to be erased.
Retrieves the coordinates of the smallest rectangle that completely encloses the update region. If CWnd was created with the CS_OWNDC style and the mapping mode is not MM_TEXT, the GetUpdateRect member function gives the rectangle in logical coordinates. Otherwise, GetUpdateRect gives the rectangle in client coordinates. If there is no update region, GetUpdateRect sets the rectangle to be empty (sets all coordinates to 0).
The bErase parameter specifies whether GetUpdateRect should erase the background of the update region. If bErase is TRUE and the update region is not empty, the background is erased. To erase the background, GetUpdateRect sends the WM_ERASEBKGND message.
The update rectangle retrieved by the BeginPaint member function is identical to that retrieved by the GetUpdateRect member function.
The BeginPaint member function automatically validates the update region, so any call to GetUpdateRect made immediately after a call to BeginPaint retrieves an empty update region.
Specifies the status of the update region. The value is TRUE if the update region is not empty; otherwise FALSE.
CWnd::BeginPaint, ::GetUpdateRect