CDC* GetDC( );
Identifies the device context for the CWnd client area if successful; otherwise, the return value is NULL. The pointer may be temporary and should not be stored for later use.
Retrieves a pointer to a common, class, or private device context for the client area depending on the class style specified for the CWnd. For common device contexts, GetDC assigns default attributes to the context each time it is retrieved. For class and private contexts, GetDC leaves the previously assigned attributes unchanged. The device context can be used in subsequent graphics device interface (GDI) functions to draw in the client area.
Unless the device context belongs to a window class, the ReleaseDC member function must be called to release the context after painting. Since only five common device contexts are available at any given time, failure to release a device context can prevent other applications from accessing a device context.
A device context belonging to the CWnd class is returned by the GetDC member function if CS_CLASSDC, CS_OWNDC, or CS_PARENTDC was specified as a style in the WNDCLASS structure when the class was registered.
See Also CWnd::GetDCEx, CWnd::ReleaseDC, CWnd::GetWindowDC, https://msdn.microsoft.com/en-us/library/ms533241(v=vs.60) , CClientDC