void GetClientRect( LPRECT lpRect ) const;
lpRect
Points to a RECT structure or a CRect to receive the client coordinates.
Copies the client coordinates of the CWnd client area into the structure pointed to by lpRect. The client coordinates specify the upper-left and lower-right corners of the client area. Since client coordinates are relative to the upper-left corners of the CWnd client area, the coordinates of the upper-left corner are (0,0).
CWnd::GetWindowRect, ::GetClientRect