CMDIFrameWnd::MDINext

// CMainFrame::OnActivateNextWindow() is a menu command handler for 
// CMainFrame class, which in turn is a CMDIFrameWnd-derived class. 
// It activates the child window immediately behind the currently  
// active child window and places the currently active child window  
// behind all other child windows.  
void CMainFrame::OnActivateNextWindow() 
{
   MDINext();
}