Effectively, the default
FocusinPicture::output(). See Picture Reference; Outputting; Functions. It's not really the default, but the version ofoutput()that doesn't take aFocusargument calls another version that does take one, passingdefault_focusto the latter as itsFocusargument.It's necessary to do this in such a roundabout way, because
Picture::output()must be declared beforeclass Focusis completely defined anddefault_focusis declared.The declaration ‘Focus& f = default_focus;’ makes
fa reference todefault_focus, i.e., it makesfanother name fordefault_focus. This may be convenient, if you don't feel like typingdefault_focus.