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