Effectively, the default |
See Picture Reference; Outputting; Functions.
It's not really the default, but the version of
output() that doesn't take a
Focus argument calls another version
that does take one, passing
default_focus to the
latter as its
It's necessary to do this in such a roundabout way,
Picture::output() must be declared before
class Focus is completely defined and
default_focus is declared.
Focus& f = default_focus; makes
default_focus, i.e., it makes
f another name
default_focus. This may be convenient, if you don't feel