[sigc] slots without functors

[Paul Davis <paul linuxaudiosystems com>]

libsigc++ 1.2 formed the basis of a very, very powerful and flexible
undo/redo system that is used within ardour (http://ardour.org/). at the
core of it (developed with Brett Viren), was the use of

	typedef SigC::Slot0<void> UndoAction;

We then simply stored a series of UndoActions in a pair of lists, one
for undo, one for redo. Anything you could package up into the functor
represented by a SigC::Slot0<void> (i.e. anything) could be inserted
into the lists.

We are just switching to the current version of libsigc++, and i am
finding that it appears not to allow slots to be declared without a
functor argument. ie, given the above typedef,

	UndoAction something_to_undo (mem_fun (*object,  &Object::method)); 

is fine, but
 
	UndoAction something_to_undo;

	something_to_undo = mem_fun (*object, &Object::method);

is not, because something_to_undo has no zero-argument constructor.

This same design is also the basis of a similarly flexible key binding
system, in which key events in a GUI trigger actions stored as
SigC::Slot0<void>.

while the no-zero-argument-constructor might arguably be a good design,
it is a bit limiting for some cases like this. why? because we have
objects that have UndoActions and/or KeyActions as member variables that
are created without any arguments, then assigned later in the life of
the object.

is there a way to create a sigc::slot<void> with no arguments?

thanks,
--p