[sigc] Return values in libsigc++ 2.0

["Timothy M. Shead" <tshead k-3d com>]

I am evaluating what the upgrade from 1.0/1.2 to 2.0 would look like for 
an existing application, and ran across one small snag.  In libsigc++ 
1.0/1.2 it was possible for a custom marshaller to specify the default 
value returned when a signal doesn't have any connections.  In 2.0 the 
behavior is to return a default-constructed object (the accumulator 
object is never even created in this case).  Unfortunately in my case 
(slots that return bool) this is undesirable.  While I could work around 
this by testing signal.empty() before calling signal.emit(), this would 
mean spreading some of my signal-related logic throughout client code, 
and is a bit of a step backwards.  Is there an explicit mechanism for 
handling this, or should I derive from signal and override emit() ?

Many thanks,
Tim Shead