I've experienced the same problem discussed on June 3rd titled "Bug, signals, use of default Combiner" http://thread.gmane.org/gmane.comp.lib.boost.devel/103903 . I would have just replied except its a little old, thus a new thread. Doug Gregor wrote:

We intentionally did not return a default-constructed value because the returned type might not have a default constructor. Granted, this case might be sufficiently rare to justify using your version of last_value.

I would like to say a few things about this and offer an alternative solution. First, I think not returning a default constructed value is the correct approach with the last_value combiner for the reasons you've mentioned. I don't think the bug is with the last_value combiner, but with the fact that the last_value combiner is the default combiner (ok an exception rather than an assert would probably be better). However, I would venture to claim that a signal being called without any slots is not a rare case. In fact I'd guess that might be the most likely case. Certainly, as a library writer, I cannot assume that all the signals I provide to my users are going to be used in every application. My suggestion is to provide a default combiner that does the bare minimum, just call the slots: namespace boost { struct default_combiner { typedef void result_type; template<typename InputIterator> result_type operator()(InputIterator first, InputIterator last) const { while (first != last) *first++; } }; } I believe this would solve the problem without restricting use of default signals to objects with default constructors, or to signals with at least one slot connected.