When signalling, with no slots connected, an assertion is raised (if the return type of the slot is not void). For example: boost::signal<void ()> sig; sig(); does "nothing" while boost::signal<int ()> sig; sig(); asserts (if the default combiner is used). I guess that makes sense, because there can be no "combined" return value if there are no slots. However, an assertion seems a bit harsh. I am really not sure what should happen, but maybe an exception is a better alternative, or maybe an alternative interface can define a default value to be returned if there are no slots connected), or maybe something else... Anyway, I guess I was wanting to know why this is the case, and what is the usual method of handling this circumstance. I assume something like: if (!sig.empty()) { result = sig(); } but that just does not look pretty...