[Nat] The difference between a void slot and one with a return value is that in the latter case, the signal needs a combiner to figure out what to do with multiple returns from multiple slots. See:
http://www.boost.org/doc/html/ch06s02.html#id2520177
I haven't actually tried your example, but it looks to me as though the default combiner isn't working for you. But then you probably want something other than the default anyway -- unless you don't care about the values returned by your slots -- in which case why not use void slots?
But I think the signal doesn't care about whether or not there is any slot connected to it, it just fire the signal and handle the return value if there is any. Why it will crash when there is no slot connected to it. Wang Yun