The Boost.Signals2 documentation at http://www.boost.org/doc/libs/1_45_0/doc/html/signals2/thread-safety.html says: "Suppose a user writes a slot which connects another slot to the invoking signal. Will the newly connected slot be run during the same signal invocation in which the new connection was made? The answer is no." I'm trying to find out what Boost.Signals has to say about it. A quick test program shows that Boost.Signals does call the newly connected slot. I'm not sure though if this is undefined behavior as connecting another slot might invalidate the iterators used by the combiner? Boris