In Visual Studio 9 at warning level 4, I get the following warning when I compile the Boost.Signals library via the header <boost/signals.hpp>: 1>c:\it\personal\rocket_test\sdk\boost\boost\signals\trackable.hpp(163) : warning C4512: 'boost::signals::detail::bound_objects_visitor' : assignment operator could not be generated 1> c:\it\personal\rocket_test\sdk\boost\boost\signals\trackable.hpp(66) : see declaration of 'boost::signals::detail::bound_objects_visitor'