libsigc++ (a.k.a. libsigcplusplus) implements a typesafe callback system for
standard C++. It allows you to define signals and to connect those signals to
any callback function, either global or a member function, regardless of whether
it is static or virtual.

libsigc++ is used by gtkmm to wrap the GTK signal system. It does not depend on
GTK or gtkmm.  sigc++-2.0 and sigc++-3.0 are different parallel-installable
ABIs.
