libsigc++ 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.

It also contains adaptor classes for connection of dissimilar
callbacks and has an ease of use unmatched by other C++ callback
libraries. Libsigc++ is licensed under the GNU Library General
Public License, LGPL.