This library implements a full callback system for use in widget
libraries, abstract interfaces, and general programming. Originally
part of the Gtk-- widget set, libsigc++ is now a separate library to
provide for more general use. It is the most complete library of its
kind with the ability to connect an abstract callback to a class
method, function, or function object. It contains adaptor classes for
connection of dissimilar callbacks and has an ease of use unmatched by
other C++ callback libraries.

Package GTK-- (gtkmm), which is a C++ binding to the GTK+ library,
starting with version 1.1.2, uses libsigc++.