Description:

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++20 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++20.