gtkmm (previously known as Gtk--) is a C++ interface for the popular GUI library GTK+. It provides a convenient interface for C++ programmers to create graphical user interfaces with GTK+'s framework. Highlights include typesafe callbacks, widgets which are extensible by inheritance, and many classes that can be easily combined to quickly create complex user interfaces.
libsigc++ implements a full callback system for use in abstract interfaces and general programming. 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 connecting dissimilar callbacks and has an ease of use unmatched by other C++ callback libraries.