gtkmm is the official C++ interface for the popular GUI library
GTK+. Highlights include typesafe callbacks, and a comprehensive set
of widgets that are easily extensible via inheritance. You can create
user interfaces either in code or with the Glade User Interface
designer, using libglademm. There's extensive documentation,
including API reference and a tutorial.

This package requires cairomm, which is also available from the
SlackBuilds.org project.
