gtkmm (previously known as Gtk--) is the official C++ interface for the popular GUI library GTK+. It closely follows GTK+ development. Highlights include typesafe callbacks, widgets extensible via inheritance and a comprehensive set of widget classes that can be freely combined to quickly create complex user interfaces. Here's an overview, with documentation and status. gtkmm is free software distributed under the GNU Library General Public License (LGPL).
Development is currently in progress on the unstable gtkmm 2.3 branch, which will become the new stable gtkmm 2.4. gtkmm 2.3/2.4 can be installed in parallel with the previous stable gtkmm 2.2. The stable (API/ABI-frozen) gtkmm 2.4.0 will be released soon after GTK+ 2.4.0. We follow the official GNOME Platform Bindings release schedule.
See attached ebuild.
P.S.: This ebuild installs into SLOT="2.4", because it's meant to co-exist with gtkmm-2.2 branch (see previous paragraph)
Created attachment 28026 [details]
An ebuild file for gtkmm-2.3.6 package
Prerequisites for this new gtkmm are in contradiction to the second paragraph of my initial comment of this bug. It needs gtk+-2.4 installed, which is meant to replace gtk+-2.2 branch. Therefore, if you install gtk+-2.4 on your system, gtkmm-2.2 has no good reason to stay there - only the new gtkmm is of some use there.
I believe this will be sorted out by the time they get to gtkmm-2.4 (i.e. stable). In the meantime, I think the SLOT="2.4" should be preserved with gtkmm-2.3.6.ebuild, since it *is able* to coexist with 2.2 branch.
Created attachment 28048 [details]
A modified ebuild file for gtkmm-2.3.6
I have removed an invalid dependency on libsigc++-1.2.x - gtkmm itself doesn't
depend on it, glibmm (a dependency of gtkmm) does depend on a new version of
2.4 got added