About gtkmm: C++ interfaces for GTK+ and GNOME!
More Information: https://www.gtkmm.org/
Steps:

1. Open Terminal by pressing command+space, type terminal and hit Enter key.
2. Run

   ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install)" < /dev/null 2> /dev/null

3. Install

   brew install gtkmm

Done!

More about gtkmm:

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 Gtk::Builder. There’s extensive documentation, including API reference and a tutorial.

gtkmm is free software distributed under the GNU Library General Public License (LGPL).

gtkmm follows the official GNOME Platform Bindings release schedule. This guarantees API/ABI-stability and new releases on a predictable schedule, delivering C++ API for the underlying GTK+ and GNOME APIs as soon as possible.