According to the maintainer of gtkmm:
I will not continue forever with maintenance of gtkmm and other *mm modules. Unless someone else continues the work, glibmm, gtkmm and other modules will be abandoned in a near future.
Therefore, what should us C++ developers do to prepare our apps? Is there another gtk language binding for C++? Will there be someone else willing to maintain this project? Should we just created our own wrapper classes and call the C API in our code instead?