gmmproc is a script used by most of C++ bindings of GNOME stack (called also mm-modules). It is responsible for generating actual C++ headers and sources from templates. That is so, because wrappers for most C API are straightforward, so writing them manually would be just a tedious job. Instead of that we just provide C API information, specify what names should C++ wrappers have and how to get a C instance from C++ one and vice versa and all happens automagically. Even documentation from C sources is taken and translated to use C++ names.
1 or 2 years or so ago.
Some time ago I was thinking about refactoring gmmproc. I was not the only one, because there were several notions that a rewrite could be welcome. I even made a topic on gtkmm-list about it and created a page at live.gnome.org. Main points would be using GIR as a source of API information and documentation, and getting rid of m4 as it makes it hard to add new features. I had some grand plans like creating two backends - one for defs (old API information) and GIR (new API information), creating conversion files on-the-fly, some configuration file a'la Doxygen, blah blah blah. Eventually I started some coding in Python, drawed some diagrams in my sketchbook, switched to C++ and then Perl and suddenly development slowed to molasses, because I already got bored. Apparently writing API structures is really uninteresting. Especially when it goes really slow and no end of it is in sight.
Now and ongoing.
I got back to it recently with a firm decision that I will finish it. I got over API structures by generating them basing on GIR definition I had to deduce from some python code in gobject-introspection (yeah, I wrote my own definition, because there seems to be none of such thing upstream - documentation is pitifully out of date). Also, most of the parser is also generated, but most important things are hand-written. Then I switched to refactoring a WrapParser and Output, which are 10 years old Perl code, added some needed infrastructure replacing m4, converted _CLASS_GOBJECT from m4/defs to Perl/GIR, added a scanner finding C <-> C++ type equivalents. My first aim is to have any compilable code being generated. For that still much work left to be done - mostly understanding and translating rest of m4 code to Perl (booooring!) and writing C <-> C++ conversion code (the one saying how to get `GtkWidget*' from `const Gtk::Widget&'; slightly less boring, but still).
Future plans are... no no no, no future plans for now. I did that before and got baffled by amount of work that needs to be done. Slowly all needed features will be introduced.
All of the code sits in gmmproc-refactor branch of glibmm. Additions here are mostly a bunch of kilo-line commits happenning once for two-three weeks, because I work on it offline at home in my free time. In the end all of it is going to be probably squashed into single commit before merging it to master.
I am usually pushing changes to repository when my single patch grows to at least 500kb. Now it has only 200 kb, so you have to wait. :)