In gtkmm 1.2, I used g_io_add_watch() to watch file descriptors.
In gtkmm 2, I can use signal_io() and have the advantage to use
SigC::Connections instead.

However, g_io_add_watch() passes a GIOChannel to it's callback function
which can be used to read/write from. This feature is lacking with SignalIO
/ signal_io()

E.g.:

  g_io_add_watch(g_io_channel_unix_new(filedescriptor),
         GIOCondition (G_IO_IN),
         &Class::callback, this);                           

The prototype for Class::callback would then be:
static gboolean Class::callback(GIOChannel* iochannel, GIOCondition cond,
gpointer data);

Here, the iochannel can be used to read/write from.

With signal_io():

  signal_io().connect(
        slot(*this, &Class::callback),
        filedescriptor, Glib::IO_IN);

The prototype will look like this:

bool Class::callback(Glib::IOCondition cond);

In the last example, I don't have the possibility to read from the file
descriptor or a iochannel. This forces me to make 'filedescriptor' a member
of the class, and I can't reuse the same callback for many different
filedescriptors.