Why would this core dump??
LinuxChessboardWidget is defined as:
class LinuxChessboardWidget : public cwmm::ChessPositionWidget, public Gio::ActionMap
{
...
I have to derive from Gio::ActionMap because its constructor is protected.
The call to `add_action` above is to that class.
Can someone tell me what I'm doing wrong?
Carlo
Gio::ActionMap is an interface. That's why its constructor is protected. An interface can't be instantiated by itself. It must be implemented by a class which can be instantiated. If you really must implement Gio::ActionMap in your class (which I doubt), there is one or (probably) two errors.
First, interfaces must come before the widget class in the list of base classes.
class LinuxChessboardWidget : public Gio::ActionMap, public cwmm::ChessPositionWidget
This is opposite to what's done in gtkmm's classes that wrap gtk classes.
Second, your constructor must call a special Glib::ObjectBase constructor that registers your class with its own GType in glib's GType system. For instance
LinuxChessboardWidget(......) : Glib::ObjectBase("myWidgetClass") ..... // Or whatever you want to call it.
This is an unusual way of using Gio::ActionMap. I don't know if these are the only necessary changes. Much more common is to use one of the classes that already implement Gio::ActionMap: Gio::SimpleActionGroup, Gtk::Application (via Gio::Application) or Gtk::ApplicationWindow.