Re: [sigc] sigc::trackable inaccessible base error

["Andrew E. Makeev" <andrew solvo ru>]

Rupert Swarbrick wrote:

> I've seen various "inaccessible base errors" in searching on Google, but
> I couldn't quite find this one and I don't understand quite what's going on.
>
> I have a class, FileEntry (which consists of an entry widget and a
> button that spawns a file selection dialog from Gtkmm):
>
> class FileEntry : protected Gtk::HBox
> {
> public:
>  FileEntry(bool open=true,
>            std::string defaultname="");
> protected:
>  void on_button();
>  ...stuff...
> };
>
> (Note that it inherits from _protected_ HBox)
>
> Now, in FileEntry::FileEntry, I need to connect the button clicked event
> for said button to a handler. Thus I create the slot with this line:
>
> sigc::slot<void> sl = sigc::mem_fun(*this, &FileEntry::on_button);
>
> Sadly, I now get these g++ error:
>
> /usr/include/sigc++-2.0/sigc++/type_traits.h: In instantiation of 'const
> bool sigc::is_base_and_derived<sigc::trackable, rjs::FileEntry>::value':
> /usr/include/sigc++-2.0/sigc++/functors/mem_fun.h:1792:   instantiated
> from 'sigc::bound_mem_functor0<void, rjs::FileEntry>'
> FileEntry.cc:11:   instantiated from here
> /usr/include/sigc++-2.0/sigc++/type_traits.h:132: error:
> 'sigc::trackable' is an inaccessible base of 'rjs::FileEntry'
>
> Now then, I can compile happily by inheriting from public Gtk::HBox, but
> this really doesn't seem like it should be necessary - surely as we're
> in the constructor the public/private/whatever should make no difference?
>
> I'd really like to know what the heck's going on here - does mem_fun do
> something very wierd?
>  
Please, consider my answer like a guess.

"sigc::mem_fun" is a wrapper that construct sigc::slot object. Last one 
should have public access to your object sigc::trackable destructor to 
disconnect slot/signals when your object destroyed. Probably, you better 
do investigate source codes either documentation files for more information.

Regards,
-andrew