Re: [sigc] binding a arguement which is a reference

[Martin Schulze <martin-ml hippogriff de>]

Am 2004.02.03 14:07 schrieb(en) Paul Elliott:
> I found in the archives the following message: See the end of this
> post:
>
>
> I am using sigc++-1.2 and I get a similar message to that mentioned
> below
> whenever the parameter I am trying to bind is a reference
> in the method mentioned in the "slot(...)" call.
>
> Is this a restriction or a bug?? Does the code need to be fixed
> or a note put in the documentation the the parameter that you bind
> can not be a reference in the called method?

It is a restriction that will be lifted in sigc++-2.0. You might want
to test the latest beta release sigc++-1.9.13. Of course, you are
welcome to patch the documentation for sigc++-1.2!

> Could this be an example of the famous reference to reference
> problem??

If I remember correctly, the problem with sigc++-1.2 is that the
bind adaptor explicitly calls the destructors of its bound objects.
This doesn't work when the objects are references.

> I made a workaround in my code by changing the called method,
> replace the referenced type T& to boost::reference_wrapper<T>.
> I got reference_wrapper from the boost libraries.
> The referenced_wrapper type has "value" semantics in that it
> has default constructors, copy constructors and assignment operators.
>
> What is the official libsigc+ approach to this problem?

In sigc++-2.0 you would pass the object through sigc::ref()
so that sigc::bind() can deduce that the value should be bound as
a reference and not as a value.

Cheers!

 Martin



> Quoted message follows:
> ==================cut here with a chain saw==================
> From: gtkmm mailing list account <gtkmm-list bernard-hugueney org>
>
> To: Martin Schulze <martin-ml hippogriff de>
>
> Cc: gtkmm-list gnome org, libsigc-list gnome org
>
> Subject: [sigc] Re: binding by value, slot taking reference was Re:
> [gtkmm] binding arguments to slots
>
> Date: Tue, 18 Nov 2003 11:23:31 +0100
>
>
>
>
> Le Mardi 18 Novembre 2003 10:26, Martin Schulze a écrit :
> > >
> > > There is still something I don't understand with slot and bind :
> > >
> > > I would like to make a slot for a function taking arguments by
> > > reference:
> > > program :
> > > "
> > > using namespace std;
> > > using namespace SigC;
> > >
> > > void foo1(int& i){++i;}
> > >
> > > int main(int argc, char** argv){
> > > int i=1;
> > > Slot0<void>slot1=bind(slot(foo1),i);
> > >  return 0;
> > > }
> >
> > Try to specify the argument type as template parameter, i.e.:
> >
> >    Slot0<void> slot1 = bind<int&>(slot(foo1), i);
>
> I shoudl have said I already tried that :-(
> >
> > I'm not sure if it works, though - let us know!
>
> same error:
>
> /usr/include/sigc++-1.2/sigc++/bind.h: Dans static member function  
> �
>
> static    void SigC::AdaptorBindData1_<C1>::dtor(void*) [with C1 =
> int&] :
> /usr/include/sigc++-1.2/sigc++/bind.h:167:   instantiated from
> `SigC::Slot0<R> SigC::bind(const SigC::Slot1<R, C1>&, A1) [with A1 =
> int&, R = void, C1 = int&]'
> testLibSigC.cxx:9:   instantiated from here
> /usr/include/sigc++-1.2/sigc++/bind.h:116: error: destructor name
> `~int&' does    not match type `int' of expression
>
> >
> > BTW in libsigc++-2.0 you will be able to write:
> >
> >    sigc::slot<void> slot1 = sigc::bind(foo1, sigc::ref(i));
> >
> > instead.
>
> Cool, I'll be missing my boost libs less :-)
> (For the record, I also tried boost::ref(i) to no avail :
> /usr/include/sigc++-1.2/sigc++/bind.h: Dans static member function
> static   void SigC::AdaptorBindData1_<C1>::dtor(void*) [with C1 =
> int&] :
> /usr/include/sigc++-1.2/sigc++/bind.h:167:   instantiated from
> `SigC::Slot0<R> SigC::bind(const SigC::Slot1<R, C1>&, A1) [with A1 =
> boost::reference_wrapper<int>, R = void, C1 = int&]'
> testLibSigC.cxx:11:   instantiated from here
> /usr/include/sigc++-1.2/sigc++/bind.h:116: error: destructor name
> `~int&' does not match type `int' of expression
>
> Now I'm really holding my breath for libsigc++-2.0 in gtkmm.
>
> Thanks again for your kind help !
>
> Bernard
>
> --
> Paul Elliott                       1(512)837-1096
> pelliott io com                    PMB 181, 11900 Metric Blvd Suite J
> http://www.io.com/~pelliott/pme/   Austin TX 78758-3117