[sigc] Re: signals and copy constructors

[Martin Schulze <martin-ml hippogriff de>]

Am 04.01.2005 15:16:51 schrieb(en) Antonio Coralles:
> Carl Nygard wrote:
>
> > On Mon, 2005-01-03 at 23:46 +0100, Antonio Coralles wrote:
> >> i know that this is more a libsigc++ question; but as gtkmm uses
> >> libsigc++ heavily and the libsigc++ mailinglist is not very
> populated i
> >> ask this question here too:
> >>
> >> is there an elegant way to "change  *p_obj in  mem_fun(*p_obj,
> ...)"
> >> after the signal is allredy attached to a concrete object , so  
> that
> when
> >> the copy constructor of that object is called,
> some_signal.emit(...)
> >> calls the method in the object created by this copy constructor
> call ?
> >> or is it better to think of a different design, so that this
> question
> >> doesn't come up ?
> >>
> >> to avoid misunderstandigs, here is somekind of example:
> >>
> >> //just a useless signal
> >> sigc::signal<void> someSignal;
> >>
> >> //just a useless class
> >> class Useless : public sigc::trackable
> >> {
> >>     public:
> >>     void call_me() { ... }
> >> };
> >>
> >> //a useless scope
> >> ....
> >> {
> >>     Useless u1;
> >>     someSignal.connect(sigc::mem_fun(u1,  &Useless::call_me));
> >>    //ok now connected - but how can i automatically change the
> target
> >> object to u2 - so that u2.call_me() is called due someSignal.emit 
> ()
> ?
> >>    Useless u2(u1);  //imagine this call is made at a comletly
> different
> >> part in my programm - for example in a clone method ....
> >>    ....
> >> }
> >>  ....
> >
> > This is a bad idea simply from the standpoint that C++ creates
> temporary
> > copies in a variety of situations, many of which you are not
> consciously
> > aware of.  I'd find a different design.
> >
> > BTW, you can also disconnect the signal if you store the connection
> info
> > returned when you call connect(), so you can manually disconnect
> and
> > reconnect (so long as you have the original signal).
> >
> Well, i guess you are right, I should find another way, allthough
> temporary copies aren't a problem, because i could make the copy
> constructor private and use it only in a virtual clone method ....;
> but
> to store the the connection and the signal in the object to wich the
> signal was connected seems messy and error prone.

I'm afraid that your initial idea is not supported by libsigc++2. You  
would need a list of signals your object was connected to. This is not  
available.

Regards,

 Martin