Re: signals and copy constructors

[Antonio Coralles <noche suapie reflex at>]

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.