Re: signals and copy constructors

[Carl Nygard <cjnygard fast net>]

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).