Re: New New Callbacks Uploaded

From: William Kempf (sirwillard_at_[hidden])
Date: 2000-11-22 16:15:39

--- In boost_at_[hidden], Karl Nelson <kenelson_at_e...> wrote:
> 
> [....]
>  > > 2) Second it makes for really poor code clarity as the class 
will
> > > have to implement all of the operator() even through it uses 
> > > only one of them...
> > > 
> > >   class callback {
> > >     ...
> > >     return_type operator()() { ... }
> > >     return_type operator()(P1 p1) { ... }
> > >     return_type operator()(P1 p1,P2 p2) { ... }
> > >     return_type operator()(P1 p1,P2 p2,P3 p3) { ... }
> > >     return_type operator()(P1 p1,P2 p2,P3 p3,P3 p3) { ... }
> > >     ...
> > >   };
> > 
> > The callback class doesn't have to implement all operator() 
overloads, though the alternative requires deriving from a base class 
which has the proper overload. The first option is slightly more 
readable for the "callback" class itself, the second option gives the 
best error messages (since only one operator() exists).
> 
> I don't believe that deriving directly will work, or at least I 
haven't
> seen one without the construction I showed above.

Look at Doug's implementation.  All that would be required is to 
derive from the "impl" class instead of containing it.  He's done the 
work or factoring out the unneeded overloads using a simple bit of 
meta programming.

> > > 5) It caps the number of callback which limits the application 
> > > somewaht.  That is with the callback#,  the user can always call
> > > the macro generator to produce another set of classes for a 
> > > larger number of parameters.
> > 
> > Not necessarily. In my original event library, I used this 
technique and had scripts to generate the event classes. The only 
reason this has yet to be done for any of the current callback 
library candidates is that it is less manageable in the prototype 
stage.
> 
> 
> However, this won't link unless all the code is compiled against 
the 
> same number.   

In general, I don't think this is that big of an issue.  If you 
supply a reasonable number to begin with, it's very unlikely that 
more will be needed.  It's generally bad programming practice to 
write functions that take large numbers of arguments to begin with.  
Generate a callback that takes up to 10 arguments and I think there's 
little danger of someone needing more. 

Bill Kempf