RE: Class hierarchy XM_CALLBACKS_FILTER

Hi Franck:

Thanks for the clarification.

Stephanie

> -----Original Message-----
> From: Franck Arnaud [mailto:[email protected]]
> Sent: Freitag, 30. Juli 2004 03:06
> To: [email protected]
> Subject: Re: [gobo-eiffel] Class hierarchy XM_CALLBACKS_FILTER
> 
> Stephanie Balzer:
> 
> > I wonder why the class XM_CALLBACKS_FILTER inherits from
> > XM_CALLBACKS_SOURCE, XM_CALLBACKS and XM_FORWARD_CALLBACKS. Since
> > XM_FORWARD_CALLBACKS already inherits from XM_CALLBACKS_SOURCE and
> > XM_CALLBACKS, I'd think it were enough that XM_CALLBACKS_FILTER only
> > inherits from XM_FORWARD_CALLBACKS.
> 
> > What were the reasons for that class hierarchy?
> 
> The comment gives a hint:
> 
>         XM_FORWARD_CALLBACKS
>                 -- implementation of default behaviour:
>                 -- forwarding to 'next' processor in chain
> 
> this is to indicate that XM_CALLBACKS_FILTER is an implementation
> of XM_CALLBACKS and XM_CALLBACKS_SOURCE abstract interfaces,
> which incidentally use XM_FORWARD_CALLBACKS as a default
> implementation. It is possible to remove the
> XM_FORWARD_CALLBACKS inheritance, and for example implement
> the default behaviour directly, without breaking client
> code. In other words, it says to clients:
> 
>   a_callbacks := a_callbacks_filter -- OK
> 
>   a_forward_callbacks := a_callbacks_filter
>       -- not OK, not guaranteed to remain valid.
> 
> Arguably it's a bit pedantic and could be written in
> comments or documentation instead.
> 
> This kind of pattern is also seen, perhaps more usefully,
> in cases like:
> 
> class BLAH
> inherit
>   ANY -- BLAH is a ANY
>   FOOBAR_ROUTINES -- BLAH uses some routines of FOOBAR_ROUTINES
>      export {NONE} all end
> 
> With some compilers it's necessary as the export {NONE} all
> also exports features of ANY to NONE -- which may be the right
> thing to do I think -- making say a_blah.is_equal illegal,
> if the explicit ANY is absent.
> 
> --
> http://www.nenie.org/
> 
> 
> 
> 
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> 





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