Re: 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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