xml-dev - Re: What is delegation

[David Brownell <db@Eng.Sun.COM>]

Dean Roddey wrote:
> 
> I'm sure you'll get some definitions, but here are
> some examples that may help...

See also my response of this morning, archived at

http://www.lists.ic.ac.uk/hypermail/xml-dev/9810/0245.html

Some technical motivations for delegating:

- Coping with situations where the interface isn't
  "exactly" the same (e.g. adapters that implicitly
  add another argument, extra setup needed, etc);

- Sometimes you need lots of objects to delegate to
  the same object, and yet have distinct identity.
  That object might be a flyweight, but to use it
  in some cases you might need to restore its weight.

- Base classes differ; client A needs something to
  call object B but needs its callback to derive from
  some concrete class.  It delegates to object C which
  is so derived, and which calls B.

- As Dean noted, "composite" objects include others
  with "has-a".  Distributed systems often work that
  way ... a user account object "has-a" home directory,
  a password, a set of files.  But those are objects in
  their own right, so "is-a" (subtyping) is wrong.

I've not found "aggregation" to be helpful except when
talking about COM, but that may be because COM has rules
about aggregating that don't apply to more classic object
composition/delegation schemes.  And of course, everyone
has their own ways to slice'n'dice design problems.

- Dave

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