Re: Mutually dependent private types

[John Volan <johnv@ac3i.dseg.ti.com>]

Jerry van Dijk wrote:
> 
> So perhaps you just found a new class:
> 
>         Office_Employee_List
> 
> :-)

This was offered in jest, but let's consider it seriously:

One way to implement an cross-referencing association between two
classes like Employee and Office could be to keep both classes ignorant
of the association and just have a third class that is entirely
responsible for keeping track of all the cross-references (in some sort
of table structure).  Yes, that is a valid design alternative, but there
are tradeoffs, and it doesn't work for all applications.  In particular,
if Office and Employee have primitive/inheritable/overridable operations
that take parameters of each other's type, then there is no way that the
two classes can avoid knowing about the association.
 
Also, this kind of cross-referencing association isn't the only way two
classes might be forced into mutual dependency.  My Doctors and Patients
example in my FAQ didn't involve any cross-referencing between Doctor
and Patient objects, but it did involve mutually-dependent primitive
operations.

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