pointers & OOP

[tmoran@bix.com (Tom Moran)]

In GNAT chat, John Robinson said:
> It is difficult to do anything really useful with Ada 95 OOP features
> without the use of pointers.
Robert Dewar pointed to finalization actions as just one example
of using OOP without pointers, and Matthew Heaney said:
> ... it would be helpful to have a list of typical examples.
so here's another:
  Sometimes, perhaps for task-safety or resource ownership
reasons, the task that wants something done can't do it, but
needs to ask a different service task to do the job.  Rather
than have a proliferation of 'entry's in the service task, one
per type of job, make a single service entry that takes a
"Job:in out Job_To_Do'Class" parameter and whose 'accept' merely
calls a "Do_It(Job)" procedure.  Any time some new type of job
needs to be done, simply declare a child of Job_To_Do, with
appropriate places in the record extension for parameters, and
overide procedure Do_It(Job : in out Job_To_Do).  Then a call
to the service entry will dispatch to the desired Do_It routine
so the service task will actually be the one executing the Do_It
code.  Essentially you are using Job_To_Do's dispatch table to
pass a procedure pointer, and using the record extension to
hold the parameters, but you are getting the compiler to build,
and check, everything instead of relying on screwup-able
explicit pointers.