Re: Container reqs

[John English <je@brighton.ac.uk>]

Ted Dennison wrote:
> 
> In article <3BCD7C50.647ADBD0@brighton.ac.uk>, John English says...
> >
> >of controlled). And you can use tricks to get around the library-level
> >problem, e.g. have a library-level parent package which defines a private
> >controlled type and generic children which make use of those controlled
> >objects as components in other composite types...
> 
> This sounds a bit like the trick Dr. Dewar keeps referring to. I know this is
> off the thread's topic, but could you give a specific example of how this works?

I do something like this in JEWL. There's a reference-counted tagged
type in package JEWL (non-generic parent):

  type Reference_Counted_Type is tagged limited
    record
      Count : Natural := 1;
    end record;

  procedure Cleanup (Object : in out Reference_Counted_Type);

  type Reference_Counted_Ptr is access all Reference_Counted_Type'Class;

  type Controlled_Type is new Ada.Finalization.Controlled with
    record
      Pointer : Reference_Counted_Ptr;
    end record;

  procedure Finalize (Object : in out Controlled_Type);
  procedure Adjust   (Object : in out Controlled_Type);

and then JEWL.Windows (which is generic) uses this Controlled_Type as
a component (actually *the* component) in all the window types, so that
all windows are implemented as pointers to reference-counted types.
The actual object that a particular type of window points to is one
of a variety of possible derivations which are defined in a private
child and produced by constructor (factory) functions.

> I could never see how one could use a controlled-type record field to manage
> anything in its parent object, about which it theoreticly knows nothing.

You can also use access discriminants to get at the containing record,
but I didn't need to in this case... This is presumably what you're
after? There's a description of the technique near the end of section
4.6 in part 2 of the Rationale:
  http://burks.bton.ac.uk/burks/language/ada/rat95/rat2_4.htm#6

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