Re: Assignment Overload in Ada-9X

[eachus@dr_no.mitre.org (Robert I. Eachus)]

In article <11815@prijat.cs.uofs.edu> beidler@guinness.cs.uofs.edu (Jack Beidler) writes:

  > I recently received a copy of the "Ada 9X Project Report dated
  > February 1993.  On the top of page 48 in this report there is a
  > statement about overloading assignment, specifically,

  >	   ".., in the case of non-limited control types, assignment
  >	   can be redefined by the user."

  > I curious about the form of the overload.  Is it something like

  > procedure ":=" (Left : in out Some_Type ; Right : in   Some_Type) ?

  > or have I misread this statement?

     No, it is not.  And yes, you have misread the statement: control
should be controlled, but that is a detail.

     The way it works is that there are three operations inherited by
non-limited controlled types from the root type: INITIALIZE, FINALIZE
and CLONE.  (Well CLONE in ILS 1.2, I believe Tucker intends to change
the name.)  In any case, by redefining these operations when deriving
from CONTROLLED, you can obtain the effect of modifying assignment.
Of course, when defining these procedures, there are a lot of
operations on the new type that will cause indefinite recursion, etc.
But used properly, it allows for nice packaged abstractions where
assignment does all the special things required.  For example, it is
(relatively) easy to define a reference-counting type or a general
garbage collected type using this mechanism.

     Actually, now that this feature is relatively stable, I may post
those examples for experts to shoot at.  In either case it it about 20
lines of very sophisticated code that looks simple.  It might even be
a good idea to have both of these classes (oops! :-) as child packages
in the standard.


--

					Robert I. Eachus

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