Re: [BLACKBOX] Sorting Extended Records

From: [at]} <Alexander>
Date: Tue, 29 Mar 2011 00:12:43 +0600

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Hello!

SRT>> In Component Pascal I wrote the common part of the records as an
SRT>> Extensible (base) record type, and then each of the three record
SRT>> types as extensions of that base type.

AI> Why use extenstions and not the base type?

  Sorry, I misquoted, that question was aimed at the following sentence:

SRT> I refer to each of the three binary trees by a root variable
SRT> which is a pointer to the corresponding extended record type.

  Why use extenstions and not the base type?

SRT> HOWEVER, in general sorting and balancing a binary tree causes
SRT> the root to change, and Component Pascal will not allow assigning
SRT> the value of a pointer to a base record, back to a pointer to an
SRT> extended record. Apparently this means I have to include three
SRT> separate but nearly-identical versions of the sorting routine,
SRT> one for each extended record type, which seems incredibly ugly,
SRT> inefficient, and error-prone. I am having trouble seeing how
SRT> this "feature" of BlackBox Component Pascal is an improvement.
SRT> Has anybody found a way to handle this issue? Any suggestions
SRT> would be much appreciated.

  Use type guard:
  VAR base: BaseType; extended: ExtendedType;
  BEGIN
      base := extended; (* Always works. *)
      extended := base(ExtendedType); (* Works only if 'base' is
      actually a pointer to an 'ExtendedType' instance, traps
      otherwise. *)

---===---
 Alexander

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Received on Mon Mar 28 2011 - 20:12:43 UTC