Re: [BLACKBOX] Polymorphism

From: [at]} <Alexander>
Date: Sun, 23 Nov 2008 18:07:51 +0600

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Hello!

AMAUN> I replaced the VAR record statements with pointers. Primarily I did
AMAUN> this because I can't call a method from an uninitialized pointer, so
AMAUN> there are fewer tests needed for good data.

  On the other hand, static RECORDs (not POINTERs) are allocated on
  stack, so there is zero dynamic memory overhead (no NEW/GC
  involved). The stack memory is released upon the exit from the
  procedure where the RECORD local variable is declared. Using static
  RECORDs is almost the same as passing their contents as parameters
  to the Do procedure, which they are, essentially, in this case.

  Yes, using POINTERs you will be "caught by the hand" if you pass an
unitialized NIL
  parameter, but even if you allocate the Helper with NEW, there is
  still no guarantee that it is initialized (you may forget to call
  Init, or whatever).

  There are two ways around it:
  1) always use a creation routine which performs initialization:
    PROCEDURE NewHelper (params): Helper;
    VAR res: Helper;
    BEGIN
         NEW (res);
         res.params := params (* initialization *)
         RETURN res
    END NewHelper;

    If you don't create instances of the Helper class outside the
    NewHelper procedure, this way guarantees that every existing
    (#NIL) instance is properly initialized;

  2) use ABSTRACT procedure Init, thus forcing every descendant class
  to implement it. But then you have to use a special creation procedure
  that makes sure the Init is actually called after NEW.

  The second approach sounds like too much for such a small and
  insignificant object with very short life span. The first approach,
  on the other hand, also works for the VAR-objects:
    PROCEDURE NewHelper (VAR res: Helper; params);
    BEGIN
         res.params := params (* initialization *)
    END NewHelper;

  In short: there is next to no added value in using POINTER TO
  objects in this case, but there is the added dynamic memory overhead
  (find a memory block, then release the memory block; memory fragmentation).

>> You may also consider "Action" instead of "Doer".
AMAUN> I decided on "Helper" but I'm not fully satisfied.

  "Iterator" is another option.

---=====---
 Alexander

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Received on Sun Nov 23 2008 - 13:07:51 UTC