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From: Adam Beneschan <adam@irvine.com>
Newsgroups: comp.lang.ada
Subject: Re: Table of pointers question
Date: Thu, 24 Sep 2009 07:55:42 -0700 (PDT)
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Date: 2009-09-24T07:55:42-07:00
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On Sep 23, 11:59=A0pm, Georg Bauhaus <rm.tsoh.plus-
bug.bauh...@maps.futureapps.de> wrote:
> Rob Solomon wrote:
> > I am working my way thru Ada As A Second Language by Norman Cohen (c)
> > 1996
>
> > This confuses me.
>
> > It is a simple sorting routine that swaps pointers rather than the
> > data. =A0Note that the variables are more like Modula-2 syntax as I am
> > very comfortable with that. =A0And it is easier to type.
>
> Has something gone wrong when (I'm guessing here) the
> program was rewritten for Ada? =A0The compiler shows some
> errors. =A0After correcting these formally, i.e. without thinking,
> it confirms Adam's observation, and hints to others:
>
> Compiling: printdirectory.adb (source file time stamp: 2009-09-24 06:42:5=
0)
>
> =A0 =A0 25. =A0 EntryList =A0 =A0 =A0 =A0: array (1..MaxEntries) of Direc=
toryEntryType;
> =A0 =A0 =A0 =A0 =A0 |
> =A0 =A0 =A0 =A0 >>> warning: variable "EntryList" is not referenced
>
> =A0 =A0 33. =A0 =A0 NewEntry.NamePart :=3D To_Bounded_String(Buffer(1..Le=
ngth));
> =A0 =A0 =A0 =A0 =A0 =A0 |
> =A0 =A0 =A0 =A0 >>> warning: "NewEntry" may be null
>
> Here, the "correction" was
> 1 - =A0END IF; =A0-- semicolon, not colon
> ...
> 2, 3 - =A0 =A0EntryPointerList(NumberOfEntries) :=3D -- *new*
> DirectoryEntry*Pointer*Type'(NewEntry); =A0-- MY QUESTION HERE
>
> (NewEntry is of a pointer type already.)

That's the error.  NewEntry should not be a pointer type.  The intent
appears to be that a record of type DirectoryEntryType is built, and
then a copy of it allocated on the heap and the pointer stored in the
array.  (I'm assuming that's the case; I don't have Cohen's book
handy.)

So the declaration of NewEntry is wrong.  Perhaps that has to do with
a misunderstanding of the relationship between pointers and data; but
I think it's more likely just a typographical error, made harder to
spot by the lack of underscores.

                                     -- Adam