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From: Simon Wright <simon@pushface.org>
Newsgroups: comp.lang.ada
Subject: Re: Pop function
Date: Thu, 15 Dec 2011 18:34:53 +0000
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Date: 2011-12-15T18:34:53+00:00
List-Id: <comp.lang.ada>

Adam Beneschan <adam@irvine.com> writes:

> On Dec 15, 4:43 am, Simon Wright <si...@pushface.org> wrote:

>> I don't see why Pop takes T_List_Ptr as an argument. I would have
>> written
>>
>>     procedure Pop (Sender : in out T_List;
>>                    T_Item : out Integer) is
>>
>> and then it would be primitive (assuming T_List is tagged).

> You could quibble about whether T_List is an appropriate name to
> represent a list element.  Fine.  I would too.  But I think our first
> objective should get our OP to help write code that works, which
> involves making sure he (I assume) understands the Ada language issues
> involved, and not adding to any confusion he might have.  We can
> quibble about the names, readability issues, encapsulation issues,
> etc., later.

I see what you mean; you've realised that OP is probably confused,
whereas I just got confused myself.

I agree about writing code that works, but that does sometimes involve
working out how best to state the problem. I don't think that it's
quibbling to point to unclear thinking; if the design is wrong that's
not an Ada language issue, and using inappropriate names is a red flag
for design problems.

Here, for example, if I write

      L : aliased T_List;
      ...
   begin
      ...
      J := Pop (L'Access);

I'm going to get a crash at runtime because I'm trying to deallcate
memory that was never allocated.

I have in the past implemented lists where the 'head' element was just
another member which -- by convention -- wasn't part of the list. That
sort of thing needs to be done by consenting adults in private, though.