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From: Stephen Leake <Stephen.Leake@gsfc.nasa.gov>
Subject: Re: Please Help.
Date: 1997/09/17
Message-ID: <342005BE.5C4E@gsfc.nasa.gov>#1/1
X-Deja-AN: 273301009
References: <01bcbcde$f8a425c0$ca70fe8c@default> <dewar.873989183@merv>
 <5vaude$q20$1@goanna.cs.rmit.edu.au> <5vbiid$luu$1@goanna.cs.rmit.edu.au>
 <34197042.7CFC@gsfc.nasa.gov> <dewar.874165401@merv>
 <341D4041.4AF8@gsfc.nasa.gov> <dewar.874434176@merv>
Organization: NASA Goddard Space Flight Center -- Greenbelt, Maryland USA
Reply-To: Stephen.Leake@gsfc.nasa.gov
Newsgroups: comp.lang.ada
Date: 1997-09-17T00:00:00+00:00
List-Id: <comp.lang.ada>


Robert Dewar wrote:
> 
> Stephe said:
> 
> <<Excellent point, which I missed. It is definitely more important to
>   learn the fundamentals. On the other hand, if your learning control
>   structures, and you want to print a diagnostic message that is built
>   from several strings, Unbounded_Strings are nice.>>
> 
> What's wrong with Put_Line ("value of x is " & Integer'Image (X) & ....) ..

Ummm, nothing. I've been doing too much C++ lately, where such things
are not 
available, and I have to use dynamic strings for this purpose. I guess I
was 
thinking of that, not of the One True Way. Sigh.

> 
> <<As I thought. So Dale (if you're still reading :), will you be pointing
>   out that using Unbounded_String does have a big downside? When you get
>   to teaching about dynamic memory allocation and managment, you could
>   point out that they've been doing it all along without thinking about
>   it, just like a lot of professional programmers!.>>
> 
> What big downside? What's wrong with using Controlled types here, it is
> certainly the intended and expected implementation!

The downside is heap fragmentation, which will bite you just when it
costs the most to fix. Controlled types can guarantee no dangling
pointers; they do not guarantee no heap fragmentation. For that you need
compacting storage pools (or very careful use of Unbounded_Strings).

Is there a requirement in Ada 95 to document whether the
Unbounded_Strings implementation is compacting? This would be useful
information; I haven't seen it in Annex M for ObjectAda, but I don't
believe I've read the whole thing.

-- 
- Stephe