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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: Unbounded_String'Write et al in GNAT
Date: Wed, 15 Nov 2017 16:34:28 -0600
Organization: JSA Research & Innovation
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Date: 2017-11-15T16:34:28-06:00
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"Simon Wright" <simon@pushface.org> wrote in message 
news:lyzi7oew9c.fsf@pushface.org...
> Victor Porton <porton@narod.ru> writes:
...
> So Unbounded_String _must_ support 'Read, 'Write, 'Output, 'Input, but
> it's up to the compiler vendor to implement them, which GNAT does *in*
> *the* *private* *part* - which is exactly the way the standard expects
> any other vendor to do it. The ARG goes to a lot of trouble to ensure
> that a vendor can implement the standard's requirements.
>
> [1] http://www.ada-auth.org/standards/rm12_w_tc1/html/RM-13-13-2.html#p2

Correct. All non-limited types defined in the Standard or by 
language-defined declarations have to properly support 'Read, 'Write and the 
rest. How that happens is up to the implementation.

It might be the case that some compiler can implement Unbounded_String'Read 
and so on without any explicit code (that is, the default implementation 
does the right thing), in which case no additional code is necessary. Based 
on your message, GNAT needs to override the default implementation for 
Unbounded_String and thus does so in the private part. That's exactly the 
way it ought to work.

In general, looking at a particular implementation's source code for a 
language-defined package can give one the wrong idea about the 
implementation. One should depend only on the RM definition and its 
requirements as much as possible, as that maximizes portability.

                                      Randy.