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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: why "unconstrained subtype in component declaration" while said
 component given default value ?
Date: Tue, 29 May 2018 17:28:46 -0500
Organization: JSA Research & Innovation
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Xref: reader02.eternal-september.org comp.lang.ada:52770
Date: 2018-05-29T17:28:46-05:00
List-Id: <comp.lang.ada>

"Mehdi Saada" <00120260a@gmail.com> wrote in message 
news:a1c9fe86-bc14-48ee-bb64-24e40ae2b7e2@googlegroups.com...
> Thanks Simon and Jeffrey. You just gave both of my points some legitimacy.

Unfortunately, there would be major problems with access constraints 
(probably the worst idea overall in Ada 83, but we're obviously stuck with 
them). And access-to-specific-tagged works the same way.

There's also an issue of implementation complexity: most Ada compiler use 
fixed size allocation for all composite objects in order to avoid implicit 
heap use (not allowed by many safety-critical customers). It's not possible 
to graft in arbitrary dynamic components in that case (recall that with a 
stand-alone object that there can only be a single dynamic thing. How does 
such a compiler deal with:

    type Nasty is record
      A : String := Some_Funct (...);
      B : String := Some_Funct (...);
      C : String := Some_Funct (...);
   end record;

or worse:
    type Arr is (Positive range <>) of String;
    X : Arr := (1 .. Some_Other_Func(...) => Some_Funct (...));

The line has to be drawn somewhere for fixed allocations, and that was done 
at the stand-alone object boundary.

                                            Randy.