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Newsgroups: comp.lang.ada
Date: Mon, 25 Apr 2016 08:33:18 -0700 (PDT)
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Subject: Re: Substrings as argument to procedures/functions
From: rieachus@comcast.net
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Xref: news.eternal-september.org comp.lang.ada:30281
Date: 2016-04-25T08:33:18-07:00
List-Id: <comp.lang.ada>

On Wednesday, April 13, 2016 at 5:29:29 PM UTC-4, Randy Brukardt wrote:

> Probably a better example is to remember that the range of Positive is=20
> implementation-defined, and that the language only requires the upper bou=
nd=20
> to be at least 32767. So if you need strings that have potentially more=
=20
> characters than that (to read an entire text file, for instance), and you=
=20
> want the code to be unconditionally portable (to steal someone else's lin=
e),=20
> you need to declare a type yourself (here, assuming that a million=20
> characters are enough)...

Is it time to "fix" this?  Or are there still Ada compilers around that use=
 16-bit String indexes by default? I remember when (in the early days of Ad=
a83) the question of whether String should use 16 or 32 bit indexes was a m=
ajor implementation decision.  Today, compilers that support hardware index=
es shorter than 32-bits are probably mapping to some hardware defined offse=
t field in instructions. Using such indexes when the subtype allows is obvi=
ously an optimization worth supporting.  But in Ada defining "subtype Short=
_String is String range (1..32767);" or whatever allows using such a hardwa=
re type if available.  But there is no need to make Short_String a type.

Yes, programs may have records with character string fields limited such th=
at the index can fit into a 16-bit field in the record.  But I can't imagin=
e doing that without an explicit layout for the record or at least a size f=
or the index field. Back to the original topic here, you want/need sliding =
to work such that

    type Rec is
      Name_Length : Integer range 0..32_767;=20
      Name: String;
    end;
    for Rec use ...
    ...
    Name_Length :=3D Param'Length;
    Name(1..Name_Length) :=3D Param;

 works as expected.