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From: Simon Wright <simon@pushface.org>
Newsgroups: comp.lang.ada
Subject: Re: AI12-0218: What is the portable representation clause for
 processing IETF packets on little-endian machines?
Date: Fri, 11 May 2018 22:39:22 +0100
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Date: 2018-05-11T22:39:22+01:00
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"Jeffrey R. Carter" <spam.jrcarter.not@spam.not.acm.org> writes:

> On 05/11/2018 06:48 PM, Simon Wright wrote:
>> "Jeffrey R. Carter" <spam.jrcarter.not@spam.not.acm.org> writes:
>>
>>> On 05/11/2018 03:49 PM, Simon Wright wrote:
>>>> Lucretia <laguest9000@googlemail.com> writes:
>>>>
>>>>> Is it not possible to use derived types with different rep clauses
>>>>> to implement byte swapping on assignment, like the pack/unpack
>>>>> trick can?
>>>>
>>>> Maybe nowadays; not when that code was written.
>>>
>>> You can, and have been able to since Ada 83.
>>
>> I must be missing something.
>>
>>     type T is record
>>        J : Integer;
>>     end T;
>>
>>     type BE_T is new T;
>>     for BE_T use record
>>        ?????
>>     end record;
>
> In Ada 83, it would have been
>
> with Text_IO;
> with Unchecked_Conversion;
> procedure Byte_Swap is
>    type I16 is range -(2 ** 15) .. 2 ** 15 - 1;
>    for I16'Size use 16;
>
>    type Byte is range -128 .. 127;
>    for Byte'Size use 8;
>
>    type LE16 is record -- We'll assume this is the native order
>       LSB : Byte;
>       MSB : Byte;
>    end record;
>    for LE16'Size use 16;
>    for LE16 use record
>       LSB at 0 range 0 .. 7;
>       MSB at 1 range 0 .. 7;
>    end record;
>
>    type BE16 is new LE16;
>    for BE16'Size use 16;
>    for BE16 use record
>       LSB at 1 range 0 .. 7;
>       MSB at 0 range 0 .. 7;
>    end record;

I had somewhere got the idea that the parent type couldn't have
representation clauses.

But I see that Ada83 LRM 13.6 says

    "Hence, if an alternative representation is needed, it is necessary
    to declare a second type, derived from the first, and to specify a
    different representation for the second type."

whereas Ada2012 says

    "To convert a record from one representation to another, two record
    types with a common ancestor type need to be declared, with no
    inherited subprograms."

which seems to require _three_ types.

But in any case your code doesn't answer my question: what
representation trick could convert between

   type T is record
      J : Integer;
   end T;

and

   type BE_T is new T;
   for BE_T use record
       ?????
   end record;

One approach (the C one) would be to use htonl(), of course.