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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: stream_element array contiguity
Date: Fri, 27 Jun 2003 13:29:26 -0500
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Date: 2003-06-27T13:29:26-05:00
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Ian Leroux wrote in message ...
>>From reading previous posts, I gather that a common approach to
>dealing with bitwise input is to use Unchecked_Conversion to transform
>to packed arrays of Boolean from arrays of Stream_Element, obtained
>from an appropriate Stream package. Given that Stream_Element'Size is
>not guaranteed to be a multiple or factor of Storage_Element'Size, is
>there any way of being sure that the elements of a Stream_Element
>array will be contiguous? If not, is there any standard way of
>avoiding the problem whereby padding bits in the Stream_Element array
>corrupt the resulting boolean array?


I generally just put in a check at elaboration time that the assumptions
of the code are not violated. For instance, if you assumed that
Stream_Element'Size = Storage_Element'Size, then I'd put:

   if Stream_Element'Size /= Storage_Element'Size then
      raise Program_Error;
   end if;

into the "begin" part of the package body. If this somehow gets
violated, you hopefully will get a warning, but certainly will get a
Program_Error on the first test. (If no one bothers to run a test, like
the Ariene 5, well, not much can help that project).

What the condition will be depends on the details of your code, of
course. I often have code that looks like:
    Item_Size : constant := 8;
    ...
    if Some_Type'Size /= Item_Size then
     raise Program_Error;
   end if;
because "Some_Type'Size" isn't static, and thus cannot be used directly
in representation clauses. But we want to make sure that the rep.
clauses are accurate. (It won't do to have the size actually be 6 or
12.)

           Randy.