Re: Creating several types from a base type and conversion

[Simon Wright <simon@pushface.org>]

Optikos <ZUERCHER_Andreas@outlook.com> writes:

> Instead of depending on a big-language solution of a very-feature-rich
> compiler, there is a common industrial practice in both Ada and other
> languages that I mentioned in my 2 replies: use 2 different packages
> with same-named identifiers (especially functions or procedures) to
> link in either the little-endian implementation or big-endian
> implementation

An alternative approach (which I've now abandoned in favour of the GNAT
Scalar_Storage_Order) was, given e.g.

   type SNTP_Timestamp is delta 2.0 ** (-32) range -2.0 ** 31 .. 2.0 ** 31;
   for SNTP_Timestamp'Size use 64;

   subtype Timestamp_Slice is Ada.Streams.Stream_Element_Array (1 .. 8);

to provide conversion functions like

   function To_Timestamp_Slice
     (T : SNTP_Timestamp) return Timestamp_Slice is
      function Convert
      is new Ada.Unchecked_Conversion (SNTP_Timestamp,
                                       Timestamp_Slice);
      Tmp : constant Timestamp_Slice := Convert (T);
   begin
      if Big_Endian then
         return Tmp;
      else
         return (1 => Tmp (8),
                 2 => Tmp (7),
                 3 => Tmp (6),
                 4 => Tmp (5),
                 5 => Tmp (4),
                 6 => Tmp (3),
                 7 => Tmp (2),
                 8 => Tmp (1));
      end if;
   end To_Timestamp_Slice;

(Big_Endian was a compile-time constant; GNAT is clever enough not to
generate any object code for the "other" branch).

It got a bit more interesting where there were bit fields involved; the
two branches can declare appropriately-represented derived types and use
type conversion between base and derived type to get the compiler to do
the hard work. See e.g. [1] starting at line 43.

[1] https://sourceforge.net/p/coldframe/adasntp/code/ci/Rel_20070311/tree/SNTP.impl/sntp_support.adb