Comments on: Gem #20: Using pragma Shared_Passive for data persistence

By: Paul F. Pearson

Paul F. Pearson — Thu, 06 Dec 2007 19:48:12 +0000

Pascal,

Thanks. I’d not read about Annex E before reading this. Now my head hurts. :-)

By: Pascal Obry

Pascal Obry — Thu, 06 Dec 2007 17:40:47 +0000

Paul,

Yes you are right the output is just a stream. And it is
indeed possible to specify the streamed format. We could
link together this gem with the previous one on stream
using an XML format.

For example the following store implementation will
outout counter as an XML like stream:

with Ada.Streams;
package Store is
pragma Shared_Passive;

use Ada.Streams;

type My_Int is new Integer;

procedure Write_Int
(S : access Root_Stream_Type’Class; V : in My_Int);
for My_Int’Write use Write_Int;

Counter : My_Int := 0;
end Store;

package body Store is
procedure Write_Int
(S : access Root_Stream_Type’Class; V : in My_Int) is
begin
String’Write
(S, “” & My_Int’Image (V) & “”);
end Write_Int;
end Store;

The ‘Read implementation is left to the reader as
an exercice.

Pascal.

By: Pascal Obry

Pascal Obry — Thu, 06 Dec 2007 17:36:18 +0000

Christoph,

Thanks for the kind words. The GNAT implementation should
indeed be specified clearly.

Pascal.

By: Paul F. Pearson

Paul F. Pearson — Tue, 04 Dec 2007 16:57:35 +0000

In what format is the data stored? I presume that it’s essentially a stream output (e.g. ‘Output)? If so, it might be interesting if the ‘Output could be overridden (assuming the Ada.Streams is Pure – which I haven’t looked at).

Thanks for a cool gem!

By: Christoph Grein

Christoph Grein — Tue, 04 Dec 2007 07:33:07 +0000

Hey, that’s a really nice gem, actually. Two remarks, though.

The storing in a file is, I reckon, a GNAT implementation detail (at least the file name). This should be pointed out. AARM E.1(11.a)

Where in the RM is it specified that the initialisation of a shared passive variable is only performed the very first time? I searched the whole RM, but without avail.