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From: Martin Klaiber <martinkl@zedat.fu-berlin.de>
Newsgroups: comp.lang.ada
Subject: Re: Error-names.
Date: Mon, 1 Mar 2004 12:20:37 +0100
Organization: Freie Universitaet Berlin
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tmoran@acm.org wrote:

>  It would not be hard to leave out, or have multiple uses of, an error
> code, as in:
>    Constraint_Error_Code : constant Exception_Codes := -10;
>    Program_Error_Code    : constant Exception_Codes := -9;
>    Storage_Error_Code    : constant Exception_Codes := -9;
>    Status_Error_Code     : constant Exception_Codes := -7;
>    My_Special_Error_Code : constant Exception_Codes := -5;
>    ...
> Similarly, a new exception might be added, but someone forgets to
> add it to the if-elsif list.

> With the if-elsif style you can only hope that neither of those happens.
> But if you do

>    Coded_Exception_List : constant array(Exception_Codes)
>    of Ada.Exceptions.Exception_ID
>      := (Constraint_Error_Code => Constraint_Error'Identity,
>          Program_Error_Code    => Program_Error'Identity,
>          Storage_Error_Code    => Storage_Error'Identity,
>          IO_Status_Error_Code  => Ada.IO_Exceptions.Status_Error'Identity,
>          My_Special_Error_Code => My_Special_Exception'Identity,
>          ...
> then the compiler will warn you if two of your error codes are the
> same, or if you've left any out in the Exception_Codes range.

This is right but how can I export the constants then?  At the moment
I use in the spec-file:

   -- No error:
   No_Error_Code : constant C_Integer := 0;
   pragma export (C, No_Error_Code, "no_error_code");

   -- Fallback:
   General_Error_Code : constant C_Integer := -1;
   pragma export (C, General_Error_Code, "general_error_code");

and so on.  The reason is that I want the C-programmer to use the
constants-names instead of the values, because I can't guarantee that
the values will not change in the future.

BTW: does someone know why these constants are not recognized by the
C-program as constants?  They are declared as constants and in the C
header-file I put:

   /* error-codes */

   extern const int no_error_code;
   extern const int general_error_code;
   ...

But if I want to use it in a switch-command like that:

   int year;
   ...
   for (year = 2003; year <= 2005; ++year) {
      switch (set_year (year)) {
      case no_error_code :
         set_lzz (1);
         ...
                         
the compiler says:

   test_c.c:20: case label does not reduce to an integer constant

Is my exporting wrong?  Or is this a C-problem?  When I use "case 0"
instead of "case no_error_code" it works ok, but this is what I want
to avoid.

Thanks,
Martin