Re: Error messages again

[John Darrington]

On Thu, May 18, 2006 at 08:46:02AM -0700, Ben Pfaff wrote:
     >
     > /* Returns TRUE if error matches domain and code, FALSE otherwise. */
     > gboolean    g_error_matches                 (const GError *error,
     >                                              GQuark domain,
     >                                              gint code);
     >
     > So the (domain, code) pair should be unique.
     
     OK.  But how would a pair of matching messages be treated
     differently from a pair of differing messages, and does the
     difference in treatment warrant actually trying to make them
     unique?


It's the glib way of implementing something akin to exception
handling.  For example I can cascade function calls thus:

gboolean
my_function_that_can_fail (GError **err)
{
  g_return_val_if_fail (err == NULL || *err == NULL, FALSE);

  if (!sub_function_that_can_fail (err))
    {
       /* assert that error was set by the sub-function */
       g_assert (err == NULL || *err != NULL);  
       return FALSE;
    }

  /* otherwise continue, no error occurred */
  g_assert (err == NULL || *err == NULL);
}


and I can handle the specific failures and pass the others to the next
stack frame :


gboolean
my_other_function_that_can_fail(GError **err2)
{
  GError *err = NULL;
  my_function_that_can_fail(&err);


  if (g_error_matches (err, THIS_DOMAIN, THIS_CODE))
    {
      handle_the_mess();
      g_clear_error(&err);
      return TRUE;
    }   
  else
    {
      g_propogate_error(err2, err);
      g_clear_error(&err);
      return FALSE;
    }
}
     
     >      > I'm not sure how we'd implement that.  One idea would be to  have 
it
     >      > initialsed whenever the function msg is called: Change msg to a 
macro,
     >      > and initialise code to a hash of the __FILE__, __LINE__ 
combination.
     
     
     I'd suggest initializing "code" as a hash of the message text.  I
     don't know whether, e.g. "ABC is an unknown identifier." and "XYZ
     is an unknown identifier." are different messages, so we could
     try to get the hash before the formatting takes place, so that it
     would actually be a hash of "%s is an unknown identifier."

The problem with that approach, is that it would fall down with
internationalisation,  since the translated string would have a
different hash to the English one.

J'

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