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Date: 11 Oct 92 16:20:45 GMT
From: eru.mt.luth.se!lunic!sunic!nobeltech!dawe@bloom-beacon.mit.edu  (Daniel W
engelin)
Subject: Re: Does the LRM permit parameter passing by reference?
Message-ID: <1992Oct11.162045.5904@nobeltech.se>
List-Id: <comp.lang.ada>

Having read the debate on allowed parameter passing mechanisms, I would
like to follow up with a related question.

The LRM does (as far as I've understood) allow for an implementation to
skip the "copy-back" of parameters in case an exception is raised preventing
a normal return. What's the impact of this in the real world? Are any 
compilers using this optimization feature?

I am especially interested in the following construct:

    package Smart_Error_Example

       type Status is (Ok, This_Error, That_Error);

       type Data is record          -- Could also be private (normally)
          Current_Status : Status;
          Other_Stuff : ......
       end record;

       procedure Do_Action ( D :  in out Data);

       Any_Error : exception;
    end Smart_Error_Example;

    
    package body Smart_Error_Example is

       procedure Set_Status (The_Status : Status;On_Data : in out Data) is
       begin On_Data.Current_Status := The_Status; end Set_Status;

       procedure Do_Action (D: in out Data) is
       begin
         -- Do something useful
         if That_Error_Detected then
            Set_Status(That_Error, D); -- Set error info for later use
            raise Any_Error;
         end if;
         :
       end Do_Action;
    end Smart_Error_Example;--Unfortunately "erroneous"

This error signaling scheme would be quite suitable for a lot of
applications. It does probably work for most compilers.


Does anyone know a system that fails on the above example?

Can I prevent an optimizer from eliminating the assignment of the 
Current_Status-field by making Set_Status separate??



Thank you for considering these issues.


           Daniel Wengelin
           Systems Designer, 
           NobelTech Systems,
           Naval Sector. 

Disclaimer: This article is on behalf of myself only.