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From: Florian Weimer <fw@deneb.enyo.de>
Newsgroups: comp.lang.ada
Subject: Re: Parameter Modes, In In Out and Out
Date: 09 Jan 2001 22:46:02 +0100
Organization: Enyo's not your organization
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 <86itno316m.fsf@acm.org>
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Date: 2001-01-09T22:46:02+01:00
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Laurent Guerby <guerby@acm.org> writes:

> if your compiler uses by copy passing for integer,

Scalar types are elmentary types and are therefore passed by copy.

> you'll probably get a program_error, because the generated code for
> exiting P will set X with an unitialized stack value (the place
> reserved for the Y value).

Or a Constraint_Error, or a Program_Error raised explicitly by your
code, or nothing.  

Speaking of out-mode pass-by-copy parameters, I've encountered another
odd thing.  Given the following code:

     procedure Do_Something_With (X : Integer);

     procedure Foo (X : out Integer) is
     begin
        raise Constraint_Error;
     end Foo;
           
     declare
        Y : Integer := 0;
     begin
        begin
           Foo (Y);
        exception
           when Constraint_Error =>
              null;
        end;
       
        Do_Something_With (Y);
     end;

Is it guaranteed by the language that Y is 0 when Do_Something_With is
called?  (Or, put the other way round, can a compiler perform
dead-store elimination in this context?)