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From: pascal.obry@der.edfgdf.fr (Pascal Obry)
Subject: Re: Is 'out' different from 'in out' (Was: Uninitialized "out"
 parameters)
Date: 1996/07/24
Message-ID: <4t55jl$e4o@cf01>#1/1
X-Deja-AN: 170047224
references: <31EEACDA.64880EEB@sage.inel.gov>
 <gauthier-1907961215520001@164.81.60.62> <Duwwqn.AD0@world.std.com>
 <gauthier-2207961213160001@164.81.60.62>
 <gauthier-2307961037570001@164.81.60.62>
organization: EDF - DER
newsgroups: comp.lang.ada
Date: 1996-07-24T00:00:00+00:00
List-Id: <comp.lang.ada>


gauthier@unilim.fr (Michel Gauthier) wrote:

>They all confirm what I was not entirely sure to understand correctly.
>There is no essential difference between 'out' and 'in out' parameters,
>and it is probably worth considering them to be the same thing.

>IMHO, it was a mistake of the 9X design. It's too late to repair.

>Possible useful style rule : avoid 'out' parameters, since they
>are error-prone, and prefer 'in out'.

This is certainly true for the designer of the procedure. But 'out'
and 'in out' parameter provide one usefull information for the
caller of the procedure/function.

There is two parts in the programming job. You are sometimes designer
(you are fulfilling a demand - providing a functionality) and
sometimes client (using what has already be done). In our case 'out'
and 'in out' are differents for clients :

	'out'
	you don't have to initialize the parameter. this is done
	inside the procedure/function.

	'in out'
	you have to initialize the paramter because the value passed to
	the procedure/function will be used.


For example :

    package P is

	type T is limited private;

	procedure Init1 (Item : in out T);

	procedure Init2 (Item :    out T);

    end P;


I really prefer Init2 and I found Init1 misleading (how can I pass
an initialized parameter ?). So I don't found the rule 'avoid out
parameter' a good one.

Pascal.

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