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From: Mark Lorenzen <mark.lorenzen@surfpost.dk>
Newsgroups: comp.lang.ada
Subject: Re: private types
Date: Mon, 13 Mar 2006 20:27:28 +0000 (UTC)
Organization: BT Openworld
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Date: 2006-03-13T20:27:28+00:00
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ada_student@yahoo.com writes:

> Consider the following package declaration,
> 
>     package PrivateType is
> 
>         type MyInteger is private;
> 
>         procedure  Read(O : out myInteger);
>         procedure Write(I : in MyInteger);
> 
>     private
> 
>         type MyInteger is range 1 .. 2**31 - 1;
> 
>     end;
> 
>     Why does Ada allow MyInteger to be made visible outside the
>     scope of PrivateType ? Doesnt it make the code less secure(for
>     example,consider an "uninitialized" object of type
>     PrivateType.MyInteger).

First of all: If you want to learn about Ada, I can recommend the book
"Programming in Ada 95" by John Barnes.

I think that I understand what you are trying to do... You want to
prevent the user of your package from declaring uninitialised
variables of type PrivateType.MyInteger, right? In this case I would
declare the partial view of the type (the non-private part) to be
indefinite:

        type MyInteger (<>) is private;

And then declare a function to provide an initial value of that type:

        function Nil returns MyInteger;

The user of your package can then only declare a variable of type
PrivateType.MyInteger if she initialises it at the same time:

with PrivateType;

[...]

My_Var : PrivateType.MyInteger := PrivateType.Nil;

Again: A good textbook on the subject will give you a much better
understanding of the language than just trying to find Ada equivalents
of C++ intricacies.

- Mark