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From: wojtek@power.com.pl (Wojtek Narczynski)
Newsgroups: comp.lang.ada
Subject: Re: Representing data differently
Date: 7 Feb 2003 16:24:25 -0800
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dallex@erols.com (Daniel Allex) wrote in message news:<686be06c.0302070615.3943b629@posting.google.com>...

> In C I can represent the same data multiple ways using 
> structs and unions. 

Ada doesn't have unions, because they are unsafe. For example consider
two simple types:

type Foo is Integer range 1..12;
type Bar is Integer range 20..100;

If two variables, one of each type, were using the same chunk of
memory the value of one would always be invalid.

So if there were unions in Ada the compiler could not check your
program for you.

>How and can I do this in Ada? 

Often you can use a variant record instad of a union.

There are also tricks, but of limited applicability. For example:

declare
  F : Foo := 3;
  B : Bar;
  for B'Address use F'Address;
begin
  -- Et violla! Constraint_Error
  B := B + 1;
end;

Another is to do Uncecked_Conversion between (constrained) access
types.

So this can be done, but you really need to know what you are doing.

Regards,
Wojtek