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From: =?iso-8859-15?Q?Yannick_Duch=EAne_=28Hibou57=29?=
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Newsgroups: comp.lang.ada
Subject: Re: S-expression I/O in Ada
Date: Fri, 20 Aug 2010 23:04:26 +0200
Organization: Ada At Home
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Date: 2010-08-20T23:04:26+02:00
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> I thought discriminant really was like C's union, allocating enough room
> for any possible sub-object to fit

Depends if it has a default discriminant or not. If it has, and you  
declare an object of such a discriminated type somewhere, without  
initialization, you will got nearly the same, except that in Ada you will  
not be allowed to change arbitrary members without regards to the actual  
discriminant. If you want to change it, you will have to assign the object  
as a whole, to give both the new discriminant and the value of the members  
which will comes into live with it.

If on the other hand, if an object is declared using a constrained  
subtype, either via initialization (this gives the object the subtype of  
the value used to initialize it) at declaration or because it provides a  
discriminant at the declaration (which is required if there is no default  
value for the discriminant), then this have nothing in common with C's  
union.

The structure of the object may be frozen or not (will not in the first  
case, will be in the second case). And if it is frozen, you cannot change  
it, so cannot make (and need not) make assumption about its actual  
internal layout.