Re: Heterogenous array

["Jean-Pierre Rosen" <rosen@adalog.fr>]

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"Bj�rn Persson" <spam-away@nowhere.nil> a �crit dans le message de news:_U71c.51050$mU6.207186@newsb.telia.net...
>Steve wrote:
>> The way I look at it, the default descriminant is a way of telling the
>> compiler you may want to assign a record with a different discriminant value
>> to a record you have declared.  That way enough memory is allocated to hold
>> the largest variant.  If you don't give a default descriminant, that tells
>> the compiler you're going to explicitly select the variant for each record
>> you declare, so it can size each variant in the smallest allocation - with
>> the restriction that you cannot assign a different variant after the
>> declaration.
>
>Okay, after some experimentation I understand the difference, but I
>can't help thinking that it seems somewhat unlike Ada to say "make the
>discriminant immutable in all objects of this type" by not giving it a
>default value. Putting a "constant" somewhere would have been more
>intuitive.
Acutally, it's the other way round.

What you want to do is to be able to declare an unconstrained variable, i.e.:
V :  Parameter_Definition;

Since there is no discriminant constraint, the variable is not constrained to always have the same value of the discriminant.
However, a discriminant is used by the compiler to make address computations. It would be extremely dangerous to have an
uninitialized discriminant. Therefore, a declaration that does not specify discriminants is allowed only if there are default values
for the discriminants.

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           J-P. Rosen (rosen@adalog.fr)
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