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From: "Robert I. Eachus" <eachus@mitre.org>
Subject: Re: null range from 1 element discrete type
Date: 1999/11/19
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Date: 1999-11-19T22:50:25+00:00
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Duncan Sands wrote:
> 
> Suppose I have a discrete type (eg an enumeration type) T
> containing just one element.  I would like to define a subtype
> with a null range.  If T had more than one element, I could use
> T'Last .. T'First, but this won't work here.  How to get a null
> range in this case?
> 
> As you may guess, I have a generic package that takes a discrete
> type parameter T, and I want it to work correctly in this degenerate
> case.  At some point the package declares an array type, eg
> type A is array (T range <>) of Float;
> and I want to declare a null slice of type A.  At this point
> I run into the above problem...

    Technically there is no "perfect" solution, as the compiler is
allowed to choose a size of zero bits for such a type.  But if the
(sub)type is a generic formal type, you shouldn't have to worry about
this possibility.

    Since you always want a null range try:  T'Val(1) .. T'Val(0)  Since
'Val returns the base type of T, as long as the base type of T contains
at least one bit you are safe.

-- 

                                        Robert I. Eachus

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