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Date: Tue, 11 Sep 2012 11:11:44 -0700 (PDT)
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Subject: Re: 'Size of an object
From: AdaMagica <christ-usch.grein@t-online.de>
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Date: 2012-09-11T11:11:44-07:00
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On Tuesday, September 11, 2012 5:52:03 PM UTC+2, Adam Beneschan wrote:
> The question has to do with X'Size where X is an object.  RM 13.3(40)
> says this "denotes the size in bits of the representation of the
> object".  Maybe I'm being dense, but this doesn't seem to clarify
> things.
> 
> Suppose you have an enumeration type with eight literals, and no other
> representation or aspect clauses apply to the type:
> 
>    type Enum is (E0, E1, E2, E3, E4, E5, E6, E7);
> 
> I think Enum'Size should be 3 in most or all implementations.
It must be 3.
Take Integer, for instance. For the subtype Natural'Size = Integer'Size -1,
because it doesn't need the sign bit. This was a much disputed decision during
the Ada9X process (Ada 83 compilers had different values for 'Size, and Ada9X
fixed this - it was on Tucker Taft's vote IIRM). GNAT implemented therefore the
'Object_Size attribute.

Now stand-alone objects normally have a size fitting in a standard word
depending on the OS. The RM says, T'Size is used for objects in packed records
and Unchecked_Conversion only.