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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: To collection (GNAT bug)
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Date: Wed, 4 Oct 2006 10:01:48 +0200
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Date: 2006-10-04T10:00:28+02:00
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On Tue, 03 Oct 2006 19:10:57 GMT, Jeffrey R. Carter wrote:

> Dmitry A. Kazakov wrote:
>> 
>> No. These aren't equivalent. In many cases the programmer does not know
>> what he is doing on the level of bits and bytes. Otherwise he probably
>> wouldn't instantiate Unchecked_Conversion, but wrote a conversion of his
>> own. He only knows the semantics of an object and asks the compiler to
>> convert the types according to this semantics. It is a deal - the
>> programmer knows the semantics, the compiler does the representation.
> 
> I think you misunderstand the meaning of unchecked. 'Unchecked_Access 
> tells the compiler that this access value will not violate the 
> accessibility rules, even though the compiler thinks it will. 
> Unchecked_Deallocation tells the compiler that there are no other 
> references to this designated object, even if there are. And 
> Unchecked_Conversion means to interpret the bit pattern starting with 
> the bits of this value as being of that type, whether it makes sense or 
> not. It does not involve a change of representation. In terms of object 
> code, Unchecked_Deallocation does nothing; it merely provides a way 
> around the restrictions of the language.

IFF "The representation of S is a representation of an object of the target
subtype" [13.9(10)]

BUT "Otherwise, the effect is implementation defined; in particular, the
result can be abnormal" [13.9(11)]

For example, when an array indexed by a 8-bit modular type is converted to
an array indexed by 32-bit integer, then surely the dope will have a
different bit pattern, and size. 13.9(14) even suggests that the dope size
does not count as "size."

ARM does not prohibit reasonable implementations of Unchecked_Conversion.
OK, it does not enforce them either.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de