Re: setting 'Size and unchecked conversion safe here?

[Martin Krischik <martin@krischik.com>]

Marius Amado Alves wrote:

> 
> On 12 Feb 2005, at 06:55, Martin Krischik wrote:
> 
>> Marius Amado Alves wrote:
>>
>>>>    ARM 13.9(5 .. 11) says
>>>> "If all of the following are true,
>>>>  ...
>>>>  S'Size = Target'Size
>>>
>>> I know. This happens, because I forced the access type to be 64-bit,
>>> same as the modular. What I'm not sure about is whether the unused 32
>>> bits are harmless or not (by definition). Thanks.
>>
>> There are not harmless since there is no requironment to set them to
>> 16#0000000#.

> If there were such a requirement is there a simple way to specify it?

A combination of checked and unchecked convertions. First an unchecked
convertion to an access compatible modular type:

http://en.wikibooks.org/wiki/Programming:Ada:Types:access#where_is_void.2A.3F

then a checked convertion to the 64 bit type.
 
>> In order to help you we have to know why it is that the access is 32
>> bit and
>> the modular 64. I can only guess it is for some compatiblity reason -
>> but
>> what do you need to keep compatible?
> 
> The access type is 32 bits because the compiler says so.
> The modular 64 is my choice to represent small values (integers,
> floats, tiny strings, access values) in the vertices of a Mneson graph:
> 
>     type Vertex is record
>        Tip : Natural_16;
>        Cue : Modular_64;
>     end record;
>     for Vertex'Size use 80;
>     pragma Pack (Vertex);
> 
> I know this might be incompatible with some unusual architectures but
> I'm prepared to deal with that when and if the time comes.

I would use a variant record:

http://en.wikibooks.org/wiki/Programming:Ada:Types:record#Variant_Record

But that's is just me - and may not be suitable for your database.

Martin

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