Re: generic function and overloading

[Simon Wright <simon.j.wright@mac.com>]

Robert A Duff <bobduff@shell01.TheWorld.com> writes:

> Simon Wright <simon.j.wright@mac.com> writes:
>
>> Robert A Duff <bobduff@shell01.TheWorld.com> writes:

>>> Alternatively, a packed array of type Bit, where:
>>>
>>>     type Bit is range 0..1;
>>>
>>> If the individual bit numbers have some specific meaning,
>>> the index type could be an enumeration type.
>>
>> Will give surprising results on a big-endian machine.
>
> Only if you're doing something that exposes endianness.
> Endianness should be completely invisible to most code.
> (E.g., if you set the N'th bit of an array of Boolean,
> and get the N'th bit, you get back what you set.
> The same is true with masking modular integers.)

As Adam said, this will likwly be hardware-related -- or, a near
equivalent, dealing with a C peer, possibly over a network.

>>... At least code
>> using shifts and masks is portable if gruesome
>
> Well, it can be portable, but you can still get into endianness
> trouble if you do things like splitting the integer into
> individual bytes (e.g. to transmit them across a network).

We have used GNAT's XDR-based streams on eg Interfaces.Unsigned_32 and
done the bit-twiddling afterwards (when we are reading, of course).

>>... -- and no more gruesome
>> than code I've written which says
>>
>>    type Unrepresented is ..
>>    Internal : Unrepresented;
>>    ...
>>
>>    if System.Default_Bit_Order = System.High_Order_First then
>>       declare
>>          type Represented is ..
>>          for Represented use ..
>>             -- record rep with big-endian usage
>>          External : constant Represented := Internal;
>
> Do you mean "type Represented is new Unrepresented;"?
> Then you need a type conversion in the line above.

Yes, I was writing from memory.

> Nitpick: I'd use a case statement rather than an if statement, here.

Is this for symmetry or in case some later Ada standard allows a third
option? (I could buy the symmetry/clarity argument, good point).