Re: (Num_Types.Mod_4 range 1..8) ------->why not (1..8)?

[convergence82 <tiamiata@gmail.com>]

On May 18, 4:53 pm, Georg Bauhaus <rm.tsoh.plus-
bug.bauh...@maps.futureapps.de> wrote:
> convergence82 wrote:
> > Thanks.  I'm looking at a Ada.Unchecked_Conversion function that
> > converts from Data_Array_1 to Data_Array_2:
> > type Data_Array_1 is array (Num_Types.Mod_4 range 1..8) of
> > Num_Types.Mod_8;
> > type Data_Array_2 is array (1..8) of Num_Types.Mod_8;
>
> Uhm, converting is what instances of Unchecked_Conversion
> do *not* do, in some (paradoxical) sense.
> To convert X of type T to X of type D, write D(X).
> An instance of Unchecked_Conversion will just leave
> the bits unchanged. The programmer should then know the
> meaning of the "converted" data.  An Ada array, however,
> is a bit more than a sequence of storage cells,
> when it includes bounds an other properties of the
> corresponding array type.  Hence, arrays of different
> type may be stored differenty.
>
> In addition, if you choose "Integer range 1 .. 8" for
> the index type (as per Bob's suggestion) the normal
> representation of values of type integer may be
> different from that of values of type Mod_4.
> I imagine that even the instructions generated for indexing
> may be different, e.g. indicating different register word
> sizes for smaller or larger types. So Unchecked_Conversion
> may yield unexpected results.
>
> You can think of Ada arrays as tables where the keys
> are values taken from the Index type. The keys will
> not be strings, though, but either numbers, or letters,
> or enumeration literals.  Maybe this helps to see why
> one array is not like the other.
>
> > type Data_Array_1 is array (Num_Types.Mod_4 range 1..8) of
> > Num_Types.Mod_8;
> > type Data_Array_2 is array (1..8) of Num_Types.Mod_8;
>
> > I would think that the way the array is indexed is (integer, mod_4, or
> > whatever) wouldn't matter (it's the datatype of the elements of the
> > array that matter).
>
> No, that's not true for Ada arrays. Imagine
> some book pages numbered i, ii, iii, iv, ...
> while some others are numbered 1, 2, 3, 4, ...
> Book printers will carefully observe this difference:
> preface, TOC, etc. numbered i, ii, ... and then
> the book's "proper text", numbered 1, 2, ...
> This difference can be mapped to arrays index types,
> reducing the risk of mixing the sequences of book
> pages.
>
> > So, will the convert function really in effect DO NOTHING but copy the
> > data over?
>
> It shouldn't have to copy at all, that's the point :-)

Thanks you guys!  Maybe one of the reasons for this awkward use of
Unchecked_Conversion is plain ole "cut-n-paste".  There must be 200+
data types being converted (or more), for the project I am on.  So, I
WILL read your posts more thoroughly in the next day or so.

But for now, thanks for the input,

Steph
PS.  Continued debate is welcome, of course!