Re: Looping over a tagged record?

["DuckE" <nospam_steved94@home.com>]

"M. A. Alves" <maa@liacc.up.pt> wrote in message
news:mailman.993045741.6638.comp.lang.ada@ada.eu.org...
[snip]
>
> Often I also want to merge record and array i.e. have named _and_
> iteratable "components". Theoretically this comes down to "types as
> objects", which Ada doesn't have. Practically, a "standard" Ada idiom for
> this is an heterogenous array indexed on an enumeration e.g. (not tested):

Could you please elaborate on exactly what you mean by "types as objects"?

>
>   ...
>   type Field_Ptr is access Field'Class;
>   type Field_Names is (Field_1, Field_2, Field_3);
>   type Record_Type is array(Field_Names) of Field_Ptr;
>   A_Record: Record_Type := (
>     Field_1 => Field_Constructor_A(...),
>     Field_2 => Field_Constructor_A(...),
>     Field_3 => Field_Constructor_B(...));
>
> Now we have a record-like object A_Record with "components" Field_1,
> Field_2 of "type" A and Field_3 of "type" B. This example assumes
> class-wide programming (including dispatching), with (tagged) type Field
> being the root of some derivation class. Iterating over A_Record's
> "components":
>
>   for I in A_Record'Range loop
>     Do_Something(A_Record(I).all); -- dispatching call
>   end loop;
>
> Accessing "components" by name:
>
>   Do_Something(A_Record(Field_1).all);
>
> This particular approach has a notorious shortcoming: field types are not
> checked (Field_Ptr values can reference _any_ derived type in the class).
> The remedy for this is left as an exercise ;-)

I believe this will be caught a runtime.  But it really depends on how
Do_Something is defined.

SteveD

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