Re: Implementing pointers to pointers in Ada

[bobduff@world.std.com (Robert A Duff)]

In article <512mt3$ret@goodnews.voicenet.com>,
John Demby <jdemby@voicenet.com> wrote:
>Vasilios Tourloupis <vasilios@insect.sd.monash.edu.au> wrote:
>
>>Dear Ada users,
>
>>As the title suggests, how would I go about implementing pointers to
>>pointers in Ada?
>>For example, I have declared the following variable in C:
>>    unsigned **a;
>>How would this be expressed in Ada terms?
>
>in general :
>
>type unsigned is blah;
>type unsigned_star is access unsigned;
>type unsigned_star_star is access unsigned_star;

Several people have suggested this, but I think it's highly unlikely to
be the answer the original poster is looking for.  In C, "unsigned **a"
means you're dealing with a pointer-to-pointer-to-unsigned, or a
pointer-to-array-of-unsigned, or an array-of-pointers-to-unsigned.  We
can't tell which it is, without knowing whether the code does a++ or
(*a)++ or whatever.  Or without knowing where the value of a comes from.

In any case, the "equivalent" Ada probably involves an array type, since
Ada has two separate features -- access types and arrays -- where C has
a single feature (pointers, which can point at single objects, or can
point at the first element of an array, or the middle of an array).
(Well, that's an oversimplification -- in C, arrays and pointers are not
*quite* the same thing, but they are in many circumstances, which is
quite confusing.)

Alternatively, maybe the "unsigned **a" is being used as a parameter.
If so, then the equivalent Ada probably involves an in-out parameter.

Perhaps the original poster can post more context, so we can see what
SORT of C pointers are being used, and advise whether they should
translate into arrays or in-out parameters, or whatever.

In Ada, I think I use access-to-access types approximately once per
decade, so I really don't think that's likely to be the right answer.

- Bob