Re: What is a `**ppEnum`?

From	Keith Thompson <Keith.S.Thompson+u@gmail.com>
Newsgroups	comp.lang.c++
Subject	Re: What is a `**ppEnum`?
Date	2022-11-16 18:40 -0800
Organization	None to speak of
Message-ID	<87v8nemhfw.fsf@nosuchdomain.example.com> (permalink)
References	(4 earlier) <tkt73c$1o60i$2@dont-email.me> <tl2bj1$1en4$1@gioia.aioe.org> <tl3t9g$2f98u$1@dont-email.me> <ZcKdnWa04MvjEej-nZ2dnZfqnPadnZ2d@brightview.co.uk> <tl44mh$2fqlq$1@dont-email.me>

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T <T@invalid.invalid> writes:
> On 11/16/22 17:21, Mike Terry wrote:
>> C++
>> HRESULT QueryVolumes(
>>    [out] IEnumVdsObject **ppEnum
>> );
>
> My (mis) understanding is that **ppEnum points to a
> pointer that points to an array of HRESULT, which is
> a 32 bit word.   Seems I don't understand.
>
> I am expecting the result to give me a long table
> of volumes.  Again, I think I misunderstand.

A (valid non-null) pointer value of type `foo*` always points to a
single object of type `foo`.  (Handwave: Ignore for now the fact that a
valid pointer can point just past the end of an array object.)

That single object may or may not be an element of an array of `foo`.
If it's just a single object, it can be treated as the lone element of a
single-element array.

In the example above, ppEnum *might* be intended to be used to
manipulate an array of pointers.  The declaration isn't enough to tell
us that.

Given a pointer value that points to an element of an array object, you
can access the other elements of that array object by performing pointer
arithmetic on the pointer value.  (The [] operator is syntactic sugar
for this; x[y] means *(x+y).)  In C, most array manipulation is done via
pointers to the elements of the array.

It's less common to have to deal with this in C++ than in C, since C++
defines container classes like std::vector that hide the raw pointer
operations.  But C++ still inherits just about all of C's (often
counterintuitive) pointer and array semantics.

Recommended reading:
Section 6 of the comp.lang.c FAQ <https://www.c-faq.com/>.

-- 
Keith Thompson (The_Other_Keith) Keith.S.Thompson+u@gmail.com
Working, but not speaking, for XCOM Labs
void Void(void) { Void(); } /* The recursive call of the void */

Thread

What is a `**ppEnum`? T <T@invalid.invalid> - 2022-11-14 00:39 -0800
  Re: What is a `**ppEnum`? "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2022-11-14 01:29 -0800
    Re: What is a `**ppEnum`? T <T@invalid.invalid> - 2022-11-14 01:46 -0800
      Re: What is a `**ppEnum`? "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2022-11-14 01:52 -0800
        Re: What is a `**ppEnum`? "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2022-11-14 01:55 -0800
      Re: What is a `**ppEnum`? Öö Tiib <ootiib@hot.ee> - 2022-11-14 02:30 -0800
        Re: What is a `**ppEnum`? T <T@invalid.invalid> - 2022-11-14 02:38 -0800
          Re: What is a `**ppEnum`? Öö Tiib <ootiib@hot.ee> - 2022-11-14 02:48 -0800
  Re: What is a `**ppEnum`? Öö Tiib <ootiib@hot.ee> - 2022-11-14 01:39 -0800
    Re: What is a `**ppEnum`? T <T@invalid.invalid> - 2022-11-14 01:48 -0800
      Re: What is a `**ppEnum`? "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2022-11-14 02:01 -0800
      Re: What is a `**ppEnum`? Öö Tiib <ootiib@hot.ee> - 2022-11-14 02:33 -0800
        Re: What is a `**ppEnum`? T <T@invalid.invalid> - 2022-11-14 02:59 -0800
          Re: What is a `**ppEnum`? Öö Tiib <ootiib@hot.ee> - 2022-11-14 03:33 -0800
            Re: What is a `**ppEnum`? T <T@invalid.invalid> - 2022-11-14 11:10 -0800
          Re: What is a `**ppEnum`? Juha Nieminen <nospam@thanks.invalid> - 2022-11-16 09:47 +0000
            Re: What is a `**ppEnum`? T <T@invalid.invalid> - 2022-11-16 15:55 -0800
              Re: What is a `**ppEnum`? scott@slp53.sl.home (Scott Lurndal) - 2022-11-17 00:19 +0000
              Re: What is a `**ppEnum`? Mike Terry <news.dead.person.stones@darjeeling.plus.com> - 2022-11-17 01:21 +0000
                Re: What is a `**ppEnum`? T <T@invalid.invalid> - 2022-11-16 18:01 -0800
                Re: What is a `**ppEnum`? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2022-11-16 18:40 -0800
                Re: What is a `**ppEnum`? Mike Terry <news.dead.person.stones@darjeeling.plus.com> - 2022-11-17 04:18 +0000
              Re: What is a `**ppEnum`? Juha Nieminen <nospam@thanks.invalid> - 2022-11-17 08:25 +0000
                Re: What is a `**ppEnum`? T <T@invalid.invalid> - 2022-11-17 00:31 -0800
                Re: What is a `**ppEnum`? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2022-11-17 10:17 -0800
              Re: What is a `**ppEnum`? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2022-11-17 13:03 +0000
                Re: What is a `**ppEnum`? T <T@invalid.invalid> - 2022-11-17 10:28 -0800
          Re: What is a `**ppEnum`? Mike Terry <news.dead.person.stones@darjeeling.plus.com> - 2022-11-17 03:36 +0000
            Re: What is a `**ppEnum`? T <T@invalid.invalid> - 2022-11-16 20:57 -0800
  Re: What is a `**ppEnum`? "Alf P. Steinbach" <alf.p.steinbach@gmail.com> - 2022-11-14 14:34 +0100
    Re: What is a `**ppEnum`? "Alf P. Steinbach" <alf.p.steinbach@gmail.com> - 2022-11-14 14:44 +0100
    Re: What is a `**ppEnum`? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2022-11-14 09:12 -0800
    Re: What is a `**ppEnum`? T <T@invalid.invalid> - 2022-11-14 11:02 -0800
      Re: What is a `**ppEnum`? Ralf Fassel <ralfixx@gmx.de> - 2022-11-15 10:17 +0100
        Re: What is a `**ppEnum`? Öö Tiib <ootiib@hot.ee> - 2022-11-15 04:24 -0800
      Re: What is a `**ppEnum`? "Alf P. Steinbach" <alf.p.steinbach@gmail.com> - 2022-11-15 14:04 +0100
        Re: What is a `**ppEnum`? T <T@invalid.invalid> - 2022-11-15 08:28 -0800
          Re: What is a `**ppEnum`? "Alf P. Steinbach" <alf.p.steinbach@gmail.com> - 2022-11-17 16:21 +0100
            Re: What is a `**ppEnum`? T <T@invalid.invalid> - 2022-11-17 16:39 -0800
              Re: What is a `**ppEnum`? "Alf P. Steinbach" <alf.p.steinbach@gmail.com> - 2022-11-19 14:16 +0100

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