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Re: pointers

From BartC <bc@freeuk.com>
Newsgroups comp.lang.c
Subject Re: pointers
Date 2016-10-24 20:58 +0100
Organization A noiseless patient Spider
Message-ID <nulp42$nqg$1@dont-email.me> (permalink)
References (7 earlier) <lnpompbvcj.fsf@kst-u.example.com> <nulcl7$sr1$1@dont-email.me> <lneg35bsfq.fsf@kst-u.example.com> <nulfjf$qgb$1@dont-email.me> <ln60ohblkv.fsf@kst-u.example.com>

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On 24/10/2016 20:20, Keith Thompson wrote:
> BartC <bc@freeuk.com> writes:
>> On 24/10/2016 17:52, Keith Thompson wrote:
>>> BartC <bc@freeuk.com> writes:
>>>> And yes I can see this being used, even if it's just to illustrate an
>>>> example like this (you'll need to imagine the spaces between &s):
>>>>
>>>>   int **pointer;
>>>>
>>>>       **pointer;     // 3 net derefs (one is always implicit with
>>>>                      // variables in HLLs); result: int
>>>>      &**pointer;     // 2 net derefs; int*   ( *pointer)
>>>>     &&**pointer;     // 1 net deref;  int**  (  pointer)
>>>>    &&&**pointer;     // 0 net derefs; int*** ( &pointer)
>>>>   &&&&**pointer;     // -1 net derefs: error (&&pointer)
>>>
>>> So the only reason you can think of to use those constructs is to
>>> illustrate them?  That's hardly a good reason to add them to the
>>> language.
>>>
>>> The unary & operator yields the address of its operand.
>>
>> What does that mean?
>
> The operand must be an lvalue, which means it's an expression that
> designates an object.  It yields the address of that object.

But &**pointer yields the address of the int value designated by **pointer.

That address is itself stored in an object and can also be written as 
*pointer.

So &**pointer and *pointer yield the same value, the same type, and the 
same object.

Why then I can only put a & in front of *pointer, but not in front of 
&**pointer which is the same thing?

> &**pointer is equivalent to *pointer,

So you agree...

  except
> that the constraints on the operators still apply and the result is not
> an lvalue.

... then what IS the difference between the two? Is it just the C 
standard decided there ought to be a difference just for the hell of it?

> Both **pointer and *pointer are lvalues.  Where did you get the idea
> that *pointer isn't an lvalue?

What's &**pointer then?

> You seem to be inventing a principle that the unary * and & operators
> should cancel each other out when combined in arbitrary ways.  There is
> no such principle in C.

The trouble is, that makes sense, and leads to consistency and 
orthogonality. Doing anything else doesn't.

I wouldn't be bothered so much if C insisted that & can only be followed 
by the name of an object. That's a reasonable simplification especially 
for 1970.

But since C appears to allow this:

   *&*&*&*&*&*&*&*&*&*&*&*&pointer=0;

then why not more silliness!

-- 
bartc

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Thread

pointers asetofsymbols@gmail.com - 2016-10-23 08:11 -0700
  Re: pointers Richard Heathfield <rjh@cpax.org.uk> - 2016-10-23 16:26 +0100
    Re: pointers asetofsymbols@gmail.com - 2016-10-23 10:35 -0700
      Re: pointers Ike Naar <ike@iceland.freeshell.org> - 2016-10-23 17:47 +0000
        Re: pointers asetofsymbols@gmail.com - 2016-10-25 12:55 -0700
      Re: pointers "James R. Kuyper" <jameskuyper@verizon.net> - 2016-10-23 13:58 -0400
        Re: pointers BartC <bc@freeuk.com> - 2016-10-24 00:55 +0100
          Re: pointers Keith Thompson <kst-u@mib.org> - 2016-10-23 17:27 -0700
            Re: pointers BartC <bc@freeuk.com> - 2016-10-24 11:53 +0100
              Re: pointers supercat@casperkitty.com - 2016-10-24 07:29 -0700
                Re: pointers Keith Thompson <kst-u@mib.org> - 2016-10-24 08:55 -0700
                Re: pointers Tim Rentsch <txr@alumni.caltech.edu> - 2016-10-24 14:02 -0700
                Re: pointers Keith Thompson <kst-u@mib.org> - 2016-10-24 14:42 -0700
                Re: pointers Tim Rentsch <txr@alumni.caltech.edu> - 2016-10-25 08:56 -0700
                Re: pointers Keith Thompson <kst-u@mib.org> - 2016-10-25 12:17 -0700
                Re: pointers Tim Rentsch <txr@alumni.caltech.edu> - 2016-10-26 06:37 -0700
              Re: pointers Keith Thompson <kst-u@mib.org> - 2016-10-24 08:49 -0700
                Re: pointers BartC <bc@freeuk.com> - 2016-10-24 17:25 +0100
                Re: pointers Keith Thompson <kst-u@mib.org> - 2016-10-24 09:52 -0700
                Re: pointers BartC <bc@freeuk.com> - 2016-10-24 18:15 +0100
                Re: pointers Keith Thompson <kst-u@mib.org> - 2016-10-24 12:20 -0700
                Re: pointers BartC <bc@freeuk.com> - 2016-10-24 20:58 +0100
                Re: pointers Keith Thompson <kst-u@mib.org> - 2016-10-24 13:16 -0700
                Re: pointers BartC <bc@freeuk.com> - 2016-10-24 23:53 +0100
                Re: pointers Keith Thompson <kst-u@mib.org> - 2016-10-25 13:38 -0700
                Re: pointers BartC <bc@freeuk.com> - 2016-10-25 23:06 +0100
                Re: pointers Keith Thompson <kst-u@mib.org> - 2016-10-25 15:39 -0700
                Re: pointers BartC <bc@freeuk.com> - 2016-10-26 00:26 +0100
                Re: pointers Keith Thompson <kst-u@mib.org> - 2016-10-25 16:44 -0700
                Re: pointers BartC <bc@freeuk.com> - 2016-10-26 01:02 +0100
                Re: pointers Keith Thompson <kst-u@mib.org> - 2016-10-25 17:35 -0700
                Re: pointers Ike Naar <ike@iceland.freeshell.org> - 2016-10-26 06:27 +0000
                Re: pointers BartC <bc@freeuk.com> - 2016-10-26 12:47 +0100
                Re: pointers Keith Thompson <kst-u@mib.org> - 2016-10-26 09:05 -0700
                Re: pointers BartC <bc@freeuk.com> - 2016-10-26 18:46 +0100
                Re: pointers supercat@casperkitty.com - 2016-10-26 11:44 -0700
                Re: pointers BartC <bc@freeuk.com> - 2016-10-26 20:12 +0100
                Re: pointers BartC <bc@freeuk.com> - 2016-10-26 20:30 +0100
                Re: pointers supercat@casperkitty.com - 2016-10-26 13:02 -0700
                Re: pointers Keith Thompson <kst-u@mib.org> - 2016-10-26 11:52 -0700
                Re: pointers Ike Naar <ike@iceland.freeshell.org> - 2016-10-26 19:11 +0000
                Re: pointers BartC <bc@freeuk.com> - 2016-10-26 21:25 +0100
                Re: pointers "James R. Kuyper" <jameskuyper@verizon.net> - 2016-10-25 18:56 -0400
                Re: pointers "James R. Kuyper" <jameskuyper@verizon.net> - 2016-10-25 19:12 -0400
                Re: pointers Keith Thompson <kst-u@mib.org> - 2016-10-25 16:31 -0700
                Re: pointers Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-10-24 21:36 +0100
                Re: pointers BartC <bc@freeuk.com> - 2016-10-25 16:54 +0100
                Re: pointers Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-10-25 19:09 +0100
                Re: pointers supercat@casperkitty.com - 2016-10-24 11:33 -0700
                Re: pointers Keith Thompson <kst-u@mib.org> - 2016-10-24 12:22 -0700
          Re: pointers James Kuyper <jameskuyper@verizon.net> - 2016-10-24 10:23 -0400
  Re: pointers "James R. Kuyper" <jameskuyper@verizon.net> - 2016-10-23 12:31 -0400
    Re: pointers Richard Heathfield <rjh@cpax.org.uk> - 2016-10-23 17:38 +0100
  Re: pointers Lorinczy Zsigmond <zsiga@nospam.for.me> - 2016-10-24 14:02 +0200
  Re: pointers John Bode <jfbode1029@gmail.com> - 2016-10-24 13:34 -0700

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