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Re: Compatibility question

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Steve Thompson <stevet810@gmail.com> writes:
> On Tue, Oct 23, 2012 at 12:26:05PM -0700, Keith Thompson wrote:
>> Steve Thompson <stevet810@gmail.com> writes:
>> > On Mon, Oct 22, 2012 at 09:06:39PM +0100, Ben Bacarisse wrote:
>> [...]
>> >> Whatever goes on under the hood (and I'd dispute that 'a' is "really a
>> >> pointer") it's much easier to learn a language in its own terms, at
>> >> least as far as that's possible.
>> >
>> > Well, no.  'a' is never a pointer as such in the terms of the language
>> > syntax, but its treatment by the compiler internally is nominally
>> > equivalent to they way it handles a pointer, with the exception that
>> > the language grammar forbids its use in the program text as a pointer.
>> 
>> I don't believe that's true.  Do you have some evidence that it is?
>
> The distinction is largely moot.  In a practical sense, all variables
> have a value which is stored somewhere.  The compiler is making an
> explicit use of a pointer when it does something with the value.  The
> problem is that "use" in this context may as well be referring to ether
> or memory and there's no easy way of knowing which it is.

When you say "explicit use of a pointer", do you mean that there's
a pointer *object* somewhere, or are you talking about a pointer
*value*, also known as an "address"?

How much do you know about the internal workings of compilers?

>> > In a simplistic sense, all variables are pointers to a segment of
>> > program data, modifiable or otherwise.

No, a variable is not a pointer to a chunk of memory, it *is* a chunk of
memory (which can contain a value at execution time).

More precisely, the C standard doesn't define the word "variable", but I
presume that a "variable" is a kind of "object", which is defined as a
"region of data storage in the execution environment, the contents of
which can represent values".  Not a pointer in sight.

>> >                                         In the real world, variable
>> > values may move around in and from RAM to CPU registers and back while
>> > being identified as a single consistent entity.  This helps to
>> > illustrate the difference between a language and its implementation.
>> 
>> So a char object is really a pointer to char, an int object is
>> really a pointer to int, and a char** object is really a char**?

Sorry, what I meant to write is that a char** object is really (in your
model) a char***, i.e., a pointer to a char**.

> No, a char ** object is a pointer to a char ** object.

Was that really what you meant to type?  I messed up the number of *s,
so I can hardly criticize you for doing the same thing, but I'd like to
know what you meant.

In your model, as I understand it, an int object is really a pointer to
int, or an int*.  It would follow that a char** object is really a
pointer to a char**, or a char***.  Or am I misunderstanding you?

In fact, an int object is an int object.  I find that so obvious
that it seems odd to have to state it.  For an operation on such
an object, a compiler might have to generate code that determines
that object's address.  That does not in any sense imply that an
int object is "really" a pointer.  You can construct a pointer to
an int object, but the pointer and the object it points to are two
different things.

>> I think you're simply adding an extra -- and unnecessary -- level
>> of indirection, and of confusion.
>> 
>> There may also be some confusion between pointer *objects* and
>> pointer *values* (i.e., addresses).
>
> Let me say again that I am not talking about addresses as might be
> referenced in object code.  Perhaps it will clarify that I am thinking
> of something like the phase space of the compiler as it is building
> the tree representing the source code of the program.

And we're getting even farther afield from the actual semantics of a C
program.

An object doesn't even exist during compilation.  Memory for it
is allocated as the program is executed.  Pointers (either pointer
objects or pointer values) do not exist until the program executes.
The "phase space" of the compiler is irrelevant.

When you claim that a struct object is "really" a pointer, are you
referring to something that exists internally to a compiler during
compilation?  If so, of what possible use is it to understanding
what a struct object is?

>> If I define an object of struct type and perform some operation
>> on it (say, a simple assignment), the generated code might very
>> well refer to the address of the object.  There is no *object*
>> of pointer type, either explicitly or implicitly.
>
> That could depend on the way the compiler is designed.

Perhaps.  But an object of pointer-to-struct type is a region
of memory holding the address of the struct object.  Why would a
compiler allocate and initialize such a pointer object, or generate
code to do so, in the absence of C code to take the addrss of the
struct object?

>> Struct objects are very often manipulated via pointers (simulating
>> by-reference argument passing, for example), but that's a coding
>> convention, not anything inherent to the language.
>
> Agreed.

So where do these "pointers" you keep talking about come from?
In what sense do they exist?

[big snip]

> See my comment above regarding the compiler's phase space, although
> I'm not quite sure that is the best term to use in describing what I
> am thinking.

I'm pretty sure it isn't.

[...]

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
    Will write code for food.
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"

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Compatibility question Steve Thompson <stevet810@gmail.com> - 2012-10-20 03:08 +0000
  Re: Compatibility question Ian Collins <ian-news@hotmail.com> - 2012-10-20 16:20 +1300
    Re: Compatibility question Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-10-20 11:50 +0100
      Re: Compatibility question Keith Thompson <kst-u@mib.org> - 2012-10-20 11:41 -0700
        Re: Compatibility question Steve Thompson <stevet810@gmail.com> - 2012-10-21 23:04 +0000
          Re: Compatibility question Barry Schwarz <schwarzb@dqel.com> - 2012-10-21 20:27 -0700
    Re: Compatibility question Steve Thompson <stevet810@gmail.com> - 2012-10-20 04:01 +0000
      Re: Compatibility question Ian Collins <ian-news@hotmail.com> - 2012-10-21 09:26 +1300
        Re: Compatibility question Steve Thompson <stevet810@gmail.com> - 2012-10-21 22:54 +0000
          Re: Compatibility question Ian Collins <ian-news@hotmail.com> - 2012-10-22 12:29 +1300
            Re: Compatibility question Steve Thompson <stevet810@gmail.com> - 2012-10-21 23:50 +0000
              Re: Compatibility question Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-10-22 02:27 +0100
                Re: Compatibility question Steve Thompson <stevet810@gmail.com> - 2012-10-22 02:28 +0000
                Re: Compatibility question Eric Sosman <esosman@comcast-dot-net.invalid> - 2012-10-22 11:18 -0400
                Re: Compatibility question Steve Thompson <stevet810@gmail.com> - 2012-10-22 16:19 +0000
                Re: Compatibility question Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-10-22 17:37 +0100
                Re: Compatibility question Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-10-22 17:40 +0100
                Re: Compatibility question Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-10-22 17:30 +0100
                Re: Compatibility question Steve Thompson <stevet810@gmail.com> - 2012-10-22 17:23 +0000
                Re: Compatibility question Keith Thompson <kst-u@mib.org> - 2012-10-22 10:52 -0700
                Re: Compatibility question Steve Thompson <stevet810@gmail.com> - 2012-10-22 18:42 +0000
                Re: Compatibility question Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-10-22 21:06 +0100
                Re: Compatibility question Steve Thompson <stevet810@gmail.com> - 2012-10-22 22:52 +0000
                Re: Compatibility question Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-10-23 03:33 +0100
                Re: Compatibility question Steve Thompson <stevet810@gmail.com> - 2012-10-23 12:28 +0000
                Re: Compatibility question "BartC" <bc@freeuk.com> - 2012-10-23 16:45 +0100
                Re: Compatibility question Steve Thompson <stevet810@gmail.com> - 2012-10-23 17:53 +0000
                Re: Compatibility question Keith Thompson <kst-u@mib.org> - 2012-10-23 12:41 -0700
                Re: Compatibility question "BartC" <bc@freeuk.com> - 2012-10-23 21:28 +0100
                Re: Compatibility question Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-10-23 20:57 +0100
                Re: Compatibility question Keith Thompson <kst-u@mib.org> - 2012-10-23 12:26 -0700
                Re: Compatibility question Steve Thompson <stevet810@gmail.com> - 2012-10-23 20:39 +0000
                Re: Compatibility question Keith Thompson <kst-u@mib.org> - 2012-10-23 15:53 -0700
                Re: Compatibility question "BartC" <bc@freeuk.com> - 2012-10-24 00:57 +0100
                Re: Compatibility question Keith Thompson <kst-u@mib.org> - 2012-10-23 20:06 -0700
                Re: Compatibility question "BartC" <bc@freeuk.com> - 2012-10-24 11:18 +0100
                Re: Compatibility question Keith Thompson <kst-u@mib.org> - 2012-10-24 12:04 -0700
                Re: Compatibility question "BartC" <bc@freeuk.com> - 2012-10-24 23:10 +0100
                Re: Compatibility question Keith Thompson <kst-u@mib.org> - 2012-10-24 16:58 -0700
                Re: Compatibility question "BartC" <bc@freeuk.com> - 2012-10-25 01:46 +0100
                Re: Compatibility question Keith Thompson <kst-u@mib.org> - 2012-10-24 19:40 -0700
                Re: Compatibility question Steve Thompson <stevet810@gmail.com> - 2012-10-24 20:20 +0000
                Re: Compatibility question Keith Thompson <kst-u@mib.org> - 2012-10-24 14:38 -0700
                Re: Compatibility question James Kuyper <jameskuyper@verizon.net> - 2012-10-24 17:55 -0400
                Re: Compatibility question Keith Thompson <kst-u@mib.org> - 2012-10-24 16:29 -0700
                Re: Compatibility question Greg Martin <greg@softsprocket.com> - 2012-10-24 15:21 -0700
                Re: Compatibility question Steve Thompson <stevet810@gmail.com> - 2012-10-26 03:30 +0000
                Re: Compatibility question Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-10-26 13:02 +0100
                Re: Compatibility question Steve Thompson <stevet810@gmail.com> - 2012-10-27 00:04 +0000
                Re: Compatibility question Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-10-27 20:15 +0100
                Re: Compatibility question Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2012-10-27 16:36 -0600
                Re: Compatibility question Steve Thompson <stevet810@gmail.com> - 2012-10-28 17:36 +0000
                Re: Compatibility question Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2012-10-27 22:42 +0300
                Re: Compatibility question Greg Martin <greg@softsprocket.com> - 2012-10-27 14:30 -0700
                Re: Compatibility question "BartC" <bc@freeuk.com> - 2012-10-27 22:57 +0100
                Re: Compatibility question Keith Thompson <kst-u@mib.org> - 2012-10-27 17:04 -0700
                Re: Compatibility question Keith Thompson <kst-u@mib.org> - 2012-10-27 17:14 -0700
                Re: Compatibility question Keith Thompson <kst-u@mib.org> - 2012-10-27 18:04 -0700
                Re: Compatibility question Keith Thompson <kst-u@mib.org> - 2012-10-24 16:46 -0700
                Re: Compatibility question Steve Thompson <stevet810@gmail.com> - 2012-10-25 01:18 +0000
                Re: Compatibility question Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2012-10-27 21:08 +0300
                Re: Compatibility question Steve Thompson <stevet810@gmail.com> - 2012-10-28 17:38 +0000
                Re: Compatibility question "BartC" <bc@freeuk.com> - 2012-10-23 00:38 +0100
                Re: Compatibility question Keith Thompson <kst-u@mib.org> - 2012-10-22 16:50 -0700
                Re: Compatibility question Steve Thompson <stevet810@gmail.com> - 2012-10-23 12:33 +0000
                Re: Compatibility question Öö Tiib <ootiib@hot.ee> - 2012-10-22 14:23 -0700
                Re: Compatibility question James Kuyper <jameskuyper@verizon.net> - 2012-10-22 17:26 -0400
                Re: Compatibility question Ian Collins <ian-news@hotmail.com> - 2012-10-23 10:55 +1300
                Re: Compatibility question Steve Thompson <stevet810@gmail.com> - 2012-10-22 23:27 +0000
                Re: Compatibility question "BartC" <bc@freeuk.com> - 2012-10-22 22:44 +0100
                Re: Compatibility question Les Cargill <lcargill99@comcast.com> - 2012-10-22 17:15 -0500
                Re: Compatibility question "BartC" <bc@freeuk.com> - 2012-10-23 00:13 +0100
                Re: Compatibility question James Kuyper <jameskuyper@verizon.net> - 2012-10-22 18:35 -0400
                Re: Compatibility question "BartC" <bc@freeuk.com> - 2012-10-23 00:14 +0100
                Re: Compatibility question Steve Thompson <stevet810@gmail.com> - 2012-10-22 23:42 +0000
                Re: Compatibility question Ian Collins <ian-news@hotmail.com> - 2012-10-23 13:35 +1300
                Re: Compatibility question Keith Thompson <kst-u@mib.org> - 2012-10-22 17:01 -0700
                Re: Compatibility question "BartC" <bc@freeuk.com> - 2012-10-23 01:47 +0100
              Re: Compatibility question Keith Thompson <kst-u@mib.org> - 2012-10-21 19:27 -0700

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