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Re: is "x *= ++f * ++f" a valid statement ?

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Tim Rentsch <tr.17687@z991.linuxsc.com> writes:
> Keith Thompson <Keith.S.Thompson+u@gmail.com> writes:
>> Keith Thompson <Keith.S.Thompson+u@gmail.com> writes:
>>> Tim Rentsch <tr.17687@z991.linuxsc.com> writes:
>>>> Keith Thompson <Keith.S.Thompson+u@gmail.com> writes:
>>>>> James Kuyper <jameskuyper@alumni.caltech.edu> writes:
>>>> [...]
>>>>
>>>>>> A minor detail is that a variable must be declared, whereas memory
>>>>>> locations can, for instance, be part of allocated memory for which
>>>>>> no declaration exists - it is still undefined behavior to write
>>>>>> code that applies unsequence side-effects to such memory locations.
>>>>>
>>>>> Digression:  I'm not even sure what "variable" means in C++.  The
>>>>> standard defines the term, but not in a way that really tells us
>>>>> what it means.
>>>>>
>>>>> "A *variable* is introduced by the declaration of a reference other
>>>>> than a non-static data member or of an object.  The variable's name,
>>>>> if any, denotes the reference or object."
>>>>
>>>> What part do you find confusing or hard to understand?
>>>
>>> The missing part that should tell us what a variable *is*.
>>>
>>> It says that certain declarations "introduce" a variable.  That's a
>>> statement about variables, but it doesn't say what a variable is.
>>>
>>> Given:
>>>
>>>     int n;
>>>
>>> we know that the declaration introduces a variable.  Is the object
>>> itself a "variable"?  That's the obvious meaning, and it's consistent
>>> with what the standard says.  Or is a "variable" some kind of logical
>>> binding between an object and a name?  That's also consistent with what
>>> the standard says.  Under the latter interpretation, the "variable" has
>>> a name, and that name denotes an object, but the variable is not the
>>> object.
>>>
>>> Given the above declaration, is the introduced variable an object?  If
>>> so, or if not, how does your answer follow from what the standard says?
>
> [...]
>
>> But the following paragraph says:
>>
>>     A *local entity* is a variable with automatic storage duration
>>     (6.7.5.4), a structured binding (9.6) whose corresponding variable
>>     is such an entity, or the *this object (7.5.3).
>>
>> Storage duration is an attribute of objects.  If a variable can have
>> automatic storage duration, then apparently a variable is an object.
>
> It seems clear that the quoted sentence is meant to be read as
>
>     A *local entity* is a variable [associated with an object that
>     has] automatic storage duration, [etc].

It seems clear to you.  It's not clear to me.  If the only way to
understand a passage is to assume it includes extra words, it is at best
poorly written -- and I can't be sure that any assumptions about what it
*really* means are correct.

> I'm not sure if C++ references also have storage durations, in which
> case the word "object" in that sentence might need to be replaced with
> "object or reference".  The key point though is that the property of
> having automatic storage duration is meant to be associated with the
> affiliated object or reference rather than with the variable itself.

Perhaps so, but that still doesn't tell me what a "variable" is.

Apparently a "variable" is something *associated with* an object.  But
what is it, and how is the concept useful when we already have the
concept of an "object"?

I tried following your advice and searchin for uses of "variable" in the
C++ standard (which, as James Kuyper pointed out, should not be
necessary to understand the definition).  It was not helpful.  For
example, "Variables with static storage duration are initialized as a
consequence of program initiation.".  So a variable is a thing that can
have a storage duration and be initialized?  And it's not an object?

I get the impression (quite possibly mistaken) that the authors of the
standard implicitly assume in some places that a variable is an object,
and in other places that it's something else.

The most obvious meaning is that a "variable" is an object that has a
name introduced by a declaration.  (I'll gloss over the question of
whether something that's const-qualified is a "variable", but I'm
willing to accept that "variable" doesn't have to mean that something
can vary.)  But that's not what the standard says.

I'll note that the C standard does not define or use the term "variable"
(though it does use the word in its non-tecnical sense).  Instead it
refers to "objects".  Perhaps C++ could have done the same.

If I were to ask you "What is a variable in C++?", could you give a
clear answer?  What would that answer be?  Is a variable an object or
is it not?

-- 
Keith Thompson (The_Other_Keith) Keith.S.Thompson+u@gmail.com
void Void(void) { Void(); } /* The recursive call of the void */

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is "x *= ++f * ++f" a valid statement ? Bonita Montero <Bonita.Montero@gmail.com> - 2024-05-23 16:14 +0200
  Re: is "x *= ++f * ++f" a valid statement ? Andrey Tarasevich <andreytarasevich@hotmail.com> - 2024-06-18 18:53 -0700
    Re: is "x *= ++f * ++f" a valid statement ? Bonita Montero <Bonita.Montero@gmail.com> - 2024-06-19 17:40 +0200
    Re: is "x *= ++f * ++f" a valid statement ? PLO olcott <polcott333@gmail.com> - 2024-06-21 16:55 -0500
      Re: is "x *= ++f * ++f" a valid statement ? PLO Richard Damon <richard@damon-family.org> - 2024-06-21 18:10 -0400
        Re: is "x *= ++f * ++f" a valid statement ? PLO olcott <polcott333@gmail.com> - 2024-06-21 18:06 -0500
          Re: is "x *= ++f * ++f" a valid statement ? PLO Richard Damon <richard@damon-family.org> - 2024-06-21 20:14 -0400
          Re: is "x *= ++f * ++f" a valid statement ? PLO Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-06-21 17:35 -0700
            Re: is "x *= ++f * ++f" a valid statement ? PLO olcott <polcott333@gmail.com> - 2024-06-21 22:58 -0500
              Re: is "x *= ++f * ++f" a valid statement ? PLO Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-06-21 23:18 -0700
                Re: is "x *= ++f * ++f" a valid statement ? PLO olcott <polcott333@gmail.com> - 2024-06-22 07:38 -0500
                Re: is "x *= ++f * ++f" a valid statement ? PLO Richard Damon <richard@damon-family.org> - 2024-06-22 09:45 -0400
                Re: is "x *= ++f * ++f" a valid statement ? PLO olcott <polcott333@gmail.com> - 2024-06-22 09:13 -0500
                Re: is "x *= ++f * ++f" a valid statement ? PLO Richard Damon <richard@damon-family.org> - 2024-06-22 10:23 -0400
                Re: is "x *= ++f * ++f" a valid statement ? PLO Mikko <mikko.levanto@iki.fi> - 2024-06-23 11:48 +0300
                Re: is "x *= ++f * ++f" a valid statement ? PLO Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-06-22 17:54 -0700
          Re: is "x *= ++f * ++f" a valid statement ? PLO James Kuyper <jameskuyper@alumni.caltech.edu> - 2024-06-21 23:42 -0400
            Re: is "x *= ++f * ++f" a valid statement ? PLO olcott <polcott333@gmail.com> - 2024-06-21 23:02 -0500
              Re: is "x *= ++f * ++f" a valid statement ? PLO David Brown <david.brown@hesbynett.no> - 2024-06-22 13:09 +0200
                Re: is "x *= ++f * ++f" a valid statement ? PLO olcott <polcott333@gmail.com> - 2024-06-22 07:40 -0500
                Re: is "x *= ++f * ++f" a valid statement ? PLO Richard Damon <richard@damon-family.org> - 2024-06-22 09:56 -0400
                Re: is "x *= ++f * ++f" a valid statement ? PLO "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2024-06-22 12:14 -0700
                Re: is "x *= ++f * ++f" a valid statement ? PLO David Brown <david.brown@hesbynett.no> - 2024-06-22 16:52 +0200
                Re: is "x *= ++f * ++f" a valid statement ? PLO olcott <polcott333@gmail.com> - 2024-06-22 11:27 -0500
                Re: is "x *= ++f * ++f" a valid statement ? PLO Richard Damon <richard@damon-family.org> - 2024-06-22 13:20 -0400
                Re: is "x *= ++f * ++f" a valid statement ? PLO Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-06-22 17:55 -0700
                Re: is "x *= ++f * ++f" a valid statement ? PLO David Brown <david.brown@hesbynett.no> - 2024-06-23 14:46 +0200
              Re: is "x *= ++f * ++f" a valid statement ? PLO James Kuyper <jameskuyper@alumni.caltech.edu> - 2024-06-22 15:48 -0400
  Re: is "x *= ++f * ++f" a valid statement ? Richard Damon <richard@damon-family.org> - 2024-06-18 22:27 -0400
  Re: is "x *= ++f * ++f" a valid statement ? olcott <polcott333@gmail.com> - 2024-06-22 21:51 -0500
    Re: is "x *= ++f * ++f" a valid statement ? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-06-22 21:51 -0700
    Re: is "x *= ++f * ++f" a valid statement ? Richard Damon <richard@damon-family.org> - 2024-06-23 07:32 -0400
      Re: is "x *= ++f * ++f" a valid statement ? olcott <polcott333@gmail.com> - 2024-06-23 07:59 -0500
        Re: is "x *= ++f * ++f" a valid statement ? Richard Damon <richard@damon-family.org> - 2024-06-23 14:25 -0400
          Re: is "x *= ++f * ++f" a valid statement ? olcott <polcott333@gmail.com> - 2024-06-23 14:18 -0500
            Re: is "x *= ++f * ++f" a valid statement ? Richard Damon <richard@damon-family.org> - 2024-06-23 16:23 -0400
              Re: is "x *= ++f * ++f" a valid statement ? David Brown <david.brown@hesbynett.no> - 2024-06-24 09:11 +0200
  Re: is "x *= ++f * ++f" a valid statement ? what@tf.com (testuseri2p) - 2024-07-22 17:51 +0000
    Re: is "x *= ++f * ++f" a valid statement ? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-07-22 14:58 -0700
    Re: is "x *= ++f * ++f" a valid statement ? James Kuyper <jameskuyper@alumni.caltech.edu> - 2024-07-22 21:57 -0400
      Re: is "x *= ++f * ++f" a valid statement ? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-07-22 20:40 -0700
        Re: is "x *= ++f * ++f" a valid statement ? James Kuyper <jameskuyper@alumni.caltech.edu> - 2024-07-23 10:15 -0400
        Re: is "x *= ++f * ++f" a valid statement ? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-07-23 07:28 -0700
          Re: is "x *= ++f * ++f" a valid statement ? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-07-23 14:42 -0700
            Re: is "x *= ++f * ++f" a valid statement ? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-07-23 15:05 -0700
              Re: is "x *= ++f * ++f" a valid statement ? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-08-11 06:22 -0700
                Re: is "x *= ++f * ++f" a valid statement ? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-08-11 14:09 -0700
            Re: is "x *= ++f * ++f" a valid statement ? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-08-11 06:11 -0700
              Re: is "x *= ++f * ++f" a valid statement ? Chris Ahlstrom <OFeem1987@teleworm.us> - 2024-08-11 09:25 -0400
              Re: is "x *= ++f * ++f" a valid statement ? James Kuyper <jameskuyper@alumni.caltech.edu> - 2024-08-11 14:17 -0400
      Re: is "x *= ++f * ++f" a valid statement ? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-07-23 07:32 -0700
    Re: is "x *= ++f * ++f" a valid statement ? Andrey Tarasevich <andreytarasevich@hotmail.com> - 2024-07-22 19:53 -0700
    Re: is "x *= ++f * ++f" a valid statement ? James Kuyper <jameskuyper@alumni.caltech.edu> - 2024-07-23 10:01 -0400

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