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Re: Array sizes and const

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"BartC" <bc@freeuk.com> writes:
> "Keith Thompson" <kst-u@mib.org> wrote in message
> news:lny5o4dsqr.fsf@nuthaus.mib.org...
>> "BartC" <bc@freeuk.com> writes:
>
>>> why ... didn't someone just add a feature like:
>>>
>>> constant int d = 4000;
>
>> In fact someone did do that: Bjarne Stroustrup, when he was designing
>> C++.
>>
>> Except that he didn't introduce a new keyword; he just tweaked the
>> semantics of "const".
>>
>> In C++, if you declare
>>
>>    const some_type x = init_expr;
>>
>> x becomes a constant expression *if* some_type is arithmetic *and*
>> init_expr is a constant expression.  Otherwise, it has pretty much the
>> same meaning it has in C.  (I probably haven't gotten that 100% right,
>> but that's the general idea.)
>
> It's taking a slightly tricky concept, a 'const' attribute, and making it
> even harder. Now it means read-only variables, read-only parameters, *and*
> named literals.

Variables and parameters are both objects, and "const" means the same
thing for both, so it's not as tricky as you imply.

>                 Provided it can be agreed whether a particular right-hand
> expression is a constant or not; some compilers might be clever enough to
> evaluate the results of certain functions, others won't. And it's still not
> clear whether they are allowed as l-values or not.

The C standard defines "constant expressions" unambiguously in section
6.6.  A constant expression isn't merely an expression that some
particular compiler is able to evaluate at compile time.

The C++ standard does make it perfectly clear whether such constants are
allowed as lvalues.  (I haven't found the relevant wording in the C++
standard, but g++ -pedantic allows it.)

> Although I suppose, compared with the rest of C++, no-one would really
> notice..
>
> Using a new keyword and a new concept would keep things simple: it would
> only be used to apply a type and name to a literal. The right-hand-size
> *must* be a constant (on any compiler). You can't use them as l-values. And
> it would almost be the same as using the literal itself (the declaration may 
> introduce a cast).

Using the same mechanism that's been used in C++ for years would keep
things even simpler.

I certainly don't argue that C should adopt C++ features wholesale, but
in this case the way C++ does it is good enough, and *unnecessary*
incompatibilities are, well, unnecessary.

[...]

-- 
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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Array sizes and const Les Cargill <lcargill99@comcast.com> - 2012-06-02 15:43 -0500
  Re: Array sizes and const Ian Collins <ian-news@hotmail.com> - 2012-06-03 08:56 +1200
  Re: Array sizes and const Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-06-02 16:59 -0400
    Re: Array sizes and const Les Cargill <lcargill99@comcast.com> - 2012-06-02 16:35 -0500
      Re: Array sizes and const jgk@panix.com (Joe keane) - 2012-06-08 20:05 +0000
      Re: Array sizes and const jgk@panix.com (Joe keane) - 2012-06-09 21:36 +0000
    Re: Array sizes and const Les Cargill <lcargill99@comcast.com> - 2012-06-02 16:43 -0500
  Re: Array sizes and const Keith Thompson <kst-u@mib.org> - 2012-06-02 14:59 -0700
    Re: Array sizes and const Les Cargill <lcargill99@comcast.com> - 2012-06-02 17:12 -0500
      Re: Array sizes and const Keith Thompson <kst-u@mib.org> - 2012-06-02 18:27 -0700
        Re: Array sizes and const Les Cargill <lcargill99@comcast.com> - 2012-06-02 22:27 -0500
      Re: Array sizes and const "BartC" <bc@freeuk.com> - 2012-06-03 11:24 +0100
        Re: Array sizes and const Keith Thompson <kst-u@mib.org> - 2012-06-03 13:51 -0700
          Re: Array sizes and const "BartC" <bc@freeuk.com> - 2012-06-04 12:20 +0100
            Re: Array sizes and const Jens Gustedt <jens.gustedt@loria.fr> - 2012-06-04 13:54 +0200
            Re: Array sizes and const Keith Thompson <kst-u@mib.org> - 2012-06-04 14:32 -0700
  Re: Array sizes and const Nobody <nobody@nowhere.com> - 2012-06-02 23:50 +0100
    Re: Array sizes and const Ian Collins <ian-news@hotmail.com> - 2012-06-03 10:53 +1200
      Re: Array sizes and const Nobody <nobody@nowhere.com> - 2012-06-03 14:35 +0100
    Re: Array sizes and const James Kuyper <jameskuyper@verizon.net> - 2012-06-02 21:11 -0400
      Re: Array sizes and const Malcolm McLean <malcolm.mclean5@btinternet.com> - 2012-06-03 07:39 -0700
        Re: Array sizes and const James Kuyper <jameskuyper@verizon.net> - 2012-06-03 16:03 -0400
        Re: Array sizes and const Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-06-03 21:49 +0100
          Re: Array sizes and const Keith Thompson <kst-u@mib.org> - 2012-06-03 14:49 -0700
          Re: Array sizes and const Malcolm McLean <malcolm.mclean5@btinternet.com> - 2012-06-03 15:02 -0700
            Re: Array sizes and const Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-06-04 01:21 +0100
        Re: Array sizes and const Ian Collins <ian-news@hotmail.com> - 2012-06-04 09:08 +1200
          Re: Array sizes and const Malcolm McLean <malcolm.mclean5@btinternet.com> - 2012-06-03 15:08 -0700
            Re: Array sizes and const Ian Collins <ian-news@hotmail.com> - 2012-06-04 10:37 +1200
              Re: Array sizes and const "BartC" <bc@freeuk.com> - 2012-06-03 23:47 +0100
                Re: Array sizes and const Ian Collins <ian-news@hotmail.com> - 2012-06-04 10:55 +1200
                Re: Array sizes and const Malcolm McLean <malcolm.mclean5@btinternet.com> - 2012-06-03 16:04 -0700
                Re: Array sizes and const Ian Collins <ian-news@hotmail.com> - 2012-06-04 11:14 +1200
                Re: Array sizes and const "BartC" <bc@freeuk.com> - 2012-06-04 00:23 +0100
                Re: Array sizes and const Ian Collins <ian-news@hotmail.com> - 2012-06-04 11:30 +1200
                Re: Array sizes and const "BartC" <bc@freeuk.com> - 2012-06-04 00:52 +0100
                Re: Array sizes and const Ian Collins <ian-news@hotmail.com> - 2012-06-04 12:04 +1200
                Re: Array sizes and const Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-06-04 01:39 +0100
                Re: Array sizes and const Malcolm McLean <malcolm.mclean5@btinternet.com> - 2012-06-04 02:35 -0700
                Re: Array sizes and const Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-06-04 01:33 +0100
                Re: Array sizes and const Ian Collins <ian-news@hotmail.com> - 2012-06-04 12:35 +1200
                Re: Array sizes and const pacman@kosh.dhis.org (Alan Curry) - 2012-06-04 21:28 +0000
        Re: Array sizes and const James Kuyper <jameskuyper@verizon.net> - 2012-06-04 12:08 -0400
        Re: Array sizes and const jgk@panix.com (Joe keane) - 2012-06-04 22:49 +0000
    Re: Array sizes and const David Brown <david.brown@hesbynett.no> - 2012-06-03 15:46 -0500
  Re: Array sizes and const Tim Rentsch <txr@alumni.caltech.edu> - 2012-06-08 02:08 -0700
    Re: Array sizes and const Les Cargill <lcargill99@comcast.com> - 2012-06-08 06:45 -0500
      Re: Array sizes and const Tim Rentsch <txr@alumni.caltech.edu> - 2012-06-08 10:49 -0700
        Re: Array sizes and const Les Cargill <lcargill99@comcast.com> - 2012-06-08 18:05 -0500
    Re: Array sizes and const Keith Thompson <kst-u@mib.org> - 2012-06-08 13:31 -0700
      Re: Array sizes and const Tim Rentsch <txr@alumni.caltech.edu> - 2012-06-08 20:30 -0700

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