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Re: Two different Results between C and C++

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Richard Damon <Richard@Damon-Family.org> writes:

> On 6/21/20 2:40 PM, Keith Thompson wrote:
>
>> Tim Rentsch <tr.17687@z991.linuxsc.com> writes:
>>
>>> Keith Thompson <Keith.S.Thompson+u@gmail.com> writes:
>>
>> [...]
>>
>>>> All these statements are true, but none are particularly relevant.
>>>> You can construct an infinite number of valid C or C++ programs by
>>>> playing games with the grammar.  There are an infinite number of
>>>> legal C programs that use conditional operators nested 10,000
>>>> levels deep, but I doubt that any such programs exist in the wild
>>>> (except *maybe* as compiler capacity tests).  I think it's more
>>>> useful to look at actual code rather than contrived infinite sets
>>>> of programs.
>>>>
>>>> There are *some* programs that are legal C but not legal C++.  The
>>>> most common causes of this are probably C++'s tighter restrictions
>>>> on implicit conversions involving void*, C code that uses C++
>>>> keywords as identifiers, and C features that haven't been
>>>> incorporated into C++, such as VLAs, _Generic, and compound
>>>> literals.  But almost all programs that are legal C but not legal
>>>> C++ can be made into legal C++ with a small effort.
>>>
>>> This claim sounds like an article of faith, being offered without
>>> either proof or evidence.  If meant as just an empirical statement
>>> then of course there is no way anyone can know that.
>>
>> When I wrote "almost all programs that are legal C but not legal
>> C++", I was referring to real-world programs, not the infinite set
>> of all possible programs.  I probably could have made that clearer.
>>
>> My statement was not an "article of faith".  It was somewhat
>> speculative.  I think it was correct, but it's not worth the effort
>> to perform a major research project to verify it.
>>
>> [...]
>
> My experiance would be a bit different, most C programs from the
> 'wild' would likely fail as C++ precisely because of the implicit
> conversion from void* that does not happen in C++.  These programs
> can trivially be converted to programs which are valid in both C
> and C++ by adding the explicit cast.  [...]

Lots of C programs fail as C++ because of constraint violations
or syntax errors.  The problems are (obviously) easy to find,
and mostly I think easy to fix.

The bigger problem is C code that is legal C++ but has different
semantics in C++ than in C.  There are more of these than most
C or C++ developers think there are.  I don't know anyone,
including myself, who can say with confidence that they know
all the different ways that source code can have a different
meaning in C++ compared to C.  Clearly if someone doesn't know
what all the potential discrepancies are there is no way they
could find and fix them.

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Re: Two different Results between C and C++ Tim Rentsch <tr.17687@z991.linuxsc.com> - 2020-09-02 23:00 -0700
  Re: Two different Results between C and C++ Öö Tiib <ootiib@hot.ee> - 2020-09-03 09:57 -0700
    Re: Two different Results between C and C++ David Brown <david.brown@hesbynett.no> - 2020-09-03 20:04 +0200
    Re: Two different Results between C and C++ Tim Rentsch <tr.17687@z991.linuxsc.com> - 2020-09-04 06:44 -0700
      Re: Two different Results between C and C++ Öö Tiib <ootiib@hot.ee> - 2020-09-05 06:51 -0700
        Re: Two different Results between C and C++ Tim Rentsch <tr.17687@z991.linuxsc.com> - 2020-09-08 07:34 -0700
          Re: Two different Results between C and C++ Öö Tiib <ootiib@hot.ee> - 2020-09-10 23:47 -0700
            Re: Two different Results between C and C++ James Kuyper <jameskuyper@alumni.caltech.edu> - 2020-09-11 10:17 -0400
              Re: Two different Results between C and C++ Öö Tiib <ootiib@hot.ee> - 2020-09-11 08:32 -0700
                Re: Two different Results between C and C++ James Kuyper <jameskuyper@alumni.caltech.edu> - 2020-09-11 12:03 -0400
                Re: Two different Results between C and C++ Öö Tiib <ootiib@hot.ee> - 2020-09-12 02:49 -0700
  Re: Two different Results between C and C++ James Kuyper <jameskuyper@alumni.caltech.edu> - 2020-09-03 12:46 -0700
    Re: Two different Results between C and C++ Tim Rentsch <tr.17687@z991.linuxsc.com> - 2020-09-10 06:20 -0700
      Re: Two different Results between C and C++ James Kuyper <jameskuyper@alumni.caltech.edu> - 2020-09-10 07:37 -0700
      Re: Two different Results between C and C++ James Kuyper <jameskuyper@alumni.caltech.edu> - 2020-09-10 10:44 -0400

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