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Re: Few questions about casting away const.

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Daniel <daniel@inventati.org> writes:
> const char *s = "abc";
> char *t = s;
>
> printf("%s", s);
>
> Firstly, the direct cast from 'const char *' to 'char *' which is seen
> on 
> the second line. I've read that this is illegal in standard C, is that 
> true?

As Ben said, yes, this is invalid (a constraint violation, requiring a
diagnostic), and there is no cast.  There may or may not be an implicit
conversion, but since it's a constraint violation, the initialization
has no actual semantics, and a compiler is free to reject it.

> Secondly, given that printf's 's' specifier matches a 'char *', the    
> comment in section 6.5.2.2p6 of C11 seems to indicate that these types 
> are interchangeable in this situation. If the direct cast is invalid, 
> would this be a legal alternative?:
>
>
> #define unconst(p) (unconst_(0, p))
>
> char *unconst_(int unused, ...)
> {
>         va_list args;
>         char *r;
>
>         va_start(args, unused);
>         r = va_arg(args, char *);
>         va_end(args);
>         return r;
> }
>  
> const char *s = "abc";
> char *t = unconst(s);

Given your declarations, both of these are valid:

    printf("%s", s);
    printf("%s", t);

The standard doesn't say that "%s" requires a char* argument; it says
"the argument shall be a pointer to the initial element of an array of
character type", and both s and t qualify.

> Or, are there other possibilities?

What problem are you trying to solve?

If you want to convert a const char* to char*, just cast it:

    const char *s = "abc";
    char *t = (char*)s;

But consider carefully whether you need to do that.  Creating a
non-const pointer to const data risks undefined behavior if you later
modify the pointed-to object.

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Working, but not speaking, for JetHead Development, Inc.
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"

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Few questions about casting away const. Daniel <daniel@inventati.org> - 2014-03-04 11:18 +0000
  Re: Few questions about casting away const. Ben Bacarisse <ben.usenet@bsb.me.uk> - 2014-03-04 12:39 +0000
  Re: Few questions about casting away const. Kaz Kylheku <kaz@kylheku.com> - 2014-03-04 17:05 +0000
  Re: Few questions about casting away const. Keith Thompson <kst-u@mib.org> - 2014-03-04 10:16 -0800
    Re: Few questions about casting away const. Daniel <daniel@inventati.org> - 2014-03-05 03:45 +0000
      Re: Few questions about casting away const. Ben Bacarisse <ben.usenet@bsb.me.uk> - 2014-03-05 04:20 +0000
  Re: Few questions about casting away const. James Kuyper <jameskuyper@verizon.net> - 2014-03-04 15:18 -0500
    Re: Few questions about casting away const. Daniel <daniel@inventati.org> - 2014-03-05 04:14 +0000
      Re: Few questions about casting away const. Ben Bacarisse <ben.usenet@bsb.me.uk> - 2014-03-05 04:35 +0000
        Re: Few questions about casting away const. Daniel <daniel@inventati.org> - 2014-03-05 06:11 +0000
          Re: Few questions about casting away const. Ben Bacarisse <ben.usenet@bsb.me.uk> - 2014-03-05 12:14 +0000
            Re: Few questions about casting away const. Daniel Harrison <daniel@inventati.org> - 2014-03-07 04:31 +0000
              Re: Few questions about casting away const. Barry Schwarz <schwarzb@dqel.com> - 2014-03-06 21:23 -0800
                Re: Few questions about casting away const. Kaz Kylheku <kaz@kylheku.com> - 2014-03-07 05:59 +0000
              Re: Few questions about casting away const. James Kuyper <jameskuyper@verizon.net> - 2014-03-07 07:18 -0500
              Re: Few questions about casting away const. Tim Rentsch <txr@alumni.caltech.edu> - 2014-03-10 00:30 -0700
                Re: Few questions about casting away const. Keith Thompson <kst-u@mib.org> - 2014-03-10 09:00 -0700
                Re: Few questions about casting away const. Tim Rentsch <txr@alumni.caltech.edu> - 2014-03-10 11:13 -0700
                Re: Few questions about casting away const. Keith Thompson <kst-u@mib.org> - 2014-03-10 12:10 -0700
                Re: Few questions about casting away const. Kaz Kylheku <kaz@kylheku.com> - 2014-03-11 07:38 +0000
                Re: Few questions about casting away const. Tim Rentsch <txr@alumni.caltech.edu> - 2014-03-29 09:02 -0700
          Re: Few questions about casting away const. Tim Rentsch <txr@alumni.caltech.edu> - 2014-03-10 02:29 -0700
            Re: Few questions about casting away const. Keith Thompson <kst-u@mib.org> - 2014-03-10 10:52 -0700
              Re: Few questions about casting away const. Tim Rentsch <txr@alumni.caltech.edu> - 2014-03-29 08:50 -0700

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