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Simultaneous read and write on a variable in C++

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  Simultaneous read and write on a variable in C++ Novice Coder <extngg1@gmail.com> - 2012-01-11 09:08 -0800
    Re: Simultaneous read and write on a variable in C++ Victor Bazarov <v.bazarov@comcast.invalid> - 2012-01-11 12:28 -0500
      Re: Simultaneous read and write on a variable in C++ Victor Bazarov <v.bazarov@comcast.invalid> - 2012-01-12 13:07 -0500
        Re: Simultaneous read and write on a variable in C++ Prasoon Saurav <prasoonsaurav.nit@gmail.com> - 2012-01-12 10:11 -0800
          Re: Simultaneous read and write on a variable in C++ Leigh Johnston <leigh@i42.co.uk> - 2012-01-12 18:46 +0000
            Re: Simultaneous read and write on a variable in C++ Prasoon Saurav <prasoonsaurav.nit@gmail.com> - 2012-01-12 11:18 -0800
              Re: Simultaneous read and write on a variable in C++ Leigh Johnston <leigh@i42.co.uk> - 2012-01-12 19:28 +0000
                Re: Simultaneous read and write on a variable in C++ Leigh Johnston <leigh@i42.co.uk> - 2012-01-12 19:41 +0000
                  Re: Simultaneous read and write on a variable in C++ Prasoon Saurav <prasoonsaurav.nit@gmail.com> - 2012-01-12 11:46 -0800
                Re: Simultaneous read and write on a variable in C++ Prasoon Saurav <prasoonsaurav.nit@gmail.com> - 2012-01-12 11:44 -0800
                  Re: Simultaneous read and write on a variable in C++ Leigh Johnston <leigh@i42.co.uk> - 2012-01-12 19:46 +0000
                    Re: Simultaneous read and write on a variable in C++ Leigh Johnston <leigh@i42.co.uk> - 2012-01-12 19:53 +0000
                      Re: Simultaneous read and write on a variable in C++ Prasoon Saurav <prasoonsaurav.nit@gmail.com> - 2012-01-12 12:01 -0800
                        Re: Simultaneous read and write on a variable in C++ Leigh Johnston <leigh@i42.co.uk> - 2012-01-12 20:10 +0000
                          Re: Simultaneous read and write on a variable in C++ Prasoon Saurav <prasoonsaurav.nit@gmail.com> - 2012-01-12 12:13 -0800
                            Re: Simultaneous read and write on a variable in C++ Leigh Johnston <leigh@i42.co.uk> - 2012-01-12 20:31 +0000
                        Re: Simultaneous read and write on a variable in C++ scott@slp53.sl.home (Scott Lurndal) - 2012-01-13 18:12 +0000
                Re: Simultaneous read and write on a variable in C++ Prasoon Saurav <prasoonsaurav.nit@gmail.com> - 2012-01-12 11:37 -0800
                  Re: Simultaneous read and write on a variable in C++ Leigh Johnston <leigh@i42.co.uk> - 2012-01-12 19:56 +0000
                    Re: Simultaneous read and write on a variable in C++ Prasoon Saurav <prasoonsaurav.nit@gmail.com> - 2012-01-12 11:58 -0800
                      Re: Simultaneous read and write on a variable in C++ Leigh Johnston <leigh@i42.co.uk> - 2012-01-12 20:04 +0000
                Re: Simultaneous read and write on a variable in C++ Juha Nieminen <nospam@thanks.invalid> - 2012-01-13 06:34 +0000
    Re: Simultaneous read and write on a variable in C++ SG <s.gesemann@gmail.com> - 2012-01-11 13:11 -0800
    Re: Simultaneous read and write on a variable in C++ Calum <spambox@calumgrant.net> - 2012-01-11 13:18 -0800
    Re: Simultaneous read and write on a variable in C++ Prasoon Saurav <prasoonsaurav.nit@gmail.com> - 2012-01-12 03:11 -0800
      Re: Simultaneous read and write on a variable in C++ "Fred Zwarts \(KVI\)" <F.Zwarts@KVI.nl> - 2012-01-12 16:35 +0100
        Re: Simultaneous read and write on a variable in C++ Prasoon Saurav <prasoonsaurav.nit@gmail.com> - 2012-01-12 08:06 -0800
      Re: Simultaneous read and write on a variable in C++ gwowen <gwowen@gmail.com> - 2012-01-12 07:48 -0800
        Re: Simultaneous read and write on a variable in C++ Prasoon Saurav <prasoonsaurav.nit@gmail.com> - 2012-01-12 07:55 -0800
          Re: Simultaneous read and write on a variable in C++ Gareth Owen <gwowen@gmail.com> - 2012-01-12 19:36 +0000
            Re: Simultaneous read and write on a variable in C++ Victor Bazarov <v.bazarov@comcast.invalid> - 2012-01-12 16:31 -0500
      Re: Simultaneous read and write on a variable in C++ Nick Keighley <nick_keighley_nospam@hotmail.com> - 2012-01-13 02:05 -0800
      Re: Simultaneous read and write on a variable in C++ Goran <goran.pusic@gmail.com> - 2012-01-13 04:24 -0800
        Re: Simultaneous read and write on a variable in C++ Prasoon Saurav <prasoonsaurav.nit@gmail.com> - 2012-01-13 04:52 -0800
    Re: Simultaneous read and write on a variable in C++ Leigh Johnston <leigh@i42.co.uk> - 2012-01-12 12:39 +0000
    Re: Simultaneous read and write on a variable in C++ Leigh Johnston <leigh@i42.co.uk> - 2012-01-12 12:50 +0000
      Re: Simultaneous read and write on a variable in C++ Leigh Johnston <leigh@i42.co.uk> - 2012-01-12 12:58 +0000
        Re: Simultaneous read and write on a variable in C++ Prasoon Saurav <prasoon@parallocity.com> - 2012-01-12 05:55 -0800
          Re: Simultaneous read and write on a variable in C++ Leigh Johnston <leigh@i42.co.uk> - 2012-01-12 13:59 +0000
            Re: Simultaneous read and write on a variable in C++ Prasoon Saurav <prasoon@parallocity.com> - 2012-01-12 06:14 -0800
              Re: Simultaneous read and write on a variable in C++ Leigh Johnston <leigh@i42.co.uk> - 2012-01-12 14:30 +0000
              Re: Simultaneous read and write on a variable in C++ Juha Nieminen <nospam@thanks.invalid> - 2012-01-12 14:34 +0000
      Re: Simultaneous read and write on a variable in C++ Prasoon Saurav <prasoon@parallocity.com> - 2012-01-12 05:54 -0800
    Re: Simultaneous read and write on a variable in C++ BCFD36 <bcfd36@cruzio.com> - 2012-01-14 01:52 -0800

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#13282

On 12/01/2012 14:14, Prasoon Saurav wrote:
> On Jan 12, 6:59 pm, Leigh Johnston<le...@i42.co.uk>  wrote:
>> On 12/01/2012 13:55, Prasoon Saurav wrote:
>>
>>
>>
>>
>>
>>
>>
>>
>>
>>> On Jan 12, 5:58 pm, Leigh Johnston<le...@i42.co.uk>    wrote:
>>>> On 12/01/2012 12:50, Leigh Johnston wrote:
>>
>>>>> On 11/01/2012 17:08, Novice Coder wrote:
>>>>>> Here is the code
>>
>>>>>> int main()
>>>>>> {
>>>>>> char *p;
>>>>>> char a[]={"Hello"};
>>>>>> (p=a)[0]; // This line
>>>>>> return 0;
>>>>>> }
>>
>>>>>> Is the behaviour of this code undefined in C++?
>>
>>>>> I vote that it is not UB.
>>
>>>>> (p=a)[0] is equivalent to *(p=a); if *(p=a) is UB then surely *(p2++) is
>>>>> also UB? Clearly nonsense. (p=a)[0] + (p=a+1)[0] is UB however.
>>
>>>> I meant '*(++p2)' not '*(p2++)'.
>>
>>>> /Leigh
>>
>>> Yes I believe *(++p2) is UB too.
>>
>> Bullshit; are you trolling?
>>
>> /Leigh
>
> Huh?

The following piece of code is not UB:

int main()
{
	const char a[] = "Hello, world!";
	const char* p = a;
	do
	{
		std::cout << *p;
	} while(*++p);
}

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#13283

Prasoon Saurav <prasoon@parallocity.com> wrote:
> On Jan 12, 6:59 pm, Leigh Johnston <le...@i42.co.uk> wrote:
>> Bullshit; are you trolling?
>>
>> /Leigh
> 
> Huh?

  Don't worry. He's one of those people who seem to think that
"trolling" means "disagrees with me" (and mostly uses it as an
insult).

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#13275

On Jan 12, 5:50 pm, Leigh Johnston <le...@i42.co.uk> wrote:
> On 11/01/2012 17:08, Novice Coder wrote:
>
> > Here is the code
>
> > int main()
> > {
> >      char *p;
> >      char a[]={"Hello"};
> >      (p=a)[0]; // This line
> >      return 0;
> > }
>
> > Is the behaviour of this code undefined in C++?
>
> I vote that it is not UB.
>
> (p=a)[0] is equivalent to *(p=a); if *(p=a) is UB then surely *(p2++) is
> also UB?  Clearly nonsense. (p=a)[0] + (p=a+1)[0] is UB however.
>
> /Leigh

*(p2++) isn't UB. p++ evaluates to 'p'(a copy) because the result of
post increment is the "value"(old) of the operand. In (p=a) the access/
dereference is on p (lvalue) itself and not on a copy of 'p'.

Assignment operation is defined in section 5.17

"The result of the assignment operation is the value stored in the
left operand after the assignment has taken place; the result
is an lvalue."

This is very confusing. :P

HTH
Prasoon

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#13366

On 1/11/12 9:08 AM, Novice Coder wrote:
> Here is the code
>
> int main()
> {
>      char *p;
>      char a[]={"Hello"};
>      (p=a)[0]; // This line
>      return 0;
> }
>
> Is the behaviour of this code undefined in C++?

I read the whole thread. I'll never get those minutes back. Whether or 
not it is well defined is not relevant except maybe in an academic 
sense. The code is crap. Posit that it is well defined. So what. It is 
obscure. It is opaque. There are other ways to make it clearer what you 
want to do. This looks like something designed to try to break the compiler.

Just because something is legal, well defined, and you saw it somewhere 
doesn't make it a good idea.

-- 
Dave Scruggs

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