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Re: Bitwise Not Operator

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On 06/26/2012 02:40 PM, aleksa wrote:
> On Tuesday, June 26, 2012 7:06:01 PM UTC+2, Ike Naar wrote:
>> On 2012-06-26, aleksa <aleksazr@gmail.com> wrote:
>>> unsigned char X, Y;
>>> if (X != (unsigned char) ~Y)
>>>
>>> why do I get a warning here?
>>> comparison of promoted ~unsigned with unsigned
>>
>> This is unexpected.
>> Looking a the format of the warning, it seems you're using gcc.
>> But gcc 4.1.2 only gives this warning when the cast to unsigned char
>> is missing, as in (X != ~Y). If the cast is in place, the compiler
>> remains silent. Which version of gcc do you use?
> 
> 4.6.2.
> 
> Actually, the original warning message is ended with [-Wsign-compare]
> don't know why I haven't included that part...
> (that option is included with -Wextra).
> 
> -Wsign-compare:
> "Warn when a comparison between signed and unsigned values..."
> but, aren't both values here unsigned?

Both values start out as unsigned, but if UCHAR_MAX <= INT_MAX, both X
and (unsigned char)~Y get promoted to int before the comparison is
performed. However, regardless of whether it's looking at the original
types or the promoted types, and regardless of whether UCHAR_MAX >
INT_MAX, the signedness is the same on both sides of the expression, so
I don't understand why the compiler would generate that particular
warning. It's the ~Y expression that needs warning about. It's generally
a bad idea to perform bitwise operations on signed types, and due to the
integer promotions, that's precisely what ~Y does (unless UCHAR_MAX >
INT_MAX). What you probably want is something like:

	X != UCHAR_MAX & ~(unsigned)Y

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Thread

Bitwise Not Operator aleksa <aleksazr@gmail.com> - 2012-06-26 08:35 -0700
  Re: Bitwise Not Operator Ike Naar <ike@iceland.freeshell.org> - 2012-06-26 17:06 +0000
    Re: Bitwise Not Operator aleksa <aleksazr@gmail.com> - 2012-06-26 11:40 -0700
      Re: Bitwise Not Operator Jens Gustedt <jens.gustedt@loria.fr> - 2012-06-26 20:57 +0200
      Re: Bitwise Not Operator James Kuyper <jameskuyper@verizon.net> - 2012-06-26 15:17 -0400
  Re: Bitwise Not Operator Les Cargill <lcargill99@comcast.com> - 2012-06-26 12:31 -0500

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