c++, gcc3.2.2 and bitwise operations

c++, gcc3.2.2 and bitwise operations

[Chris Croswhite]

How can I play with the guts of a variable one bit at a time in c++?  That
is, I want to do something like this,

long long val =0x4000000000000001;
((int*)(&val)[1]) &= 0x0;

(this should set the first bit to zero).  This worked with gcc 2.95.x, but I
understand now that the casting (&val)[1] (to an array) is "undefined" and
therefore not predictible.

Could anyone give me hand?

TIA,
Chris

Re: c++, gcc3.2.2 and bitwise operations

[Sebastian Huber]

On Wednesday 19 February 2003 02:33, Chris Croswhite wrote:
> How can I play with the guts of a variable one bit at a time in c++?  That
> is, I want to do something like this,
>
> long long val =0x4000000000000001;
> ((int*)(&val)[1]) &= 0x0;
>
> (this should set the first bit to zero).
> [...]

Hi,
the standard way to to this is:
val &= ~(0x1LL)

Bye

Re: c++, gcc3.2.2 and bitwise operations

[Nathan Sidwell]

Chris Croswhite wrote:
> How can I play with the guts of a variable one bit at a time in c++?  That
> is, I want to do something like this,
> 
> long long val =0x4000000000000001;
> ((int*)(&val)[1]) &= 0x0;
> 
> (this should set the first bit to zero).  This worked with gcc 2.95.x, but I
> understand now that the casting (&val)[1] (to an array) is "undefined" and
> therefore not predictible.
> 
> Could anyone give me hand?
a) read the documentation about type based aliasing
b) don't do that :)

nathan

-- 
Nathan Sidwell    ::   http://www.codesourcery.com   ::     CodeSourcery LLC
          The voices in my head said this was stupid too
nathan@codesourcery.com : http://www.cs.bris.ac.uk/~nathan/ : nathan@acm.org

Re: c++, gcc3.2.2 and bitwise operations

[John Love-Jensen]

Hi Chris,

> How can I play with the guts of a variable one bit at a time in c++?

The C++ way is to use the #include <bitset>, as describe in Stroustrup's
C++PL (3rd) in section 17.5.3.

Sincerely,
--Eljay