Help with bit-field semantics in C and C++

[Gabriel Dos Reis]

Joe Buck <Joe.Buck@synopsys.COM> writes:

| On Wed, Aug 25, 2004 at 12:26:24PM +0100, Dave Korn wrote:
| >   It seems to me that "unspecified" and "undefined" are repeatedly being
| > confused in this discussion.  They are both clearly defined technical terms
| > in the standard; they have different meanings; everyone participating in
| > this thread should make very sure they know which one they're referring to
| > at any time.
| > 
| >   "undefined" is the one that covers rm -rf'ing your hard drive.
| > 
| >   "unspecified" is what the standard says that the result of the truncating
| > cast we've been discussing is.  That means (IIUIC) that the result has to be
| > one of the enumeration values, but that the standard does not specify which
| > _particular_ one.
| 
| If you are correct, then it appears that every C++ compiler I know of gets
| this wrong.  That is, it seems that every C++ compiler happily allows
| out-of-range integers to be assigned to enums, and does so in a way that
| the effect is observable.
| 
| If your interpretation is correct, then I can write the following test for
| a conforming compiler; g++ fails this test.  I'm deliberately putting
| is_zero and is_one in separate functions, to catch a compiler that might
| allow storing the 3, but that would later assume that global must be zero
| or one and optimize the "if" away.


There is a very simple principle : if you initialize an integer
variable, or bit-field of such type, its value is always in range --
even if the value you used to initialize the variable was out-of-range
(therefore generated an implementation-defined value of the
destination type).  Enumerations are no exception.

Do you expect any of the following assertions to fail?


     #include <assert.h>
     #include <limits.h>

     // assume 'signed char' has limits -128 and 127
     int main()
     {
        signed char i = 200;
        assert(i >= -128);
        assert(i <= 127);
        assert(i != 200);
     }
   

-- Gaby