Entry 32265 (Berkeley CSUA MOTD)

Berkeley CSUA MOTD:Entry 32265

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2025/07/09 [General] UID:1000 Activity:popular

7/9

2004/7/13-14 [Computer/SW/Languages/C_Cplusplus] UID:32265 Activity:high

7/12    Someone please explain the following?
        ((size_t )& ((SomeVar *)0)->Field);
        \_ It's a hideous abuse of C++ syntax and results in undefined
           behavior.  It appears to be an attempt to find out the offset of the
           member "Field" in the class/struct type SomeVar.  Ow, ow ow.
           \_ This is actually not undefined, in C or C++.  It is hideous,
              though; it would be nicer to #include <stddef.h> and just use
              though; it would be nicer to #include  and just use
              offsetof(SomeVar, Field).  In C++, you might also be able to
              use a pointer-to-member (&SomeVar::Field), depending on what
              you're trying to do.  --mconst
              \_ You're saying that dereferencing a null pointer is not
                 undefined? -emarkp
                 \_ Right.  In C99, you're allowed to dereference a null
                    pointer as long as all you do is take the address of
                    it afterwards; see 6.5.3.2p3.  In C++, it's not very
                    well stated in the standard, but the current view of
                    the committee is that you're allowed to do anything
                    with a dereferenced null pointer as long as you don't
                    trigger lvalue-to-rvalue conversion or try to write
                    to the memory; see http://csua.org/u/86c  --mconst
                    \_ Fascinating.  I usually follow Herb Sutter's articles to
                       keep me up-to-date.  Thanks for the info.  I just wish
                       they'd add a typeof operator and ditch the nonsensical
                       syntax of calling a static member function though an
                       instance (requiring T::func() instead of allowing T x;
                       x.func(); ). -emarkp
                    \_ Although 6.5.3.2 states that &*E is legal, it does not
                       address the issue in the original question, which
                       involves an additional structure member access.  I
                       believe that it is implementation-dependent.  It is
                       the compiler vendor's responsibility to provide an
                       offsetof macro in stddef.h that works with their own
                       implementation, but it need not be portable. --jameslin
                       \_ You're right, I take it back -- the original
                          statement is actually undefined in C99.  It works
                          for arrays, but not structs.  (It is still valid
                          C++, although it's not guaranteed to produce the
                          value you want.)  --mconst
        \_ So shouldn't your "SomeVar" be "SomeType" for clarity? That
           confused me for a bit.

2025/07/09 [General] UID:1000 Activity:popular

7/9

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