Path: utzoo!utgpu!watserv1!watmath!att!pacbell!pacbell.com!ames!ucsd!usc!cs.utexas.edu!uunet!taumet!steve
From: steve@taumet.COM (Stephen Clamage)
Newsgroups: comp.lang.c
Subject: Re: How many elements are in my arrays?
Message-ID: <237@taumet.COM>
Date: 4 Jun 90 15:42:47 GMT
References: <6644.26663D46@puddle.fidonet.org>
Reply-To: steve@taumet.UUCP (Stephen Clamage)
Organization: Taumetric Corporation, San Diego
Lines: 56

In article <6644.26663D46@puddle.fidonet.org> cspw.quagga@p0.f4.n494.z5.fidonet.org (EP Wentworth) writes:
>1) Did the 'offsetof' macro make it into the finals?  I notice

Yes.

>   it is missing in TurboC 2.0, but present in a couple of other
>   compilers that also claim ANSI compatibility.

It is in Turbo C++ 1.0 (which is also a full ANSI C compiler).

>3) I'd like to check at that two arrays have the same number of initializers.
>   I'd like this check at compile time, so I'd like to be able to write
> 
>   #if (num_elems(a) - num_elems(s))
>      cause a deliberate compilation error
>   #endif
> 
>   But, uh-huhm, the 'sizeof' and the pointer arithmetic is not permitted
>   at pre-processing time.   Any way to do this nicely?

Do it at compile time.  With any decent compiler, num_elems will be computed
at compile time, the if-expression will evaluate to a constant, and if the
condition is met, the compiler will eliminate the test and the "dead"
code it controls.  (If your compiler doesn't work this way, try to get a
better compiler.)  Of course, you cannot include illegal code to cause a
compile-time error with this technique.

ANSI corner:
Use the assert macro in the ANSI header <assert.h>.  If your compiler
doesn't support this, it looks like this:

<assert.h>:
#undef assert	/* because multiple includes of <assert.h> are allowed */
#if defined(NDEBUG)
    #define assert(ignore)	((void) 0)
#else
    extern void gripe(const char *, int, const char *);
    #define assert(expr) \
	((expr) ? ((void) 0) : _gripe_(__FILE__, __LINE__, # expr))
#endif

"gripe" just calls printf to print the file, line, and the text of the
test which failed; it then aborts the program.

To use it:

	assert(num_elems(a) == num_elems(s));

As noted above, when the test passes no code should be generated for
the assertion.

For production versions, just #define NDEBUG, and the preprocessor
deletes the whole wretched mess.
-- 

Steve Clamage, TauMetric Corp, steve@taumet.com