Re: C23 thoughts and opinions

On 24/05/2024 21:29, Keith Thompson wrote:

David Brown <david.brown@hesbynett.no> writes:

On 23/05/2024 18:40, Keith Thompson wrote:

Michael S <already5chosen@yahoo.com> writes:
[...]

Removed

[...]

7) static_assert is not provided as a macro defined in <assert.h>
(becomes a keyword)
8) thread_local is not provided as a macro defined in <threads.h>
(becomes a keyword)
>

[...]

7) bad. Breaks existing code for weak reason
8) bad. Breaks existing code for weak reason

In pre-C23, _Static_assert and _Thread_local are keywords, and
static_assert and thread_local are macros that expand to those keywords.
In C23, _Static_assert, _Thread_local, static_assert, and
thread_local
are all keywords.  Code that simply uses the old ugly keywords would not
break.
Code that does something like "#ifdef static_assert".  I suppose the
headers could have retained the old macro definitions.
      #define static_assert static_assert
      #define thread_local thread_local
>

>
The sort of code that could theoretically break is when you have
definitions like this:
>
#define STATIC_ASSERT_NAME_(line)       STATIC_ASSERT_NAME2_(line)
#define STATIC_ASSERT_NAME2_(line)      assertion_failed_at_line_##line
#define static_assert(claim, warning) \
     typedef struct { \
         char STATIC_ASSERT_NAME_(__COUNTER__) [(claim) ? 2 : -2]; \
     } STATIC_ASSERT_NAME_(__COUNTER__)
>
That works in any C version, until C23, almost as well as
_static_assert.  I used this when C11 support was rare in the tools I
used.

 You mean _Static_assert.

I meant either "static_assert" or "_Static_assert", rather than a mixture of the two!  (I consider "static_assert" part of C11 even though it needs a header.)

 

While using #define for a C keyword is undefined behaviour, in
practice I think you'd have a hard time finding code and a compiler
that used such a macro and which did not work just as well in C23
mode.
>
(I don't know if anyone is in the habit of declaring macros named
"thread_local".)

 "static_assert" is already a macro defined in <assert.h> starting in
C11.  The above code is valid in pre-C23, but will break in C11 and C17
if it includes <assert.h> directly or indirectly. 

Yes.  But including <assert.h> is optional.

You can fix it by
adding "#undef static_assert" or by picking a different name, or by
making your macro definition conditional on __STDC_VERSION__ >= 202311L.
 

The actual code I use had a number of conditional checks for different C standards and C++, so that it does not define a static_assert macro for C++ (my C++ usage for the code was always at least C++11), and for C11 onwards it was defined to _Static_assert.  (I specifically did not want to include <assert.h>.)