Re: Implementing static_assert

From	Keith Thompson <kst-u@mib.org>
Newsgroups	comp.lang.c
Subject	Re: Implementing static_assert
Date	2015-05-06 14:30 -0700
Organization	None to speak of
Message-ID	<871titl7h9.fsf@kst-u.example.com> (permalink)
References	<midqsq$81d$1@dont-email.me> <876185lach.fsf@kst-u.example.com> <midv1j$pin$1@dont-email.me>

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jacobnavia <jacob@jacob.remcomp.fr> writes:
> Le 06/05/2015 22:28, Keith Thompson a écrit :
>> jacobnavia <jacob@jacob.remcomp.fr> writes:
>>> I have added this useful construct to the lcc-win compiler.
>>>
>>> Description:
>>>
>>> #include <assert.h>
>>> static_assert(boolean expression, message)
>> [snip]
>>
>> See my reply in comp.compilers.lcc (I happened to see that first).
>>
>> Quick summary: This is standard in C11.
>>
>
> Again, thanks a lot for this. A summary review of my implementation
> and the standard looks like they are the same...
>
> I have just not understood what the static assertion in a structure
> declaration should do, but I have to read more carefully the stuff in
> the new C11 that I haven't really read that much.

I'm pretty sure having it inside a structure (or union) definition
doesn't affect anything.  It's just permitted in that context for
convenience.

> Not specified is what to do when the expression is not a constant
> expression!

The standard says "The constant expression shall be an integer constant
expression."  Since this is a "shall" outside a constraint, violating it
has undefined behavior.  (I find that a little surprising; I would think
it would make more sense for a non-constant expression to be a
constraint violation.)

One popular C compiler treats a non-constant expression as an error.
That would be my preference as well.  In C++, a non-constant expression
requires a diagnostic (if I'm reading the standard correctly).

> In my implementation I just call a run time procedure that will print
> the same error messages if the assert expression fails and return
> without doing anything if the assert expression is OK (different from
> zero).
>
> Since the behavior of a non constant expression is not specified I
> suppose that this is OK. In a strict mode I will issue a warning.
>
>
> What a surprise really. Yesterday I was looking at some C++ code and
> saw that construct and told me: "Interesting construct", and not that
> difficult to implement, so I did it today.

I think C11 may have borrowed static_assert from C++, with a change from
`static_assert` to `_Static_assert` to avoid breaking existing code.
One difference is that in C++ the message is optional.

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Working, but not speaking, for JetHead Development, Inc.
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"

Thread

Implementing static_assert jacobnavia <jacob@jacob.remcomp.fr> - 2015-05-06 21:44 +0200
  Re: Implementing static_assert Keith Thompson <kst-u@mib.org> - 2015-05-06 13:28 -0700
    Re: Implementing static_assert jacobnavia <jacob@jacob.remcomp.fr> - 2015-05-06 22:55 +0200
      Re: Implementing static_assert Keith Thompson <kst-u@mib.org> - 2015-05-06 14:30 -0700
        Re: Implementing static_assert Martin Shobe <martin.shobe@yahoo.com> - 2015-05-06 18:07 -0500
          Re: Implementing static_assert Keith Thompson <kst-u@mib.org> - 2015-05-06 17:34 -0700
          Re: Implementing static_assert David Brown <david.brown@hesbynett.no> - 2015-05-07 09:44 +0200
      Re: Implementing static_assert Tim Rentsch <txr@alumni.caltech.edu> - 2015-05-07 12:37 -0700
  Re: Implementing static_assert David Brown <david.brown@hesbynett.no> - 2015-05-07 10:11 +0200
    Re: Implementing static_assert Ian Collins <ian-news@hotmail.com> - 2015-05-07 22:13 +1200
      Re: Implementing static_assert jacobnavia <jacob@jacob.remcomp.fr> - 2015-05-07 12:24 +0200
        Re: Implementing static_assert Ian Collins <ian-news@hotmail.com> - 2015-05-07 22:33 +1200
          Re: Implementing static_assert David Brown <david.brown@hesbynett.no> - 2015-05-07 14:30 +0200
    Re: Implementing static_assert Philip Lantz <prl@canterey.us> - 2015-05-07 03:53 -0700
      Re: Implementing static_assert David Brown <david.brown@hesbynett.no> - 2015-05-07 14:21 +0200
        Re: Implementing static_assert Keith Thompson <kst-u@mib.org> - 2015-05-07 09:11 -0700
          Re: Implementing static_assert James Kuyper <jameskuyper@verizon.net> - 2015-05-07 12:30 -0400
            Re: Implementing static_assert James Kuyper <jameskuyper@verizon.net> - 2015-05-07 12:55 -0400
    Re: Implementing static_assert Keith Thompson <kst-u@mib.org> - 2015-05-07 09:06 -0700
  Re: Implementing static_assert jacobnavia <jacob@jacob.remcomp.fr> - 2015-05-07 19:43 +0200
    Re: Implementing static_assert Tim Rentsch <txr@alumni.caltech.edu> - 2015-05-07 16:10 -0700

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