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From: David Brown <david.brown@hesbynett.no>
Newsgroups: comp.lang.c
Subject: Re: Is it possible to generate a compile time error from an inline
 function?
Date: Sun, 14 Jul 2024 15:17:35 +0200
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On 13/07/2024 20:07, Richard Damon wrote:
> On 7/13/24 1:56 PM, Alan Mackenzie wrote:
>> Hello, Richard.
>>
>> Richard Damon <richard@damon-family.org> wrote:
>>> On 7/13/24 1:05 PM, Alan Mackenzie wrote:
>>>> Hello, David.
>>
>>>> Many thanks for the reply!  It's just what I was looking for.
>>
>>>> David Brown <david.brown@hesbynett.no> wrote:
>>>>> On 13/07/2024 15:04, Alan Mackenzie wrote:
>>>>>> Hello, comp.lang.c.


>>>> Thanks, these are all things I didn't know.  I will look them up in the
>>>> GCC manual.  :-)
>>
>>
>>> In C++, I would use constexpr and static_assert to do this, and your
>>> compiler might allow its use in C as an extension.
>>
>> C++ isn't an option.
> 
> The question being since most C compilers are also C++ compilers, they 
> somethings accept this sort of C++ism as an extension.

Actually, the vast majority of C compilers are /not/ also C++ compilers. 
  But the most popular C compilers, by a significant margin, are C++ 
compilers.

_Static_assert has been in C since C11, so it does not need to be an 
extension - it has been standard C for over a decade.  (And many C 
compilers allow as extensions the use of features from later C standards 
to be used along with earlier standards, as well as - as you say - 
allowing some C++-isms.)  But to make _Static_assert work here, you'd 
have to stretch things a lot further than just supporting C11/C++11 
static assertions.  Static assertions are a feature I like and find very 
useful, but they don't work in all situations I would like.

> 
>>
>>> If not, in C you could use just _Static_assert, perhaps in the expansion
>>> of a macro that generates the expression that does the testing.
>>
>> Yes, that's an even better idea, thanks.  Looking it up in
>> https://en.cppreference.com, _Static_assert has existed in C since C11,
>> that spelling being deprecated in favour of static_assert in C23.  I just
>> need to check the project I'm working in doesn't still support C < C11.
>>
> 
> If the compiler doesn't support some form of static assert, you cam make 
> one yourself with a macro.
> 
> #define static_assert(x) extern int _static_assert[(x) ? 1 : -1]
> 
> Possible adding whatever hacks you want to make the variable unique, (or 
> make it a typedef, or whatever). (a typedef doesn't add an external 
> symbol that is never used, so might be clearer, but needs something to 
> make the name unique)
> 
> The key idea is a array of negative size is a constraint error, so if x 
> is false you get the error.

That's a useful technique for those stuck with pre-C11 or pre-C++11 and 
who don't want to use gcc-isms - I have such a macro in an oft-used header.