Re: stdbool.h does not define _Bool when included by C++ code

[Jens Gustedt <jens.gustedt@inria.fr>]

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Hello,

On Mon, 31 Jul 2017 21:31:22 -0400 Rich Felker <dalias@libc.org> wrote:

> On Tue, Aug 01, 2017 at 09:06:01AM +1200, Michael Clark wrote:
> >   
> > > On 31 Jul 2017, at 9:46 PM, Jens Gustedt <jens.gustedt@inria.fr>
> > > wrote:
> > > 
> > > Hello,
> > > 
> > > On Mon, 31 Jul 2017 11:18:28 +0200 Szabolcs Nagy <nsz@port70.net>
> > > wrote: 
>  [...]  
> > > 
> > > Basically stdbool.h is already a header to accomodate C++ usage of
> > > bool, false and true to C. I makes not much sense to include this
> > > in C++ code.  
> > 
> > It’s useful exactly for the case where you have C code in your C++,
> > which is why I suspect GCC has the condition in its version of
> > stdbool.h
> >   
> > > I think that applications that want to be shure that their code
> > > compiles for both should use "bool" and should do
> > > 
> > > #ifndef __cplusplus
> > > # include <stdbool.h>
> > > #endif  
> > 
> > I think you’re missing the point. C code can use _Bool and this can
> > be put through the C++ front-end. Compiler explorer does this by
> > default, and the GCC and glibc headers already have the fix, which
> > is #define _Bool bool (in the patch) for the case where C code is
> > fed in to the C++ front end.  
> 
> Compiling C code with a C++ compiler is utterly wrong, and will fail
> in more subtle ways; erroring out early seems much more desirable. C
> is not a subset of C++, and even the syntax that's in the intersection
> of both languages is semantically different between them.

I agree, and I should have been more precise for the use case where
this actually makes sense. The only compatibility between C an C++
that is ensured is ABI compatibility. And here what I said above is a
reasonable way to write *header* files that can be included by both
languages. So such a header is *not* simply "C code that is compiled
as C++". There are several syntax problems for such headers, simple
ones as _Bool, but also _Noreturn, _Static_assert, and more difficult
ones as _Atomic or enumeration types ...

Such a header has to be carefully designed to fit to both
languages. Placing __cplusplus ifdef's into some of the headers could
be a strategy to ease the pain. Defining _Bool and stuff in some way
for C++ would just be pushing things under the carpet and hide
difficulties that reduce awareness of the problem.

Jens

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