Re: including sparse headers in C++ code

[Bernd Petrovitsch <bernd@sysprog.at>]

On Mon, 2010-10-11 at 16:37 -0700, Josh Triplett wrote:
[....] 
> Wow.  I had to double-check this because I couldn't quite believe C++
> had that degree of dain bramage, but sure enough:
> 
> /tmp$ cat test.c
> extern void *pv(void);
> 
> int *pi(void)
> {
>     return pv();
> }
> /tmp$ gcc -c test.c -o /dev/null
> /tmp$ g++ -c test.c -o /dev/null
> test.c: In function ‘int* pi()’:
> test.c:5: error: invalid conversion from ‘void*’ to ‘int*’
> (1) /tmp$

Welcome to one of the (more obvious) differences between C and C++: you
can't assign/convert/promote a "void *" to any other pointer type (and
vice versa) without explicit type-cast.
For a library written in C, that is the point where "compile it also as
C++" ends IMHO. Just keep the 'extern "C" {' stuff in headers (under
'#ifdef __cplusplus') and compile it as C (and not C++).

BTW there are other similar issues if one digs deep enough.
BTDT (read: looked after a library which was used in 2 "applications" -
one in pure C, the other C++. No way without explicit type-casts which
look somewhat funny and are surely superfluous in C as such).

> I can understand C++ having stronger typechecking, but void pointers
> *exist* for this purpose.  *Really* debatable whether Sparse should work
> around this.  Avoiding keywords, sure, but casting void pointers
> everywhere?  People *remove* these kinds of casts from C programs as a
> cleanup.

Type-casts are inherently evil. But in several situations there is in C
no other (or better) workaround. The above - kill warnings through "void
*" type-casts - makes it worse because some people (including /me) use
the type system in C and want the compiler to check as much as possible.

Bernd
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