[petsc-dev] Compiling C code with C compiler

[Lisandro Dalcin]

On 4 April 2011 23:44, Dmitry Karpeev <karpeev at mcs.anl.gov> wrote:
>
>
> On Mon, Apr 4, 2011 at 9:32 PM, Jack Poulson <jack.poulson at gmail.com> wrote:
>>
>> On Mon, Apr 4, 2011 at 9:24 PM, Dmitry Karpeev <karpeev at mcs.anl.gov>
>> wrote:
>>>
>>> struct _p_Mat {
>>>   PETSCHEADER(struct _MatOps);
>>>   PetscLayout            rmap,cmap;
>>>   ...
>>> }
>>> typedef struct _p_Mat *Mat;
>>> Mat A;
>>> Now ((PetscObject)A)->comm is kosher, because of the way C lays out the
>>> struct.
>>> I don't know whether C++ does the same (there is the issue of where it
>>> sticks the vtable, and I'm not sure what else).
>>> I imagine there will be no problem, since we've been compiling the PETSc
>>> C code with C++ compilers for a decade now,
>>> but I don't know whether the C++ standard guarantees that.  Jed is the
>>> only person I know that actually reads the standard
>>> (or maybe it's part of the ABI).
>>> Dmitry.
>>
>> I would think that most compilers would not even use a vtable for C code
>> since there are no virtual functions and no class derivations. On the other
>> hand,  it is my understanding that the way vtables are handled is not in the
>> standard and is up to the compiler. Jed may know better even though he hates
>> C++.
>
> If you take a piece of C code and compile it with a C++ compiler, then all
> structs become classes.
> Sure, if there are no virtual methods, no vtable will get generated, but the
> mere possibility of having it there might force
> the compiler to lay the rest of the crap out differently.  As I said,
> though, experience shows nothing like that happens.

But some day, some guy will add something like this to PETSc:

struct something {
...
#ifdef __cplusplus
virtual void foo()
#endif
}

Barry's rant is going to be memorable :-)

-- 
Lisandro Dalcin
---------------
CIMEC (INTEC/CONICET-UNL)
Predio CONICET-Santa Fe
Colectora RN 168 Km 472, Paraje El Pozo
3000 Santa Fe, Argentina
Tel: +54-342-4511594 (ext 1011)
Tel/Fax: +54-342-4511169