Re: [Qemu-devel] Coding style, C++ compatible code

[Markus Armbruster]

Stefan Weil <address@hidden> writes:

> Markus Armbruster schrieb:
>> Stefan Weil <address@hidden> writes:
>>
>>> Juan Quintela schrieb:
>>>> Signed-off-by: Juan Quintela <address@hidden>
>>>> ---
>>>> hw/eepro100.c | 6 +++---
>>>> 1 files changed, 3 insertions(+), 3 deletions(-)
>>>>
>>>> diff --git a/hw/eepro100.c b/hw/eepro100.c
>>>> index 0031d36..09083c2 100644
>>>> --- a/hw/eepro100.c
>>>> +++ b/hw/eepro100.c
>>>> @@ -507,7 +507,7 @@ static void nic_selective_reset(EEPRO100State * s)
>>>>
>>>> static void nic_reset(void *opaque)
>>>> {
>>>> - EEPRO100State *s = (EEPRO100State *) opaque;
>>>> + EEPRO100State *s = opaque;
>>>> logout("%p\n", s);
>>>> static int first;
>>>> if (!first) {
>>>> @@ -1544,7 +1544,7 @@ static ssize_t nic_receive(VLANClientState
>>>> *vc, const uint8_t * buf, size_t size
>>>>
>>>> static int nic_load(QEMUFile * f, void *opaque, int version_id)
>>>> {
>>>> - EEPRO100State *s = (EEPRO100State *) opaque;
>>>> + EEPRO100State *s = opaque;
>>>> int i;
>>>> int ret;
>>>>
>>>> @@ -1634,7 +1634,7 @@ static int nic_load(QEMUFile * f, void
>>>> *opaque, int version_id)
>>>>
>>>> static void nic_save(QEMUFile * f, void *opaque)
>>>> {
>>>> - EEPRO100State *s = (EEPRO100State *) opaque;
>>>> + EEPRO100State *s = opaque;
>>>> int i;
>>>>
>>>> if (s->pci_dev)
>>>>
>>> I wrote these type casts, and I think they make sense.
>>> In C++ code, they are even mandatory.
>>
>> Yes, but this isn't C++.
>>
>>> I think the arguments why C++ requires this kind of
>>> type casts apply to C code (like in Qemu) as well.
>>>
>>> If it is possible with no or very litte efforts to write
>>> code which is C and C++ compatible, I prefer to do so.
>>
>> I respectfully disagree. Casts from "void *" to "T *" are pure noise.
>> Getting into the habit of writing noise casts runs the risk of silencing
>> warnings that flag real type errors.
>
> Hello
>
> Do you only disagree with my first sentence or with both sentences?

We can certainly discuss how to write better C, but appealing to the
authority of the C++ rationale is an argument I don't buy.

I prefer not to attempt to write C code that behaves identically when
interpreted as C++ (extern "C" seems safer and easier).

Does this answer your question?

> Currently, I seem to be alone with my opinion (at least in qemu-devel).
> Nevertheless, the designers of C++ thought that casts from void * to
> T * were something very important. I don't know the history of their
> decision. I personally think that deriving a data type T from some
> bytes in memory which can contain anything is an operation which is
> worth being documented by the programmer, and this is exactly what
> the cast does.

I don't see much value in that, sorry.

> My main reason why I try to write portable code is my personal
> experience with code reuse and the future development of compilers.
> It is quite possible that some day C compilers will require
> type casts like C++ compilers do today.

I can't read the ISO C committee's mind, but I believe such an
incompatible change is extremely unlikely, given the pains they've taken
not to break existing C code.

> And even today I want to be able to share C code from QEMU with
> program code written in C++ (which makes a large part of my
> business work).

How about extern "C"?

[...]