kern/99979: Get Ready for Kernel Module in C++

[Joerg Sonnenberger]

On Tue, Jul 11, 2006 at 10:45:52PM +0800, mag at intron.ac wrote:
> Just as you said, C++ is more complicated than C. However, without
> C++ exception and other advanced features, it hasn't brought much
> complexity to C++ runtime library. Early C++ compiler even translates
> C++ code into C code before real compilation.

RTTI, allocation, exceptions and static allocators all add complexity
for the runtime library. If you really want to use C++ for a kernel, you
must likely want to use all of them as well.

> For example, I think C++ exception handling is really poorly suited for
> low-level code.

Bullshit. With a proper implementation exceptions add no overhead as
long an exception is not thrown in terms of speed. It does matter
somewhat in terms of code (or better: data) size, but the very same
information is useful to generate much better tracebacks as well.
RTTI is highly useful for debugging and semi-optional consistent checks,
which should not be neglected either. I don't care about the abuse of
explicitly comparing object types etc., but the ability to decide what
exactly a certain object is.

> But the "object model" is still obscure to understand no matter how many
> people all over the world master C++. What's more, can the "object model"
> function really as OpenDarwin's IOKit class model?
> (http://developer.apple.com/documentation/DeviceDrivers/Conceptual/IOKitFundamentals/HelperClassesChart/chapter_12_section_1.html)

The kobj implementation has the same feature set as a lightweight C++,
e.g. inheritance and virtual functions. That's basically what IOKit is
using as well. It is not obscure, just a Domain Specific Language.

> Now, OpenDarwin has owned a C++-based kernel object model. But why
> cannot FreeBSD?

Because the kernel is C and not many points to incrementally add C++
have been made, which makes it a worthwhile goal.

And if you want to complain about redundancy in drivers, carefully look
for differences which often far outweight the common functionality.

Joerg