Re: C versus C++

 James Kanze <>
Sun, 15 Jul 2007 14:17:29 -0000
On Jul 14, 7:08 pm, Zachary Turner <> wrote:

On Jul 14, 10:13 am, Erik Wikstr=F6m <> wrote:

That's why major file systems like XFS, reiser, etc., as well as
database systems are usually programmed in C, even though they *could*
just as well be programmed in C++.

I'd say that they are programmed in C because either they have to be
part of an OS written in C (for the FS) and because if something is
written in C it can be called from almost any other language (for the DB
application). That and the fact that most DB projects are either decades
old or derived from code that is, rewriting something that works just to
change the language is usually not a popular idea.

Actually most code that runs in an operating system kernel is
typically better suited to C than C++ because C++ makes no guarantee
over how and where certain types of things will be allocated.

C++ offers the same guarantees as C, where ever such guarantees
are applicable.

For example, when you declare a virtual method the compiler
allocates a vtbl and puts it in memory somewhere. Where?

That's for your compiler to document. Typically, somewhere in
the text segment, I'd hope.

The C++ standard doesn't mandate how and where to allocate a
vtbl, but this can easily matter if the code is running inside
the kernel (which obviously most file systems and device
drivers do).

C doesn't mandate how and where to allocate actual function
code, either, nor const objects. If you're writing kernel code,
you probably have to read the compiler documentation, and use
special features when linking, and maybe even when compiling.

There are other examples of this in C++ as well, and while you could
define a "white list" of C++ features that you can use when developing
such software, it effectively boils down to C with the class keyword,
so you might as well just use C and save yourself the trouble of
possibly using the wrong thing on accident.

The most important single feature of C++, regardless of what you
are doing, is private data.

In practice, in kernel code, I'd probably ban exceptions,
because they often do involve a lot of extra memory, and can
have unexpected run-time repercussions. (Kernel code is often
concerned with worst case times, not average times.) I'd also
insist that the standard operator new and operator delete be
replaced with something very specific, and only be used in the
outer layers of the kernel.

And of course, there are parts of the kernel which simply can't
be written in C++. Nor in C.

James Kanze (Gabi Software) email:
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