Date: Sun, 30 Mar 97 13:48:32 +0200 From: cracauer@wavehh.hanse.de (Martin Cracauer) To: moncrg@dominus.ma.ultranet.COM Cc: freebsd-hackers@freebsd.org Subject: Re: C++ Code in Kernel Message-ID: <9703301148.AA06383@wavehh.hanse.de> References: <01BC3BA3.6FFD23A0@dominus.ultranet.com>
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>maybe its just that I'm taking a c++ class now, but I'm >cusious as to the pro's and con's on writing kernel source >in c++. I haven't seen nor heard on this happening on FreeBSD >[but then my travels haven't been that wide] My current list of argument against C++: - The compilers in general and g++ in special are not as mature as the C equivalents. We have enough problems with gcc itself that show up in deep system space (like floating-point - handling). - The performance characteristic of C++ in general is less predictable than for pure C. It is very easy to use constructs that end up being compiled to code with hidden extra work. And even worse in practice, it is quite easy to write wrong code when trying to avoid these hidden costs. - The performance characteristic of g++ is surprisingly often different from that of gcc for the same code. g++ disables some optimizations and aligns data often quite different than gcc does. I have done some little benchmarks, let me know if you want them. It is not that g++ is really slower than gcc for C, the disabled optimizations are not too effective for my code anyway and the other effects of g++ cause faster code in some place, too. But our kernel hackers can't reuse their compiler experience that easy. - C++ is a very complicated language and using it (read: using some real C++ features) would make it fundamentally more difficult to let new developers hack on the code. The most dramatic effect (IMHO) is that error messages in C++ are very often a quite esoteric abstraction from the real problem. - You can reuse C code in other languages as you want. This is several numbers of magnitude more difficult in C++ because of name mangling, constructors and destructors of static data, initialization of virtual function tables etc etc etc. - C++ code is more diffucult to browse, to search for definitions and declarations, to present to the user in a debugger and such. Many if not all FreeBSD developers use self-constructed browse tools and couldn't reuse them for C++. Don't misunderstand me: C++ is a nice tool for a lot of things and I sheduled it for use in some projects of mine. Especially the STL, despite some bloating problems, enables quite some code compression for me. I use languages like Common Lisp and C++ when I have to start coding before I really understand the problem I have to write a program for. That is usually the case in almost every project I get payed for, noone can expect customers to give straigt outline of a project in a domain they do their first project in, which in turn applies to 70% of my work. But when I have the strong will to do something really really right, I use C and nothing else than C. For example, when I need a collection class code for several data types, I don't want templates to expand them. I think over it where casting or macros or code duplication is applicable and use the right one in every place. And build normal libraries so that the code isn't constructed everytime I compile the program. I know I didn't do it right when compile time prohibits developing on my old Sparc 1 (no joke). And last but not leat, I can use C libraries in Common Lisp and C++. I clearly want FreeBSD's kernel code to be of the way outlined in the last paragraph. Martin P.S. When some OO features are really useful, maybe for constructing device driver or network protocol families with inheritance and runtime lookup of functionality, I would think Objectve-C is a better bet. Objective-C is just a C library for message calls and some syntactic shugar to constructs calls to messages in source code. You can even leave out all the syntactic stuff and do everything ObjC and its runtime can do from plain C code. You just link in the runtime which is a plain C library and a brave citizen inside other code of every kind. -- %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% Martin_Cracauer@wavehh.hanse.de http://cracauer.cons.org Fax.: +4940 5228536 "As far as I'm concerned, if something is so complicated that you can't ex- plain it in 10 seconds, then it's probably not worth knowing anyway"- Calvin
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