[Python-3000] Python/C++ question

Guido van Rossum guido at python.org
Thu Aug 10 01:18:02 CEST 2006

On 8/9/06, Talin <talin at acm.org> wrote:
> A while back someone proposed switching to C++ as the implementation
> language for CPython, and the response was that this would make ABI
> compatibility too difficult, since the different C++ compilers don't
> have a common way to represent things like vtables and such.
> However, I was thinking - if you remove all of the ABI-breaking features
> of C++, such as virtual functions, name mangling, RTTI, exceptions, and
> so on, its still a pretty nice language compared to C - you still have
> things like namespaces, constructors/destructors (especially nice for
> stack-local objects), overloadable type conversion, automatic
> upcasting/downcasting, references, plus you don't have to keep repeating
> the word 'struct' everywhere.
> Think how much cleaner the Python source would be if just one C++
> feature - namespaces - could be used. Imagine being able to put all of
> your enumeration values in their own namespace, instead of mixing them
> in with all the other global symbols.
> Think of the gazillions of cast operators you could get rid of if you
> could assign from PyString* to PyObject*, without having to explicitly
> cast between pointer types.
> My question is, however - would this even work? That is, if you wrapped
> all the source files in 'extern "C"', turned off the exception and RTTI
> compiler switches, suppressed the use of the C++ runtime libs and
> forbade use of the word 'virtual', would that effectively avoid the ABI
> compatibility issues? Would you be able to produce, on all supported
> platforms, a binary executable that was interoperable with ones produced
> by straight C?
> I actually have a personal motivation in asking this - it has been so
> many years since I've written in C, that I've actually *forgotten how*.
> Despite the fact that my very first C program, written in 1982, was a C
> compiler, today I find writing C programs a considerable challenge,
> because I don't remember exactly where the dividing line between C and
> C++ is - and I will either end up accidentally using a C++-specific
> language feature, or worse, I'll unconsciously avoid a valid C language
> feature because I don't remember whether it's C++ specific or not. (For
> example, I don't remember whether its valid to define an enumeration
> within a struct, which is something that I do all the time in C++.)

For the majority of Python developers it's probably the other way
around. It's been 15 years since I wrote C++, and unlike C, that
language has changed a lot since then...

It would be a complete rewrite; I prefer doing a gradual
transmogrification of the current codebase into Py3k rather than
starting from scratch (read Joel Spolsky on why).

--Guido van Rossum (home page: http://www.python.org/~guido/)

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