[Python-Dev] Re: Proposal: C API Macro to decref and set to NULL

[Guido van Rossum]

> [David Abrahams, on the Py_CLEAR macro]
> > FWIW our experience in the C++ community suggests that
> > deallocate-and-NULL tends to hide bugs.  In general we prefer to use
> > deallocate-and-fill-pointer-with-garbage.  I'm not sure if the
> > experience translates here, of course.

[Tim]
> I don't think so.  Along with the macro, Jim checked in a nice
> explanation in the Extending and Embedding manual.  A decref can cause
> anything to happen if the decref'ed containee is now trash, including
> fresh accesses to the container object holding the now-trash containee
> that was decrefed.  If the container's pointer to the containee isn't
> NULLed out first, the entire implementation of the container has to be
> aware that this particular containee may hold an insane value.  Ditto
> if garbage is put in the pointer.  Ditto for NULL too, but NULL is a
> single, obvious, and easy-to-test way to say "and this containee no
> longer exists".  This isn't really so much about deallocation as about
> horrid side effects triggered by deallocation.

Plus, in many cases NULL is *already* an expected value.

We even have macros like Py_XDECREF() that are NULL-aware.

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

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