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

Tim Peters tim.peters at gmail.com
Fri Jul 16 02:01:47 CEST 2004

[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.

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.

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