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

[David Abrahams]

Guido van Rossum <guido at python.org> writes:

>> [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.

That's exactly the sort of case that covers up bugs in C++.

Our operator delete basically just ignores NULL values.  When people
write

   delete p; p = 0;

It can cover up bugs where p is unintentionally getting deleted
twice.  It might still not be applicable here, though.

-- 
Dave Abrahams
Boost Consulting
http://www.boost-consulting.com