Enhancement request: make pycore_cell.h compile cleanly under C++

[yoney]

This isn’t a “real issue” report, since pycore_cell.h (and the helpers it declares) are internal implementation details, so I understand they’re not part of the stable/public API, not guaranteed to be C++-compatible, and may change or disappear in future CPython versions. I’m still asking about this because the fix is very small.

While adapting CinderX to FT-Python, we include pycore_cell.h from a C++ (in the JIT/codegen path for LOAD_DEREF / STORE_DEREF). This currently fails to compile under C++ unless -fpermissive is used.

pycore_cell.h already has extern "C" guards, so it looks intended to be usable from C++ in at least some capacity. Would it be acceptable to add an explicit cast so it compiles cleanly under C++?``

Linked PRs

• gh-144490: Fix C++ compatibility in pycore_cell.h #144482
• gh-144490: Fix test_cppext: test the internal C API #144536
• [3.14] gh-144490: Test the internal C API in test_cppext #144547
• [3.14] gh-144490: Fix C++ compatibility in pycore_cell.h (GH-144482) #144555
• gh-144490: Fix mimalloc debug build #144620
• gh-144490: Fix test_cppext on Windows #144628

[vstinner]

What is the error message? What is your compiler (name/version)?

[colesbury]

I think most compilers error or warn for this pattern in C++: https://gcc.godbolt.org/z/dvsoqvPzd

Typically with something like:

GCC:

<source>:8:18: error: invalid conversion from 'void*' to 'Foo*' [-fpermissive]
    8 |     return malloc(sizeof(Foo));
      |            ~~~~~~^~~~~~~~~~~~~
      |                  |
      |                  void*
Compiler returned: 1

Clang:

<source>:8:12: error: cannot initialize return object of type 'Foo *' with an rvalue of type 'void *'
    8 |     return malloc(sizeof(Foo));
      |            ^~~~~~~~~~~~~~~~~~~

[yoney]

clang version 19.1.2, it should be similar for other compilers, C++ is stricter on implicit conversions.

internal/pycore_cell.h:56:13: error: assigning to 'PyObject *' (aka '_object *') from incompatible type 'void *'
   56 |     value = _Py_atomic_load_ptr(&cell->ob_ref);
      |             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1 error generated.