[New-bugs-announce] [issue47164] [C API] Add private "CAST" macros to clean up casts in C code

[STINNER Victor]

New submission from STINNER Victor <vstinner at python.org>:

Last years, I started to add "CAST" macros like:

#define _PyObject_CAST(op) ((PyObject*)(op))
#define _PyType_CAST(op) (assert(PyType_Check(op)), (PyTypeObject*)(op))

Example of usage:

#define PyObject_TypeCheck(ob, type) PyObject_TypeCheck(_PyObject_CAST(ob), type)

These macros avoids parenthesis. Example without CAST macro:

#define PyCFunction_GET_FLAGS(func) \
        (((PyCFunctionObject *)func) -> m_ml -> ml_flags)

Currently, inline cast requires adding parenthesis:

   ((PyCFunctionObject *)func)

IMO it's more readable with a CAST macro:

#define _PyCFunction_CAST(func) ((PyCFunctionObject *)func)
#define PyCFunction_GET_FLAGS(func) \
        (_PyCFunction_CAST(func)->m_ml->ml_flags)


I propose to add more CAST macros.

By the way, the current Python C API is not fully compatible with C++. "(type)expr" C syntax is seen as "old-style cast" in C++ which recommends static_cast<type>(expr), reinterpret_cast<type>(expr), or another kind of cast. But I prefer to discuss C++ in a separated issue ;-) IMO without considering C++, adding CAST macros is worth it for readability.

I am preparing pull requests for add CAST macros and use existing CAST macros.

----------
components: C API
messages: 416350
nosy: vstinner
priority: normal
severity: normal
status: open
title: [C API] Add private "CAST" macros to clean up casts in C code
versions: Python 3.11

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue47164>
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