Description

vstinner

opened on Oct 4, 2026

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Bug report

Example:

#include <Python.h>

int main()
{
    int array[] = {1, 2, 3};
    return Py_ARRAY_LENGTH(array);
}

Building this example with g++ -std=gnu++11 example.cpp or just g++ example.cpp fails with:

In file included from Include/Python.h:70,
                 from x.cpp:1:
x.cpp: In function ‘int main()’:
Include/pymacro.h:217:30: error: ‘__builtin_types_compatible_p’ was not declared in this scope
  217 |      + Py_BUILD_ASSERT_EXPR(!__builtin_types_compatible_p(typeof(array), \
      |                              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~

The example builds correctly if I use -std=c++11 (g++ -std=c++11 example.cpp).

The macro is currently defined as:

#define Py_ARRAY_LENGTH(array) \
    (sizeof(array) / sizeof((array)[0]) \
     + Py_BUILD_ASSERT_EXPR(!__builtin_types_compatible_p(typeof(array), \
                                                          typeof(&(array)[0]))))
#else
#define Py_ARRAY_LENGTH(array) \
    (sizeof(array) / sizeof((array)[0]))
#endif

I'm not sure why __builtin_types_compatible_p() is available with -std=c++11, but not with -std=gnu++11.

I made my tests using g++ (GCC) 16.2.1 on Fedora 44.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-158810: Fix Py_ARRAY_LENGTH() macro on C++ #158812
• gh-158810: Use _Py_TYPEOF in Py_ARRAY_LENGTH() #158838
• [3.15] gh-158810: Fix Py_ARRAY_LENGTH() macro on C++ #159315
• [3.14] gh-158810: Fix Py_ARRAY_LENGTH() macro on C++ #159317

Activity

1. vstinner

   added

   type-bugAn unexpected behavior, bug, or errorAn unexpected behavior, bug, or error

   topic-C-API

   on Oct 4, 2026
2. vstinner

   changed the title [-]C API: Py_ARRAY_LENGTH() macro is incompatible with g++ -std=gnu++11[/-] [+][C API] Py_ARRAY_LENGTH() macro is incompatible with g++ -std=gnu++11[/+] on Oct 4, 2026
3. da-woods commented on Oct 4, 2026

   

   da-woods

   on Oct 4, 2026

   Contributor

   More actions

   The documentation for __builtin_types_compatible_p says

   Note: This construct is only available for C.

   Which leaves a mystery about why it seems to work in -std=c++11 but which does imply you shouldn't be using it in c++
4. vstinner

   added a commit that references this issue on Oct 4, 2026

   pythongh-158810: Fix Py_ARRAY_LENGTH() macro on C++

   9d6063a
5. vstinner commented on Oct 4, 2026

   

   vstinner

   on Oct 4, 2026

   MemberAuthor

   More actions

   @da-woods: It would be nice to be able to use __builtin_types_compatible_p() on C++ when it's supported. The problem is that I don't know how to check if the built-in function is available or not.

   (...) which does imply you shouldn't be using it in c++

   Well well well, a simple fix is to do exactly that: don't use __builtin_types_compatible_p() on C++. I implemented this approach in PR gh-158812.
6. chris-eibl commented on Oct 5, 2026

   

   chris-eibl

   on Oct 5, 2026

   Member

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   Other places in the code base use _Py__has_builtin, but I don't think this buys us much here, since we know __builtin_types_compatible_p isn't supported in C++. Yet, it'll reduce the detection to #if _Py__has_builtin(__builtin_types_compatible_p).

   The most natural fit in modern C++ beginning with C++17 is std::size, and AI will for sure spit out something for earlier versions, but that's all template based, and you've mentioned in #157649 (comment) that templates did not play out nicely for Py_CLEAR et al.
7. picnixz

   added

   interpreter-core(Objects, Python, Grammar, and Parser dirs)(Objects, Python, Grammar, and Parser dirs)

   buildThe build process and cross-buildThe build process and cross-build

   on Oct 5, 2026
8. vstinner

   added a commit that references this issue on Oct 5, 2026

   gh-158810: Fix Py_ARRAY_LENGTH() macro on C++ (#158812)

   b2bfbce
9. vstinner commented on Oct 5, 2026

   

   vstinner

   on Oct 5, 2026

   MemberAuthor

   More actions

   The example builds correctly if I use -std=c++11 (g++ -std=c++11 example.cpp).

   When using -std=c++11, the __STRICT_ANSI__ macro is defined. In this case, Py_ARRAY_LENGTH() doesn't use built-in functions (it doesn't use __builtin_types_compatible_p() or typeof()).

   Other places in the code base use _Py__has_builtin, but I don't think this buys us much here, since we know __builtin_types_compatible_p isn't supported in C++. Yet, it'll reduce the detection to #if _Py__has_builtin(__builtin_types_compatible_p).

   __has_builtin(typeof) and __has_builtin(__builtin_types_compatible_p) are false using -std=c++11 and -std=gnu++11. So it cannot be used in C++ for Py_ARRAY_LENGTH().

   The most natural fit in modern C++ beginning with C++17 is std::size, and AI will for sure spit out something for earlier versions, but that's all template based, and you've mentioned in #157649 (comment) that templates did not play out nicely for Py_CLEAR et al.

   I'm sure that it's possible to write something nice using templates, but I would prefer to keep the Python C API simple, and avoid C++ templates in general.

   #define Py_ARRAY_LENGTH(array) (sizeof(array) / sizeof((array)[0])) works just fine on C++.

   It's just that it doesn't detect API misuse on C++. It would be nice to have, but it's ok to not detect API misuse in this case.

   I just merged my PR to just use #define Py_ARRAY_LENGTH(array) (sizeof(array) / sizeof((array)[0])) in C++.

   Thanks for your feedback @da-woods and @chris-eibl.

   Reacted by Chris Eibl

   Reacted by Chris Eibl
10. vstinner

    closed this as completedon Oct 5, 2026
11. vstinner

    added a commit that references this issue on Oct 6, 2026

    gh-158810: Use _Py_TYPEOF in Py_ARRAY_LENGTH() (#158838)

    85c03d4
12. vstinner

    added 2 commits that reference this issue on Oct 13, 2026

    [3.15] gh-158810: Fix Py_ARRAY_LENGTH() macro on C++ (#159315)

    152185f

    [3.14] gh-158810: Fix Py_ARRAY_LENGTH() macro on C++ (#159317)

    931322b

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