feat: enable type-safe interoperability between different independent Python/C++ bindings systems.

CMakeLists.txt

@@ -129,6 +129,7 @@ endif()
 set(PYBIND11_HEADERS
     include/pybind11/detail/class.h
     include/pybind11/detail/common.h
+    include/pybind11/detail/cpp_conduit.h
     include/pybind11/detail/descr.h
     include/pybind11/detail/init.h
     include/pybind11/detail/internals.h

include/pybind11/detail/cpp_conduit.h

@@ -0,0 +1,77 @@
+// Copyright (c) 2024 The pybind Community.
+
+#pragma once
+
+#include <pybind11/pytypes.h>
+
+#include "common.h"
+#include "internals.h"
+
+#include <typeinfo>
+
+PYBIND11_NAMESPACE_BEGIN(PYBIND11_NAMESPACE)
+PYBIND11_NAMESPACE_BEGIN(detail)
+
+// Forward declaration needed here: Refactoring opportunity.
+extern "C" inline PyObject *pybind11_object_new(PyTypeObject *type, PyObject *, PyObject *);
+
+inline bool type_is_managed_by_our_internals(PyTypeObject *type_obj) {
+#if defined(PYPY_VERSION)
+    auto &internals = get_internals();
+    return bool(internals.registered_types_py.find(type_obj)
+                != internals.registered_types_py.end());
+#else
+    return bool(type_obj->tp_new == pybind11_object_new);
+#endif
+}
+
+inline bool is_instance_method_of_type(PyTypeObject *type_obj, PyObject *attr_name) {
+    PyObject *descr = _PyType_Lookup(type_obj, attr_name);
+    return bool((descr != nullptr) && PyInstanceMethod_Check(descr));
+}
+
+inline object try_get_cpp_conduit_method(PyObject *obj) {
+    if (PyType_Check(obj)) {
+        return object();
+    }
+    PyTypeObject *type_obj = Py_TYPE(obj);
+    str attr_name("__cpp_conduit__");
+    bool assumed_to_be_callable = false;
+    if (type_is_managed_by_our_internals(type_obj)) {
+        if (!is_instance_method_of_type(type_obj, attr_name.ptr())) {
+            return object();
+        }
+        assumed_to_be_callable = true;
+    }
+    PyObject *method = PyObject_GetAttr(obj, attr_name.ptr());
+    if (method == nullptr) {
+        PyErr_Clear();
+        return object();
+    }
+    if (!assumed_to_be_callable && PyCallable_Check(method) == 0) {
+        Py_DECREF(method);
+        return object();
+    }
+    return reinterpret_steal<object>(method);
+}
+
+inline void *try_raw_pointer_ephemeral_from_cpp_conduit(handle src,
+                                                        const std::type_info *cpp_type_info) {
+    object method = try_get_cpp_conduit_method(src.ptr());
+    if (method) {
+        capsule cpp_type_info_capsule(const_cast<void *>(static_cast<const void *>(cpp_type_info)),
+                                      typeid(std::type_info).name());
+        object cpp_conduit = method(bytes(PYBIND11_PLATFORM_ABI_ID),
+                                    cpp_type_info_capsule,
+                                    bytes("raw_pointer_ephemeral"));
+        if (isinstance<capsule>(cpp_conduit)) {
+            return reinterpret_borrow<capsule>(cpp_conduit).get_pointer();
+        }
+    }
+    return nullptr;
+}
+
+#define PYBIND11_HAS_CPP_CONDUIT
+
+PYBIND11_NAMESPACE_END(detail)
+PYBIND11_NAMESPACE_END(PYBIND11_NAMESPACE)

include/pybind11/detail/internals.h

@@ -321,15 +321,17 @@ struct type_info {
 #    define PYBIND11_INTERNALS_KIND ""
 #endif
 
+#define PYBIND11_PLATFORM_ABI_ID                                                                  \
+    PYBIND11_INTERNALS_KIND PYBIND11_COMPILER_TYPE PYBIND11_STDLIB PYBIND11_BUILD_ABI             \
+        PYBIND11_BUILD_TYPE
+
 #define PYBIND11_INTERNALS_ID                                                                     \
     "__pybind11_internals_v" PYBIND11_TOSTRING(PYBIND11_INTERNALS_VERSION)                        \
-        PYBIND11_INTERNALS_KIND PYBIND11_COMPILER_TYPE PYBIND11_STDLIB                            \
-            PYBIND11_BUILD_ABI PYBIND11_BUILD_TYPE "__"
+        PYBIND11_PLATFORM_ABI_ID "__"
 
 #define PYBIND11_MODULE_LOCAL_ID                                                                  \
     "__pybind11_module_local_v" PYBIND11_TOSTRING(PYBIND11_INTERNALS_VERSION)                     \
-        PYBIND11_INTERNALS_KIND PYBIND11_COMPILER_TYPE PYBIND11_STDLIB                            \
-            PYBIND11_BUILD_ABI PYBIND11_BUILD_TYPE "__"
+        PYBIND11_PLATFORM_ABI_ID "__"
 
 /// Each module locally stores a pointer to the `internals` data. The data
 /// itself is shared among modules with the same `PYBIND11_INTERNALS_ID`.

include/pybind11/detail/type_caster_base.h

@@ -12,14 +12,17 @@
 #include <pybind11/pytypes.h>
 
 #include "common.h"
+#include "cpp_conduit.h"
 #include "descr.h"
 #include "internals.h"
 #include "typeid.h"
 #include "value_and_holder.h"
 
 #include <cstdint>
+#include <cstring>
 #include <iterator>
 #include <new>
+#include <stdexcept>
 #include <string>
 #include <type_traits>
 #include <typeindex>
@@ -611,6 +614,13 @@ class type_caster_generic {
         }
         return false;
     }
+    bool try_cpp_conduit(handle src) {
+        value = try_raw_pointer_ephemeral_from_cpp_conduit(src, cpptype);
+        if (value != nullptr) {
+            return true;
+        }
+        return false;
+    }
     void check_holder_compat() {}
 
     PYBIND11_NOINLINE static void *local_load(PyObject *src, const type_info *ti) {
@@ -742,6 +752,10 @@ class type_caster_generic {
             return true;
         }
 
+        if (convert && cpptype && this_.try_cpp_conduit(src)) {
+            return true;
+        }
+
         return false;
     }
 
@@ -769,6 +783,32 @@ class type_caster_generic {
     void *value = nullptr;
 };
 
+inline object cpp_conduit_method(handle self,
+                                 const bytes &pybind11_platform_abi_id,
+                                 const capsule &cpp_type_info_capsule,
+                                 const bytes &pointer_kind) {
+#ifdef PYBIND11_HAS_STRING_VIEW
+    using cpp_str = std::string_view;
+#else
+    using cpp_str = std::string;
+#endif
+    if (cpp_str(pybind11_platform_abi_id) != PYBIND11_PLATFORM_ABI_ID) {
+        return none();
+    }
+    if (std::strcmp(cpp_type_info_capsule.name(), typeid(std::type_info).name()) != 0) {
+        return none();
+    }
+    if (cpp_str(pointer_kind) != "raw_pointer_ephemeral") {
+        throw std::runtime_error("Invalid pointer_kind: \"" + std::string(pointer_kind) + "\"");
+    }
+    const auto *cpp_type_info = cpp_type_info_capsule.get_pointer<const std::type_info>();
+    type_caster_generic caster(*cpp_type_info);
+    if (!caster.load(self, false)) {
+        return none();
+    }
+    return capsule(caster.value, cpp_type_info->name());
+}
+
 /**
  * Determine suitable casting operator for pointer-or-lvalue-casting type casters.  The type caster
  * needs to provide `operator T*()` and `operator T&()` operators.

include/pybind11/pybind11.h

@@ -610,7 +610,8 @@ class cpp_function : public function {
         int index = 0;
         /* Create a nice pydoc rec including all signatures and
            docstrings of the functions in the overload chain */
-        if (chain && options::show_function_signatures()) {
+        if (chain && options::show_function_signatures()
+            && strcmp(rec->name, "__cpp_conduit__") != 0) {
             // First a generic signature
             signatures += rec->name;
             signatures += "(*args, **kwargs)\n";
@@ -619,7 +620,7 @@ class cpp_function : public function {
         // Then specific overload signatures
         bool first_user_def = true;
         for (auto *it = chain_start; it != nullptr; it = it->next) {
-            if (options::show_function_signatures()) {
+            if (options::show_function_signatures() && strcmp(rec->name, "__cpp_conduit__") != 0) {
                 if (index > 0) {
                     signatures += '\n';
                 }
@@ -1652,6 +1653,7 @@ class class_ : public detail::generic_type {
                     = instances[std::type_index(typeid(type))];
             });
         }
+        def("__cpp_conduit__", cpp_conduit_method);
     }
 
     template <typename Base, detail::enable_if_t<is_base<Base>::value, int> = 0>

tests/CMakeLists.txt

@@ -118,6 +118,7 @@ set(PYBIND11_TEST_FILES
     test_const_name
     test_constants_and_functions
     test_copy_move
+    test_cpp_conduit
     test_custom_type_casters
     test_custom_type_setup
     test_docstring_options
@@ -218,6 +219,7 @@ tests_extra_targets("test_exceptions.py;test_local_bindings.py;test_stl.py;test_
 # And add additional targets for other tests.
 tests_extra_targets("test_exceptions.py" "cross_module_interleaved_error_already_set")
 tests_extra_targets("test_gil_scoped.py" "cross_module_gil_utils")
+tests_extra_targets("test_cpp_conduit.py" "exo_planet_pybind11;exo_planet_c_api")
 
 set(PYBIND11_EIGEN_REPO
     "https://gitlab.com/libeigen/eigen.git"

tests/exo_planet_c_api.cpp

@@ -0,0 +1,103 @@
+// Copyright (c) 2024 The pybind Community.
+
+// THIS MUST STAY AT THE TOP!
+#include <pybind11/pybind11.h> // EXCLUSIVELY for PYBIND11_PLATFORM_ABI_ID
+// Potential future direction to maximize reusability:
+// (e.g. for use from SWIG, Cython, PyCLIF, nanobind):
+//     #include <pybind11/compat/platform_abi_id.h>
+// This would only depend on:
+//     1. A C++ compiler, WITHOUT requiring -fexceptions.
+//     2. Python.h
+
+#include "test_cpp_conduit_traveler_types.h"
+
+#include <Python.h>
+#include <typeinfo>
+
+namespace {
+
+void *get_cpp_conduit_void_ptr(PyObject *py_obj, const std::type_info *cpp_type_info) {
+    PyObject *cpp_type_info_capsule
+        = PyCapsule_New(const_cast<void *>(static_cast<const void *>(cpp_type_info)),
+                        typeid(std::type_info).name(),
+                        nullptr);
+    if (cpp_type_info_capsule == nullptr) {
+        return nullptr;
+    }
+    PyObject *cpp_conduit = PyObject_CallMethod(py_obj,
+                                                "__cpp_conduit__",
+                                                "yOy",
+                                                PYBIND11_PLATFORM_ABI_ID,
+                                                cpp_type_info_capsule,
+                                                "raw_pointer_ephemeral");
+    Py_DECREF(cpp_type_info_capsule);
+    if (cpp_conduit == nullptr) {
+        return nullptr;
+    }
+    void *void_ptr = PyCapsule_GetPointer(cpp_conduit, cpp_type_info->name());
+    Py_DECREF(cpp_conduit);
+    if (PyErr_Occurred()) {
+        return nullptr;
+    }
+    return void_ptr;
+}
+
+template <typename T>
+T *get_cpp_conduit_type_ptr(PyObject *py_obj) {
+    void *void_ptr = get_cpp_conduit_void_ptr(py_obj, &typeid(T));
+    if (void_ptr == nullptr) {
+        return nullptr;
+    }
+    return static_cast<T *>(void_ptr);
+}
+
+extern "C" PyObject *wrapGetLuggage(PyObject * /*self*/, PyObject *traveler) {
+    const auto *cpp_traveler
+        = get_cpp_conduit_type_ptr<pybind11_tests::test_cpp_conduit::Traveler>(traveler);
+    if (cpp_traveler == nullptr) {
+        return nullptr;
+    }
+    return PyUnicode_FromString(cpp_traveler->luggage.c_str());
+}
+
+extern "C" PyObject *wrapGetPoints(PyObject * /*self*/, PyObject *premium_traveler) {
+    const auto *cpp_premium_traveler
+        = get_cpp_conduit_type_ptr<pybind11_tests::test_cpp_conduit::PremiumTraveler>(
+            premium_traveler);
+    if (cpp_premium_traveler == nullptr) {
+        return nullptr;
+    }
+    return PyLong_FromLong(static_cast<long>(cpp_premium_traveler->points));
+}
+
+PyMethodDef ThisMethodDef[] = {{"GetLuggage", wrapGetLuggage, METH_O, nullptr},
+                               {"GetPoints", wrapGetPoints, METH_O, nullptr},
+                               {nullptr, nullptr, 0, nullptr}};
+
+struct PyModuleDef ThisModuleDef = {
+    PyModuleDef_HEAD_INIT, // m_base
+    "exo_planet_c_api",    // m_name
+    nullptr,               // m_doc
+    -1,                    // m_size
+    ThisMethodDef,         // m_methods
+    nullptr,               // m_slots
+    nullptr,               // m_traverse
+    nullptr,               // m_clear
+    nullptr                // m_free
+};
+
+} // namespace
+
+#if defined(WIN32) || defined(_WIN32)
+#    define EXO_PLANET_C_API_EXPORT __declspec(dllexport)
+#else
+#    define EXO_PLANET_C_API_EXPORT __attribute__((visibility("default")))
+#endif
+
+extern "C" EXO_PLANET_C_API_EXPORT PyObject *PyInit_exo_planet_c_api() {
+    PyObject *m = PyModule_Create(&ThisModuleDef);
+    if (m == nullptr) {
+        return nullptr;
+    }
+    return m;
+}

tests/exo_planet_pybind11.cpp

@@ -0,0 +1,10 @@
+// Copyright (c) 2024 The pybind Community.
+
+#if defined(PYBIND11_INTERNALS_VERSION)
+#    undef PYBIND11_INTERNALS_VERSION
+#endif
+#define PYBIND11_INTERNALS_VERSION 900000001
+
+#include "test_cpp_conduit_traveler_bindings.h"
+
+PYBIND11_MODULE(exo_planet_pybind11, m) { pybind11_tests::test_cpp_conduit::wrap_traveler(m); }

tests/extra_python_package/test_files.py

@@ -53,6 +53,7 @@
 detail_headers = {
     "include/pybind11/detail/class.h",
     "include/pybind11/detail/common.h",
+    "include/pybind11/detail/cpp_conduit.h",
     "include/pybind11/detail/descr.h",
     "include/pybind11/detail/init.h",
     "include/pybind11/detail/internals.h",

tests/test_cpp_conduit.cpp

@@ -0,0 +1,21 @@
+// Copyright (c) 2024 The pybind Community.
+
+#include "pybind11_tests.h"
+#include "test_cpp_conduit_traveler_bindings.h"
+
+#include <typeinfo>
+
+namespace pybind11_tests {
+namespace test_cpp_conduit {
+
+TEST_SUBMODULE(cpp_conduit, m) {
+    m.attr("PYBIND11_PLATFORM_ABI_ID") = py::bytes(PYBIND11_PLATFORM_ABI_ID);
+    m.attr("cpp_type_info_capsule_Traveler")
+        = py::capsule(&typeid(Traveler), typeid(std::type_info).name());
+    m.attr("cpp_type_info_capsule_int") = py::capsule(&typeid(int), typeid(std::type_info).name());
+
+    wrap_traveler(m);
+}
+
+} // namespace test_cpp_conduit
+} // namespace pybind11_tests