Adding py::smart_holder support to py::class_, purging py::classh completely.

include/pybind11/cast.h

@@ -1628,6 +1628,7 @@ using type_caster_holder = conditional_t<is_copy_constructible<holder_type>::val
                                          copyable_holder_caster<type, holder_type>,
                                          move_only_holder_caster<type, holder_type>>;
 
+template <typename T, bool Value = false> struct is_smart_holder { static constexpr bool value = Value; };
 template <typename T, bool Value = false> struct always_construct_holder { static constexpr bool value = Value; };
 
 /// Create a specialization for custom holder types (silently ignores std::shared_ptr)
@@ -1642,7 +1643,8 @@ template <typename T, bool Value = false> struct always_construct_holder { stati
 
 // PYBIND11_DECLARE_HOLDER_TYPE holder types:
 template <typename base, typename holder> struct is_holder_type :
-    std::is_base_of<detail::type_caster_holder<base, holder>, detail::type_caster<holder>> {};
+    detail::any_of<std::is_base_of<detail::type_caster_holder<base, holder>, detail::type_caster<holder>>,
+                   detail::is_smart_holder<holder>> {};
 // Specialization for always-supported unique_ptr holders:
 template <typename base, typename deleter> struct is_holder_type<base, std::unique_ptr<base, deleter>> :
     std::true_type {};

include/pybind11/detail/classh_type_casters.h

@@ -631,5 +631,41 @@ struct classh_type_caster<std::unique_ptr<T const>> : smart_holder_type_caster_l
     }                                                                                             \
     }
 
+template <>
+struct is_smart_holder<pybindit::memory::smart_holder>
+    : is_smart_holder<pybindit::memory::smart_holder, true> {
+
+    static decltype(&modified_type_caster_generic_load_impl::local_load)
+    get_type_caster_local_load_function_ptr() {
+        return &modified_type_caster_generic_load_impl::local_load;
+    }
+
+    template <typename T>
+    static void init_instance_for_type(detail::instance *inst, const void *holder_const_void_ptr) {
+        // Need for const_cast is a consequence of the type_info::init_instance type:
+        // void (*init_instance)(instance *, const void *);
+        auto holder_void_ptr = const_cast<void *>(holder_const_void_ptr);
+
+        auto v_h = inst->get_value_and_holder(detail::get_type_info(typeid(T)));
+        if (!v_h.instance_registered()) {
+            register_instance(inst, v_h.value_ptr(), v_h.type);
+            v_h.set_instance_registered();
+        }
+        using holder_type = pybindit::memory::smart_holder;
+        if (holder_void_ptr) {
+            // Note: inst->owned ignored.
+            auto holder_ptr = static_cast<holder_type *>(holder_void_ptr);
+            new (std::addressof(v_h.holder<holder_type>())) holder_type(std::move(*holder_ptr));
+        } else if (inst->owned) {
+            new (std::addressof(v_h.holder<holder_type>()))
+                holder_type(holder_type::from_raw_ptr_take_ownership(v_h.value_ptr<T>()));
+        } else {
+            new (std::addressof(v_h.holder<holder_type>()))
+                holder_type(holder_type::from_raw_ptr_unowned(v_h.value_ptr<T>()));
+        }
+        v_h.set_holder_constructed();
+    }
+};
+
 } // namespace detail
 } // namespace pybind11

include/pybind11/pybind11.h

@@ -1294,7 +1294,7 @@ public:
         /* Process optional arguments, if any */
         process_attributes<Extra...>::init(extra..., &record);
 
-        generic_type::initialize(record, &type_caster_generic::local_load);
+        generic_type_initialize(record);
 
         if (has_alias) {
             auto &instances = record.module_local ? registered_local_types_cpp() : get_internals().registered_types_cpp;
@@ -1502,6 +1502,16 @@ public:
     }
 
 private:
+    template <typename H = holder_type, detail::enable_if_t<!detail::is_smart_holder<H>::value, int> = 0>
+    void generic_type_initialize(const detail::type_record &record) {
+        generic_type::initialize(record, &detail::type_caster_generic::local_load);
+    }
+
+    template <typename H = holder_type, detail::enable_if_t<detail::is_smart_holder<H>::value, int> = 0>
+    void generic_type_initialize(const detail::type_record &record) {
+        generic_type::initialize(record, detail::is_smart_holder<H>::get_type_caster_local_load_function_ptr());
+    }
+
     /// Initialize holder object, variant 1: object derives from enable_shared_from_this
     template <typename T>
     static void init_holder(detail::instance *inst, detail::value_and_holder &v_h,
@@ -1546,6 +1556,7 @@ private:
     /// instance.  Should be called as soon as the `type` value_ptr is set for an instance.  Takes an
     /// optional pointer to an existing holder to use; if not specified and the instance is
     /// `.owned`, a new holder will be constructed to manage the value pointer.
+    template <typename H = holder_type, detail::enable_if_t<!detail::is_smart_holder<H>::value, int> = 0>
     static void init_instance(detail::instance *inst, const void *holder_ptr) {
         auto v_h = inst->get_value_and_holder(detail::get_type_info(typeid(type)));
         if (!v_h.instance_registered()) {
@@ -1555,6 +1566,11 @@ private:
         init_holder(inst, v_h, (const holder_type *) holder_ptr, v_h.value_ptr<type>());
     }
 
+    template <typename H = holder_type, detail::enable_if_t<detail::is_smart_holder<H>::value, int> = 0>
+    static void init_instance(detail::instance *inst, const void *holder_ptr) {
+        detail::is_smart_holder<H>::template init_instance_for_type<type>(inst, holder_ptr);
+    }
+
     /// Deallocates an instance; via holder, if constructed; otherwise via operator delete.
     static void dealloc(detail::value_and_holder &v_h) {
         // We could be deallocating because we are cleaning up after a Python exception.

tests/classh_module_local_0.cpp

 #include <pybind11/classh.h>
+#include <pybind11/pybind11.h>
 
 #include <string>
 

tests/classh_module_local_1.cpp

 // Identical to classh_module_local_2.cpp, except 2 replaced with 1.
 #include <pybind11/classh.h>
+#include <pybind11/pybind11.h>
 
 #include <string>
 
@@ -21,7 +22,7 @@ PYBIND11_MODULE(classh_module_local_1, m) {
     namespace py = pybind11;
     using namespace pybind11_tests::classh_module_local;
 
-    py::classh<atyp>(m, "atyp", py::module_local())
+    py::class_<atyp, py::smart_holder>(m, "atyp", py::module_local())
         .def(py::init([](const std::string &mtxt) {
             atyp obj;
             obj.mtxt = mtxt;

tests/classh_module_local_2.cpp

 // Identical to classh_module_local_1.cpp, except 1 replaced with 2.
 #include <pybind11/classh.h>
+#include <pybind11/pybind11.h>
 
 #include <string>
 
@@ -21,7 +22,7 @@ PYBIND11_MODULE(classh_module_local_2, m) {
     namespace py = pybind11;
     using namespace pybind11_tests::classh_module_local;
 
-    py::classh<atyp>(m, "atyp", py::module_local())
+    py::class_<atyp, py::smart_holder>(m, "atyp", py::module_local())
         .def(py::init([](const std::string &mtxt) {
             atyp obj;
             obj.mtxt = mtxt;

tests/test_classh_inheritance.cpp

@@ -67,8 +67,8 @@ namespace pybind11_tests {
 namespace classh_inheritance {
 
 TEST_SUBMODULE(classh_inheritance, m) {
-    py::classh<base>(m, "base");
-    py::classh<drvd, base>(m, "drvd");
+    py::class_<base, py::smart_holder>(m, "base");
+    py::class_<drvd, base, py::smart_holder>(m, "drvd");
 
     auto rvto = py::return_value_policy::take_ownership;
 
@@ -82,9 +82,10 @@ TEST_SUBMODULE(classh_inheritance, m) {
     m.def("pass_shcp_base", pass_shcp_base);
     m.def("pass_shcp_drvd", pass_shcp_drvd);
 
-    py::classh<base1>(m, "base1").def(py::init<>()); // __init__ needed for Python inheritance.
-    py::classh<base2>(m, "base2").def(py::init<>());
-    py::classh<drvd2, base1, base2>(m, "drvd2");
+    // __init__ needed for Python inheritance.
+    py::class_<base1, py::smart_holder>(m, "base1").def(py::init<>());
+    py::class_<base2, py::smart_holder>(m, "base2").def(py::init<>());
+    py::class_<drvd2, base1, base2, py::smart_holder>(m, "drvd2");
 
     m.def("rtrn_mptr_drvd2", rtrn_mptr_drvd2, rvto);
     m.def("rtrn_mptr_drvd2_up_cast1", rtrn_mptr_drvd2_up_cast1, rvto);

tests/test_classh_wip.cpp

@@ -57,7 +57,9 @@ namespace classh_wip {
 TEST_SUBMODULE(classh_wip, m) {
     namespace py = pybind11;
 
-    py::classh<atyp>(m, "atyp").def(py::init<>()).def(py::init([](const std::string &mtxt) {
+    py::class_<atyp, py::smart_holder>(m, "atyp")
+        .def(py::init<>())
+        .def(py::init([](const std::string &mtxt) {
             atyp obj;
             obj.mtxt = mtxt;
             return obj;

tests/test_unique_ptr_member.cpp

@@ -46,7 +46,9 @@ namespace pybind11_tests {
 namespace unique_ptr_member {
 
 TEST_SUBMODULE(unique_ptr_member, m) {
-    py::classh<pointee>(m, "pointee").def(py::init<>()).def("get_int", &pointee::get_int);
+    py::class_<pointee, py::smart_holder>(m, "pointee")
+        .def(py::init<>())
+        .def("get_int", &pointee::get_int);
 
     m.def("make_unique_pointee", make_unique_pointee);