[skip ci] Move "Pitfalls with raw pointers and shared ownership" section to a more prominent location. (#5611)

Author: rwgk

docs/advanced/smart_ptrs.rst

@@ -1,3 +1,5 @@
+.. _py_class_holder:
+
 Smart pointers & ``py::class_``
 ###############################
 
@@ -175,59 +177,3 @@ provides ``.get()`` functionality via ``.getPointer()``.
     The file :file:`tests/test_smart_ptr.cpp` contains a complete example
     that demonstrates how to work with custom reference-counting holder types
     in more detail.
-
-
-Be careful not to accidentally undermine automatic lifetime management
-======================================================================
-
-``py::class_``-wrapped objects automatically manage the lifetime of the
-wrapped C++ object, in collaboration with the chosen holder type.
-When wrapping C++ functions involving raw pointers, care needs to be taken
-to not inadvertently transfer ownership, resulting in multiple Python
-objects acting as owners, causing heap-use-after-free or double-free errors.
-For example:
-
-.. code-block:: cpp
-
-    class Child { };
-
-    class Parent {
-    public:
-       Parent() : child(std::make_shared<Child>()) { }
-       Child *get_child() { return child.get(); }  /* DANGER */
-    private:
-        std::shared_ptr<Child> child;
-    };
-
-    PYBIND11_MODULE(example, m) {
-        py::class_<Child, std::shared_ptr<Child>>(m, "Child");
-
-        py::class_<Parent, std::shared_ptr<Parent>>(m, "Parent")
-           .def(py::init<>())
-           .def("get_child", &Parent::get_child);  /* PROBLEM */
-    }
-
-The following Python code leads to undefined behavior, likely resulting in
-a segmentation fault.
-
-.. code-block:: python
-
-   from example import Parent
-
-   print(Parent().get_child())
-
-Part of the ``/* PROBLEM */`` here is that pybind11 falls back to using
-``return_value_policy::take_ownership`` as the default (see
-:ref:`return_value_policies`). The fact that the ``Child`` instance is
-already managed by ``std::shared_ptr<Child>`` is lost. Therefore pybind11
-will create a second independent ``std::shared_ptr<Child>`` that also
-claims ownership of the pointer, eventually leading to heap-use-after-free
-or double-free errors.
-
-There are various ways to resolve this issue, either by changing
-the ``Child`` or ``Parent`` C++ implementations (e.g. using
-``std::enable_shared_from_this<Child>`` as a base class for
-``Child``, or adding a member function to ``Parent`` that returns
-``std::shared_ptr<Child>``), or if that is not feasible, by using
-``return_value_policy::reference_internal``. What is the best approach
-depends on the exact situation.

docs/classes.rst

@@ -459,6 +459,64 @@ you can use ``py::detail::overload_cast_impl`` with an additional set of parenth
     other using the ``.def(py::init<...>())`` syntax. The existing machinery
     for specifying keyword and default arguments also works.
 
+☝️ Pitfalls with raw pointers and shared ownership
+==================================================
+
+``py::class_``-wrapped objects automatically manage the lifetime of the
+wrapped C++ object, in collaboration with the chosen holder type (see
+:ref:`py_class_holder`). When wrapping C++ functions involving raw pointers,
+care needs to be taken to not accidentally undermine automatic lifetime
+management. For example, ownership is inadvertently transferred here:
+
+.. code-block:: cpp
+
+    class Child { };
+
+    class Parent {
+    public:
+       Parent() : child(std::make_shared<Child>()) { }
+       Child *get_child() { return child.get(); }  /* DANGER */
+    private:
+        std::shared_ptr<Child> child;
+    };
+
+    PYBIND11_MODULE(example, m) {
+        py::class_<Child, std::shared_ptr<Child>>(m, "Child");
+
+        py::class_<Parent, std::shared_ptr<Parent>>(m, "Parent")
+           .def(py::init<>())
+           .def("get_child", &Parent::get_child);  /* PROBLEM */
+    }
+
+The following Python code leads to undefined behavior, likely resulting in
+a segmentation fault.
+
+.. code-block:: python
+
+   from example import Parent
+
+   print(Parent().get_child())
+
+Part of the ``/* PROBLEM */`` here is that pybind11 falls back to using
+``return_value_policy::take_ownership`` as the default (see
+:ref:`return_value_policies`). The fact that the ``Child`` instance is
+already managed by ``std::shared_ptr<Child>`` is lost. Therefore pybind11
+will create a second independent ``std::shared_ptr<Child>`` that also
+claims ownership of the pointer, eventually leading to heap-use-after-free
+or double-free errors.
+
+There are various ways to resolve this issue, either by changing
+the ``Child`` or ``Parent`` C++ implementations (e.g. using
+``std::enable_shared_from_this<Child>`` as a base class for
+``Child``, or adding a member function to ``Parent`` that returns
+``std::shared_ptr<Child>``), or if that is not feasible, by using
+``return_value_policy::reference_internal``. What is the best approach
+depends on the exact situation.
+
+A highly effective way to stay in the clear — even in pure C++, but
+especially when binding C++ code to Python — is to consistently prefer
+``std::shared_ptr`` or ``std::unique_ptr`` over passing raw pointers.
+
 .. _native_enum:
 
 Enumerations and internal types