1 files changed, 35 insertions, 1 deletions
diff --git a/ext/pybind11/docs/advanced/smart_ptrs.rst b/ext/pybind11/docs/advanced/smart_ptrs.rst
index 23072b6bf..e4a238603 100644
--- a/ext/pybind11/docs/advanced/smart_ptrs.rst
+++ b/ext/pybind11/docs/advanced/smart_ptrs.rst
@@ -123,7 +123,7 @@ Custom smart pointers
 pybind11 supports ``std::unique_ptr`` and ``std::shared_ptr`` right out of the
 box. For any other custom smart pointer, transparent conversions can be enabled
 using a macro invocation similar to the following. It must be declared at the
-level before any binding code:
+top namespace level before any binding code:
 
 .. code-block:: cpp
 
@@ -134,8 +134,42 @@ placeholder name that is used as a template parameter of the second argument.
 Thus, feel free to use any identifier, but use it consistently on both sides;
 also, don't use the name of a type that already exists in your codebase.
 
+The macro also accepts a third optional boolean parameter that is set to false
+by default. Specify
+
+.. code-block:: cpp
+
+    PYBIND11_DECLARE_HOLDER_TYPE(T, SmartPtr<T>, true);
+
+if ``SmartPtr<T>`` can always be initialized from a ``T*`` pointer without the
+risk of inconsistencies (such as multiple independent ``SmartPtr`` instances
+believing that they are the sole owner of the ``T*`` pointer). A common
+situation where ``true`` should be passed is when the ``T`` instances use
+*intrusive* reference counting.
+
 Please take a look at the :ref:`macro_notes` before using this feature.
 
+By default, pybind11 assumes that your custom smart pointer has a standard
+interface, i.e. provides a ``.get()`` member function to access the underlying
+raw pointer. If this is not the case, pybind11's ``holder_helper`` must be
+specialized:
+
+.. code-block:: cpp
+
+    // Always needed for custom holder types
+    PYBIND11_DECLARE_HOLDER_TYPE(T, SmartPtr<T>);
+
+    // Only needed if the type's `.get()` goes by another name
+    namespace pybind11 { namespace detail {
+        template <typename T>
+        struct holder_helper<SmartPtr<T>> { // <-- specialization
+            static const T *get(const SmartPtr<T> &p) { return p.getPointer(); }
+        };
+    }}
+
+The above specialization informs pybind11 that the custom ``SmartPtr`` class
+provides ``.get()`` functionality via ``.getPointer()``.
+
 .. seealso::
 
     The file :file:`tests/test_smart_ptr.cpp` contains a complete example