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+Custom type casters
+===================
+
+In very rare cases, applications may require custom type casters that cannot be
+expressed using the abstractions provided by pybind11, thus requiring raw
+Python C API calls. This is fairly advanced usage and should only be pursued by
+experts who are familiar with the intricacies of Python reference counting.
+
+The following snippets demonstrate how this works for a very simple ``inty``
+type that that should be convertible from Python types that provide a
+``__int__(self)`` method.
+
+.. code-block:: cpp
+
+ struct inty { long long_value; };
+
+ void print(inty s) {
+ std::cout << s.long_value << std::endl;
+ }
+
+The following Python snippet demonstrates the intended usage from the Python side:
+
+.. code-block:: python
+
+ class A:
+ def __int__(self):
+ return 123
+
+ from example import print
+ print(A())
+
+To register the necessary conversion routines, it is necessary to add
+a partial overload to the ``pybind11::detail::type_caster<T>`` template.
+Although this is an implementation detail, adding partial overloads to this
+type is explicitly allowed.
+
+.. code-block:: cpp
+
+ namespace pybind11 { namespace detail {
+ template <> struct type_caster<inty> {
+ public:
+ /**
+ * This macro establishes the name 'inty' in
+ * function signatures and declares a local variable
+ * 'value' of type inty
+ */
+ PYBIND11_TYPE_CASTER(inty, _("inty"));
+
+ /**
+ * Conversion part 1 (Python->C++): convert a PyObject into a inty
+ * instance or return false upon failure. The second argument
+ * indicates whether implicit conversions should be applied.
+ */
+ bool load(handle src, bool) {
+ /* Extract PyObject from handle */
+ PyObject *source = src.ptr();
+ /* Try converting into a Python integer value */
+ PyObject *tmp = PyNumber_Long(source);
+ if (!tmp)
+ return false;
+ /* Now try to convert into a C++ int */
+ value.long_value = PyLong_AsLong(tmp);
+ Py_DECREF(tmp);
+ /* Ensure return code was OK (to avoid out-of-range errors etc) */
+ return !(value.long_value == -1 && !PyErr_Occurred());
+ }
+
+ /**
+ * Conversion part 2 (C++ -> Python): convert an inty instance into
+ * a Python object. The second and third arguments are used to
+ * indicate the return value policy and parent object (for
+ * ``return_value_policy::reference_internal``) and are generally
+ * ignored by implicit casters.
+ */
+ static handle cast(inty src, return_value_policy /* policy */, handle /* parent */) {
+ return PyLong_FromLong(src.long_value);
+ }
+ };
+ }} // namespace pybind11::detail