added docs for custom type casters (fixes #298)

Author: wjakob

docs/advanced.rst

@@ -1937,3 +1937,85 @@ is always ``none``).
 
     // Evaluate the statements in an separate Python file on disk
     py::eval_file("script.py", scope);
+
+Development of 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);
+                }
+            };
+        }
+    };