DEAD END: Make make_caster unique_to_translation_unit

docs/advanced/cast/custom.rst

@@ -1,93 +1,140 @@
 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.
+Some applications may prefer custom type casters that convert between existing
+Python types and C++ types, similar to the ``list`` ↔ ``std::vector``
+and ``dict`` ↔ ``std::map`` conversions which are built into pybind11.
+Implementing custom type casters is fairly advanced usage and requires
+familiarity with the intricacies of the Python C API.
 
 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.
+type that we want to be convertible to C++ from any Python type that provides
+an ``__int__`` method, and is converted to a Python ``int`` when returned from
+C++ to Python.
 
-.. code-block:: cpp
-
-    struct inty { long long_value; };
+..
+    PLEASE KEEP THE CODE BLOCKS IN SYNC WITH
+        tests/test_docs_advanced_cast_custom.cpp
+    Ideally, change the test, run pre-commit (incl. clang-format),
+    then copy the changed code back here.
 
-    void print(inty s) {
-        std::cout << s.long_value << std::endl;
-    }
+.. code-block:: cpp
 
-The following Python snippet demonstrates the intended usage from the Python side:
+    namespace user_space {
 
-.. code-block:: python
+    struct inty {
+        long long_value;
+    };
 
-    class A:
-        def __int__(self):
-            return 123
+    std::string to_string(const inty &s) { return std::to_string(s.long_value); }
 
+    inty return_42() { return inty{42}; }
 
-    from example import print
+    } // namespace user_space
 
-    print(A())
+The necessary conversion routines are defined by a caster class, which
+is then "plugged into" pybind11 using one of two alternative mechanisms.
 
-To register the necessary conversion routines, it is necessary to add an
-instantiation of the ``pybind11::detail::type_caster<T>`` template.
-Although this is an implementation detail, adding an instantiation of this
-type is explicitly allowed.
+Starting with the example caster class:
 
 .. 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, const_name("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());
+    namespace user_space {
+
+    struct inty_type_caster {
+    public:
+        // This macro establishes the name 'inty' in function signatures and declares a local variable
+        // 'value' of type inty.
+        PYBIND11_TYPE_CASTER(inty, pybind11::detail::const_name("inty"));
+
+        // Python -> C++: convert a PyObject into an inty instance or return false upon failure. The
+        // second argument indicates whether implicit conversions should be allowed.
+        bool load(pybind11::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;
             }
-
-            /**
-             * 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);
+            // Now try to convert into a C++ int.
+            value.long_value = PyLong_AsLong(tmp);
+            Py_DECREF(tmp);
+            // Ensure PyLong_AsLong succeeded (to catch out-of-range errors etc).
+            if (PyErr_Occurred()) {
+                PyErr_Clear();
+                return false;
             }
-        };
-    }} // namespace pybind11::detail
+            return true;
+        }
+
+        // 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 often ignored by custom casters.
+        static pybind11::handle
+        cast(inty src, pybind11::return_value_policy /* policy */, pybind11::handle /* parent */) {
+            return PyLong_FromLong(src.long_value);
+        }
+    };
+
+    } // namespace user_space
 
 .. note::
 
-    A ``type_caster<T>`` defined with ``PYBIND11_TYPE_CASTER(T, ...)`` requires
+    A caster class using ``PYBIND11_TYPE_CASTER(T, ...)`` requires
     that ``T`` is default-constructible (``value`` is first default constructed
-    and then ``load()`` assigns to it).
+    and then ``load()`` assigns to it). It is possible but more involved to define
+    a caster class for types that are not default-constructible.
 
-.. warning::
+The caster class defined above can be plugged into pybind11 in two ways:
+
+* Starting with pybind11 v2.10, a new — and the recommended — alternative is to *declare* a
+  function named ``pybind11_select_caster``:
+
+  .. code-block:: cpp
+
+    namespace user_space {
+
+    inty_type_caster pybind11_select_caster(inty *);
 
-    When using custom type casters, it's important to declare them consistently
-    in every compilation unit of the Python extension module. Otherwise,
-    undefined behavior can ensue.
+    } // namespace user_space
+
+  The argument is a *pointer* to the C++ type, the return type is the caster type.
+  This function has no implementation! Its only purpose is to associate the C++ type
+  with its caster class. pybind11 exploits C++ Argument Dependent Lookup
+  (`ADL <https://en.cppreference.com/w/cpp/language/adl>`_)
+  to discover the association.
+
+  Note that ``pybind11_select_caster`` can alternatively be declared as a ``friend``
+  function of the C++ type, if that is practical and preferred:
+
+  .. code-block:: cpp
+
+    struct inty_type_caster;
+
+    struct inty {
+        ...
+        friend inty_type_caster pybind11_select_caster(inty *);
+    };
+
+* An older alternative is to specialize the ``pybind11::detail::type_caster<T>`` template.
+  Although the ``detail`` namespace is involved, adding a ``type_caster`` specialization
+  is explicitly allowed:
+
+  .. code-block:: cpp
+
+    namespace pybind11 {
+    namespace detail {
+    template <>
+    struct type_caster<user_space::inty> : user_space::inty_type_caster {};
+    } // namespace detail
+    } // namespace pybind11
+
+  .. note::
+      ``type_caster` specializations may be full (as in this simple example) or partial.
+
+.. warning::
+    With either alternative, for a given type ``T``, the ``pybind11_select_caster``
+    declaration or ``type_caster`` specialization must be consistent across all compilation
+    units of a Python extension module, to satisfy the C++ One Definition Rule
+    (`ODR <https://en.cppreference.com/w/cpp/language/definition>`_).

include/pybind11/cast.h

@@ -31,10 +31,29 @@
 PYBIND11_NAMESPACE_BEGIN(PYBIND11_NAMESPACE)
 PYBIND11_NAMESPACE_BEGIN(detail)
 
-template <typename type, typename SFINAE = void>
+namespace {
+struct unique_to_translation_unit {};
+} // namespace
+
+struct global_caster {};
+
+template <typename T, typename = unique_to_translation_unit>
+struct caster_scope {
+    using select = global_caster;
+};
+
+template <typename type, typename SFINAE = void, typename = typename caster_scope<type>::select>
 class type_caster : public type_caster_base<type> {};
-template <typename type>
-using make_caster = type_caster<intrinsic_t<type>>;
+
+template <
+    typename IntrinsicType,
+    enable_if_t<std::is_same<typename caster_scope<IntrinsicType>::select, global_caster>::value,
+                int> = 0>
+type_caster<IntrinsicType> pybind11_select_caster(IntrinsicType *, global_caster);
+
+template <typename type, typename = unique_to_translation_unit>
+using make_caster = decltype(pybind11_select_caster(static_cast<intrinsic_t<type> *>(nullptr),
+                                                    typename caster_scope<type>::select()));
 
 // Shortcut for calling a caster's `cast_op_type` cast operator for casting a type_caster to a T
 template <typename T>
@@ -49,7 +68,7 @@ cast_op(make_caster<T> &&caster) {
 }
 
 template <typename type>
-class type_caster<std::reference_wrapper<type>> {
+class type_caster<std::reference_wrapper<type>, void, typename caster_scope<type>::select> {
 private:
     using caster_t = make_caster<type>;
     caster_t subcaster;
@@ -512,7 +531,9 @@ struct type_caster<std::basic_string_view<CharT, Traits>,
 // Type caster for C-style strings.  We basically use a std::string type caster, but also add the
 // ability to use None as a nullptr char* (which the string caster doesn't allow).
 template <typename CharT>
-struct type_caster<CharT, enable_if_t<is_std_char_type<CharT>::value>> {
+struct type_caster<CharT,
+                   enable_if_t<is_std_char_type<CharT>::value>,
+                   typename caster_scope<CharT>::select> {
     using StringType = std::basic_string<CharT>;
     using StringCaster = make_caster<StringType>;
     StringCaster str_caster;
@@ -1007,8 +1028,8 @@ struct return_value_policy_override<
 };
 
 // Basic python -> C++ casting; throws if casting fails
-template <typename T, typename SFINAE>
-type_caster<T, SFINAE> &load_type(type_caster<T, SFINAE> &conv, const handle &handle) {
+template <typename T, typename Caster>
+Caster &load_type(Caster &conv, const handle &handle) {
     if (!conv.load(handle, true)) {
 #if !defined(PYBIND11_DETAILED_ERROR_MESSAGES)
         throw cast_error("Unable to cast Python instance to C++ type (#define "
@@ -1021,11 +1042,17 @@ type_caster<T, SFINAE> &load_type(type_caster<T, SFINAE> &conv, const handle &ha
     }
     return conv;
 }
+
+template <typename T, typename SFINAE>
+type_caster<T, SFINAE> &load_type(type_caster<T, SFINAE> &conv, const handle &handle) {
+    return load_type<T, type_caster<T, SFINAE>>(conv, handle);
+}
+
 // Wrapper around the above that also constructs and returns a type_caster
 template <typename T>
 make_caster<T> load_type(const handle &handle) {
     make_caster<T> conv;
-    load_type(conv, handle);
+    load_type<T>(conv, handle);
     return conv;
 }
 
@@ -1152,7 +1179,7 @@ using override_caster_t = conditional_t<cast_is_temporary_value_reference<ret_ty
 template <typename T>
 enable_if_t<cast_is_temporary_value_reference<T>::value, T> cast_ref(object &&o,
                                                                      make_caster<T> &caster) {
-    return cast_op<T>(load_type(caster, o));
+    return cast_op<T>(load_type<T>(caster, o));
 }
 template <typename T>
 enable_if_t<!cast_is_temporary_value_reference<T>::value, T> cast_ref(object &&,

tests/CMakeLists.txt

@@ -128,6 +128,7 @@ set(PYBIND11_TEST_FILES
     test_custom_type_casters
     test_custom_type_setup
     test_docstring_options
+    test_docs_advanced_cast_custom
     test_eigen
     test_enum
     test_eval
@@ -147,6 +148,8 @@ set(PYBIND11_TEST_FILES
     test_operator_overloading
     test_pickling
     test_pytypes
+    test_select_caster
+    test_select_caster_alt
     test_sequences_and_iterators
     test_smart_ptr
     test_stl