Introduce def_visitor abstraction for objects that provide custom bin…

…ding logic when passed to def() (#884)

docs/api_core.rst

@@ -2217,6 +2217,13 @@ Class binding
       where possible. See the discussion of :ref:`customizing object creation
       <custom_new>` for more details.
 
+   .. cpp:function:: template <typename Visitor, typename... Extra> class_ &def(def_visitor<Visitor> arg, const Extra &... extra)
+
+      Dispatch to custom user-provided binding logic implemented by the type
+      ``Visitor``, passing it the binding annotations ``extra...``.
+      See the documentation of :cpp:struct:`nb::def_visitor\<..\> <def_visitor>`
+      for details.
+
    .. cpp:function:: template <typename C, typename D, typename... Extra> class_ &def_rw(const char * name, D C::* p, const Extra &...extra)
 
       Bind the field `p` and assign it to the class member `name`. nanobind
@@ -2654,6 +2661,41 @@ Class binding
    See the discussion of :ref:`customizing Python object creation <custom_new>`
    for more information.
 
+.. cpp:struct:: template <typename Visitor> def_visitor
+
+   An empty base object which serves as a tag to allow :cpp:func:`class_::def()`
+   to dispatch to custom logic implemented by the type ``Visitor``. This is the
+   same mechanism used by :cpp:class:`init`, :cpp:class:`init_implicit`, and
+   :cpp:class:`new_`; it's exposed publicly so that you can create your own
+   reusable abstractions for binding logic.
+
+   To define a ``def_visitor``, you would write something like:
+
+   .. code-block:: cpp
+
+      struct my_ops : nb::def_visitor<my_ops> {
+          template <typename Class, typename... Extra>
+          void execute(Class &cl, const Extra&... extra) {
+              /* series of def() statements on `cl`, which is a nb::class_ */
+          }
+      };
+
+   Then use it like:
+
+   .. code-block:: cpp
+
+      nb::class_<MyType>(m, "MyType")
+          .def("some_method", &MyType::some_method)
+          .def(my_ops())
+          ... ;
+
+   Any arguments to :cpp:func:`class_::def()` after the ``def_visitor`` object
+   get passed through as the ``Extra...`` parameters to ``execute()``.
+   As with any other C++ object, data needed by the ``def_visitor`` can be passed
+   through template arguments or ordinary constructor arguments.
+   The ``execute()`` method may be static if it doesn't need to access anything
+   in ``*this``.
+
 
 GIL Management
 --------------

docs/changelog.rst

@@ -65,6 +65,14 @@ Version TBD (not yet released)
   not be an instance of the alias/trampoline type.
   (PR `#859 <https://github.com/wjakob/nanobind/pull/859>`__)
 
+- Added :cpp:class:`nb::def_visitor\<..\> <def_visitor>`, which can be used to
+  define your own binding logic that operates on a :cpp:class:`nb::class_\<..\>
+  <class_>` when an instance of the visitor object is passed to
+  :cpp:func:`class_::def()`. This generalizes the mechanism used by
+  :cpp:class:`init`, :cpp:class:`new_`, etc, so that you can create
+  binding abstractions that "feel like" the built-in ones.
+  (PR `#884 <https://github.com/wjakob/nanobind/pull/884>`__)
+
 Version 2.4.0 (Dec 6, 2024)
 ---------------------------
 

include/nanobind/nb_class.h

@@ -329,8 +329,18 @@ inline void *type_get_slot(handle h, int slot_id) {
 #endif
 }
 
+template <typename Visitor> struct def_visitor {
+  protected:
+    // Ensure def_visitor<T> can only be derived from, not constructed
+    // directly
+    def_visitor() {
+        static_assert(std::is_base_of_v<def_visitor, Visitor>,
+                      "def_visitor uses CRTP: def_visitor<T> should be "
+                      "a base of T");
+    }
+};
 
-template <typename... Args> struct init {
+template <typename... Args> struct init : def_visitor<init<Args...>> {
     template <typename T, typename... Ts> friend class class_;
     NB_INLINE init() {}
 
@@ -355,7 +365,7 @@ template <typename... Args> struct init {
     }
 };
 
-template <typename Arg> struct init_implicit {
+template <typename Arg> struct init_implicit : def_visitor<init_implicit<Arg>> {
     template <typename T, typename... Ts> friend class class_;
     NB_INLINE init_implicit() { }
 
@@ -455,7 +465,7 @@ template <typename Func, typename Sig = detail::function_signature_t<Func>>
 struct new_;
 
 template <typename Func, typename Return, typename... Args>
-struct new_<Func, Return(Args...)> {
+struct new_<Func, Return(Args...)> : def_visitor<new_<Func, Return(Args...)>> {
     std::remove_reference_t<Func> func;
 
     new_(Func &&f) : func((detail::forward_t<Func>) f) {}
@@ -614,21 +624,9 @@ class class_ : public object {
         return *this;
     }
 
-    template <typename... Args, typename... Extra>
-    NB_INLINE class_ &def(init<Args...> &&arg, const Extra &... extra) {
-        arg.execute(*this, extra...);
-        return *this;
-    }
-
-    template <typename Arg, typename... Extra>
-    NB_INLINE class_ &def(init_implicit<Arg> &&arg, const Extra &... extra) {
-        arg.execute(*this, extra...);
-        return *this;
-    }
-
-    template <typename Func, typename... Extra>
-    NB_INLINE class_ &def(new_<Func> &&arg, const Extra &... extra) {
-        arg.execute(*this, extra...);
+    template <typename Visitor, typename... Extra>
+    NB_INLINE class_ &def(def_visitor<Visitor> &&arg, const Extra &... extra) {
+        static_cast<Visitor&&>(arg).execute(*this, extra...);
         return *this;
     }