Allow arbitrary class_ template option ordering

docs/advanced.rst

@@ -311,7 +311,7 @@ The binding code also needs a few minor adaptations (highlighted):
     PYBIND11_PLUGIN(example) {
         py::module m("example", "pybind11 example plugin");
 
-        py::class_<Animal, std::unique_ptr<Animal>, PyAnimal /* <--- trampoline*/> animal(m, "Animal");
+        py::class_<Animal, PyAnimal /* <--- trampoline*/> animal(m, "Animal");
         animal
             .def(py::init<>())
             .def("go", &Animal::go);
@@ -325,9 +325,10 @@ The binding code also needs a few minor adaptations (highlighted):
     }
 
 Importantly, pybind11 is made aware of the trampoline trampoline helper class
-by specifying it as the *third* template argument to :class:`class_`. The
-second argument with the unique pointer is simply the default holder type used
-by pybind11. Following this, we are able to define a constructor as usual.
+by specifying it as an extra template argument to :class:`class_`.  (This can
+also be combined with other template arguments such as a custom holder type;
+the order of template types does not matter).  Following this, we are able to
+define a constructor as usual.
 
 Note, however, that the above is sufficient for allowing python classes to
 extend ``Animal``, but not ``Dog``: see ref:`virtual_and_inheritance` for the
@@ -453,9 +454,9 @@ The classes are then registered with pybind11 using:
 
 .. code-block:: cpp
 
-    py::class_<Animal, std::unique_ptr<Animal>, PyAnimal<>> animal(m, "Animal");
-    py::class_<Dog, std::unique_ptr<Dog>, PyDog<>> dog(m, "Dog");
-    py::class_<Husky, std::unique_ptr<Husky>, PyDog<Husky>> husky(m, "Husky");
+    py::class_<Animal, PyAnimal<>> animal(m, "Animal");
+    py::class_<Dog, PyDog<>> dog(m, "Dog");
+    py::class_<Husky, PyDog<Husky>> husky(m, "Husky");
     // ... add animal, dog, husky definitions
 
 Note that ``Husky`` did not require a dedicated trampoline template class at
@@ -525,7 +526,7 @@ be realized as follows (important changes highlighted):
     PYBIND11_PLUGIN(example) {
         py::module m("example", "pybind11 example plugin");
 
-        py::class_<Animal, std::unique_ptr<Animal>, PyAnimal> animal(m, "Animal");
+        py::class_<Animal, PyAnimal> animal(m, "Animal");
         animal
             .def(py::init<>())
             .def("go", &Animal::go);
@@ -939,11 +940,11 @@ This section explains how to pass values that are wrapped in "smart" pointer
 types with internal reference counting. For the simpler C++11 unique pointers,
 refer to the previous section.
 
-The binding generator for classes, :class:`class_`, takes an optional second
-template type, which denotes a special *holder* type that is used to manage
-references to the object. When wrapping a type named ``Type``, the default
-value of this template parameter is ``std::unique_ptr<Type>``, which means that
-the object is deallocated when Python's reference count goes to zero.
+The binding generator for classes, :class:`class_`, can be passed a template
+type that denotes a special *holder* type that is used to manage references to
+the object.  If no such holder type template argument is given, the default for
+a type named ``Type`` is ``std::unique_ptr<Type>``, which means that the object
+is deallocated when Python's reference count goes to zero.
 
 It is possible to switch to other types of reference counting wrappers or smart
 pointers, which is useful in codebases that rely on them. For instance, the
@@ -977,6 +978,8 @@ code?
 
 .. code-block:: cpp
 
+    PYBIND11_DECLARE_HOLDER_TYPE(T, std::shared_ptr<T>);
+
     class Child { };
 
     class Parent {
@@ -1089,7 +1092,7 @@ pybind11. The underlying issue is that the ``std::unique_ptr`` holder type that
 is responsible for managing the lifetime of instances will reference the
 destructor even if no deallocations ever take place. In order to expose classes
 with private or protected destructors, it is possible to override the holder
-type via the second argument to ``class_``. Pybind11 provides a helper class
+type via a holder type argument to ``class_``. Pybind11 provides a helper class
 ``py::nodelete`` that disables any destructor invocations. In this case, it is
 crucial that instances are deallocated on the C++ side to avoid memory leaks.
 
@@ -1224,7 +1227,7 @@ section.
     the other existing exception translators.
 
     The ``py::exception`` wrapper for creating custom exceptions cannot (yet)
-    be used as a ``py::base``.
+    be used as a base type.
 
 .. _eigen:
 
@@ -1808,16 +1811,17 @@ However, it can be acquired as follows:
         .def(py::init<const std::string &>())
         .def("bark", &Dog::bark);
 
-Alternatively, we can rely on the ``base`` tag, which performs an automated
-lookup of the corresponding Python type. However, this also requires invoking
-the ``import`` function once to ensure that the pybind11 binding code of the
-module ``basic`` has been executed.
+Alternatively, you can specify the base class as a template parameter option to
+``class_``, which performs an automated lookup of the corresponding Python
+type. Like the above code, however, this also requires invoking the ``import``
+function once to ensure that the pybind11 binding code of the module ``basic``
+has been executed:
 
 .. code-block:: cpp
 
     py::module::import("basic");
 
-    py::class_<Dog>(m, "Dog", py::base<Pet>())
+    py::class_<Dog, Pet>(m, "Dog")
         .def(py::init<const std::string &>())
         .def("bark", &Dog::bark);
 

docs/classes.rst

@@ -185,16 +185,23 @@ inheritance relationship:
         std::string bark() const { return "woof!"; }
     };
 
-There are two different ways of indicating a hierarchical relationship to
-pybind11: the first is by specifying the C++ base class explicitly during
-construction using the ``base`` attribute:
+There are three different ways of indicating a hierarchical relationship to
+pybind11: the first specifies the C++ base class as an extra template
+parameter of the :class:`class_`; the second uses a special ``base`` attribute
+passed into the constructor:
 
 .. code-block:: cpp
 
     py::class_<Pet>(m, "Pet")
        .def(py::init<const std::string &>())
        .def_readwrite("name", &Pet::name);
 
+    // Method 1: template parameter:
+    py::class_<Dog, Pet /* <- specify C++ parent type */>(m, "Dog")
+        .def(py::init<const std::string &>())
+        .def("bark", &Dog::bark);
+
+    // Method 2: py::base attribute:
     py::class_<Dog>(m, "Dog", py::base<Pet>() /* <- specify C++ parent type */)
         .def(py::init<const std::string &>())
         .def("bark", &Dog::bark);
@@ -208,11 +215,12 @@ Alternatively, we can also assign a name to the previously bound ``Pet``
     pet.def(py::init<const std::string &>())
        .def_readwrite("name", &Pet::name);
 
+    // Method 3: pass parent class_ object:
     py::class_<Dog>(m, "Dog", pet /* <- specify Python parent type */)
         .def(py::init<const std::string &>())
         .def("bark", &Dog::bark);
 
-Functionality-wise, both approaches are completely equivalent. Afterwards,
+Functionality-wise, all three approaches are completely equivalent. Afterwards,
 instances will expose fields and methods of both types:
 
 .. code-block:: pycon

include/pybind11/cast.h

@@ -798,6 +798,13 @@ template <typename type, typename holder_type> class type_caster_holder : public
     holder_type holder;
 };
 
+// PYBIND11_DECLARE_HOLDER_TYPE holder types:
+template <typename base, typename holder> struct is_holder_type :
+    std::is_base_of<detail::type_caster_holder<base, holder>, detail::type_caster<holder>> {};
+// Specialization for always-supported unique_ptr holders:
+template <typename base, typename deleter> struct is_holder_type<base, std::unique_ptr<base, deleter>> :
+    std::true_type {};
+
 template <typename T> struct handle_type_name { static PYBIND11_DESCR name() { return _<T>(); } };
 template <> struct handle_type_name<bytes> { static PYBIND11_DESCR name() { return _(PYBIND11_BYTES_NAME); } };
 template <> struct handle_type_name<args> { static PYBIND11_DESCR name() { return _("*args"); } };

include/pybind11/common.h

@@ -358,6 +358,28 @@ template <template<typename> class P, typename T, typename... Ts>
 struct any_of_t<P, T, Ts...> : conditional_t<P<T>::value, std::true_type, any_of_t<P, Ts...>> { };
 #endif
 
+// Extracts the first type from the template parameter pack matching the predicate, or Default if none match.
+template <template<class> class Predicate, class Default, class... Ts> struct first_of;
+template <template<class> class Predicate, class Default> struct first_of<Predicate, Default> {
+    using type = Default;
+};
+template <template<class> class Predicate, class Default, class T, class... Ts>
+struct first_of<Predicate, Default, T, Ts...> {
+    using type = typename std::conditional<
+        Predicate<T>::value,
+        T,
+        typename first_of<Predicate, Default, Ts...>::type
+    >::type;
+};
+template <template<class> class Predicate, class Default, class... T> using first_of_t = typename first_of<Predicate, Default, T...>::type;
+
+// Counts the number of types in the template parameter pack matching the predicate
+template <template<typename> class Predicate, typename... Ts> struct count_t;
+template <template<typename> class Predicate> struct count_t<Predicate> : std::integral_constant<size_t, 0> {};
+template <template<typename> class Predicate, class T, class... Ts>
+struct count_t<Predicate, T, Ts...> : std::integral_constant<size_t,
+    Predicate<T>::value + count_t<Predicate, Ts...>::value> {};
+
 /// Defer the evaluation of type T until types Us are instantiated
 template <typename T, typename... /*Us*/> struct deferred_type { using type = T; };
 template <typename T, typename... Us> using deferred_t = typename deferred_type<T, Us...>::type;

include/pybind11/operators.h

@@ -49,17 +49,19 @@ template <op_id, op_type, typename B, typename L, typename R> struct op_impl { }
 
 /// Operator implementation generator
 template <op_id id, op_type ot, typename L, typename R> struct op_ {
-    template <typename Base, typename Holder, typename... Extra> void execute(pybind11::class_<Base, Holder> &class_, const Extra&... extra) const {
+    template <typename Class, typename... Extra> void execute(Class &cl, const Extra&... extra) const {
+        typedef typename Class::type Base;
         typedef typename std::conditional<std::is_same<L, self_t>::value, Base, L>::type L_type;
         typedef typename std::conditional<std::is_same<R, self_t>::value, Base, R>::type R_type;
         typedef op_impl<id, ot, Base, L_type, R_type> op;
-        class_.def(op::name(), &op::execute, extra...);
+        cl.def(op::name(), &op::execute, extra...);
     }
-    template <typename Base, typename Holder, typename... Extra> void execute_cast(pybind11::class_<Base, Holder> &class_, const Extra&... extra) const {
+    template <typename Class, typename... Extra> void execute_cast(Class &cl, const Extra&... extra) const {
+        typedef typename Class::type Base;
         typedef typename std::conditional<std::is_same<L, self_t>::value, Base, L>::type L_type;
         typedef typename std::conditional<std::is_same<R, self_t>::value, Base, R>::type R_type;
         typedef op_impl<id, ot, Base, L_type, R_type> op;
-        class_.def(op::name(), &op::execute_cast, extra...);
+        cl.def(op::name(), &op::execute_cast, extra...);
     }
 };
 

include/pybind11/pybind11.h

@@ -563,7 +563,7 @@ class module : public object {
 NAMESPACE_BEGIN(detail)
 /// Generic support for creating new Python heap types
 class generic_type : public object {
-    template <typename type, typename holder_type, typename type_alias> friend class class_;
+    template <typename...> friend class class_;
 public:
     PYBIND11_OBJECT_DEFAULT(generic_type, object, PyType_Check)
 protected:
@@ -802,12 +802,46 @@ class generic_type : public object {
 
     static void releasebuffer(PyObject *, Py_buffer *view) { delete (buffer_info *) view->internal; }
 };
+
+template <template<typename> class Predicate, typename... BaseTypes> struct class_selector;
+template <template<typename> class Predicate, typename Base, typename... Bases>
+struct class_selector<Predicate, Base, Bases...> {
+    static inline void set_bases(detail::type_record &record) {
+        if (Predicate<Base>::value) record.base_type = &typeid(Base);
+        else class_selector<Predicate, Bases...>::set_bases(record);
+    }
+};
+template <template<typename> class Predicate>
+struct class_selector<Predicate> {
+    static inline void set_bases(detail::type_record &) {}
+};
+
 NAMESPACE_END(detail)
 
-template <typename type, typename holder_type = std::unique_ptr<type>, typename type_alias = type>
+template <typename type_, typename... options>
 class class_ : public detail::generic_type {
+    template <typename T> using is_holder = detail::is_holder_type<type_, T>;
+    template <typename T> using is_subtype = detail::bool_constant<std::is_base_of<type_, T>::value && !std::is_same<T, type_>::value>;
+    template <typename T> using is_base_class = detail::bool_constant<std::is_base_of<T, type_>::value && !std::is_same<T, type_>::value>;
+    template <typename T> using is_valid_class_option =
+        detail::bool_constant<
+            is_holder<T>::value ||
+            is_subtype<T>::value ||
+            is_base_class<T>::value
+        >;
+
 public:
-    typedef detail::instance<type, holder_type> instance_type;
+    using type = type_;
+    using type_alias = detail::first_of_t<is_subtype, void, options...>;
+    constexpr static bool has_alias = !std::is_void<type_alias>::value;
+    using holder_type = detail::first_of_t<is_holder, std::unique_ptr<type>, options...>;
+    using instance_type = detail::instance<type, holder_type>;
+
+    static_assert(detail::all_of_t<is_valid_class_option, options...>::value,
+            "Unknown/invalid class_ template parameters provided");
+
+    static_assert(detail::count_t<is_base_class, options...>::value <= 1,
+            "Invalid class_ base types: multiple inheritance is not supported");
 
     PYBIND11_OBJECT(class_, detail::generic_type, PyType_Check)
 
@@ -822,12 +856,14 @@ class class_ : public detail::generic_type {
         record.init_holder = init_holder;
         record.dealloc = dealloc;
 
+        detail::class_selector<is_base_class, options...>::set_bases(record);
+
         /* Process optional arguments, if any */
         detail::process_attributes<Extra...>::init(extra..., &record);
 
         detail::generic_type::initialize(&record);
 
-        if (!std::is_same<type, type_alias>::value) {
+        if (has_alias) {
             auto &instances = pybind11::detail::get_internals().registered_types_cpp;
             instances[std::type_index(typeid(type_alias))] = instances[std::type_index(typeid(type))];
         }
@@ -852,25 +888,25 @@ class class_ : public detail::generic_type {
 
     template <detail::op_id id, detail::op_type ot, typename L, typename R, typename... Extra>
     class_ &def(const detail::op_<id, ot, L, R> &op, const Extra&... extra) {
-        op.template execute<type>(*this, extra...);
+        op.execute(*this, extra...);
         return *this;
     }
 
     template <detail::op_id id, detail::op_type ot, typename L, typename R, typename... Extra>
     class_ & def_cast(const detail::op_<id, ot, L, R> &op, const Extra&... extra) {
-        op.template execute_cast<type>(*this, extra...);
+        op.execute_cast(*this, extra...);
         return *this;
     }
 
     template <typename... Args, typename... Extra>
     class_ &def(const detail::init<Args...> &init, const Extra&... extra) {
-        init.template execute<type>(*this, extra...);
+        init.execute(*this, extra...);
         return *this;
     }
 
     template <typename... Args, typename... Extra>
     class_ &def(const detail::init_alias<Args...> &init, const Extra&... extra) {
-        init.template execute<type>(*this, extra...);
+        init.execute(*this, extra...);
         return *this;
     }
 
@@ -1071,31 +1107,34 @@ template <typename Type> class enum_ : public class_<Type> {
 
 NAMESPACE_BEGIN(detail)
 template <typename... Args> struct init {
-    template <typename Base, typename Holder, typename Alias, typename... Extra,
-              typename std::enable_if<std::is_same<Base, Alias>::value, int>::type = 0>
-    void execute(pybind11::class_<Base, Holder, Alias> &class_, const Extra&... extra) const {
+    template <typename Class, typename... Extra, typename std::enable_if<!Class::has_alias, int>::type = 0>
+    void execute(Class &cl, const Extra&... extra) const {
+        using Base = typename Class::type;
         /// Function which calls a specific C++ in-place constructor
-        class_.def("__init__", [](Base *self_, Args... args) { new (self_) Base(args...); }, extra...);
+        cl.def("__init__", [](Base *self_, Args... args) { new (self_) Base(args...); }, extra...);
     }
 
-    template <typename Base, typename Holder, typename Alias, typename... Extra,
-              typename std::enable_if<!std::is_same<Base, Alias>::value &&
-                                       std::is_constructible<Base, Args...>::value, int>::type = 0>
-    void execute(pybind11::class_<Base, Holder, Alias> &class_, const Extra&... extra) const {
-        handle cl_type = class_;
-        class_.def("__init__", [cl_type](handle self_, Args... args) {
+    template <typename Class, typename... Extra,
+              typename std::enable_if<Class::has_alias &&
+                                       std::is_constructible<typename Class::type, Args...>::value, int>::type = 0>
+    void execute(Class &cl, const Extra&... extra) const {
+        using Base = typename Class::type;
+        using Alias = typename Class::type_alias;
+        handle cl_type = cl;
+        cl.def("__init__", [cl_type](handle self_, Args... args) {
                 if (self_.get_type() == cl_type)
                     new (self_.cast<Base *>()) Base(args...);
                 else
                     new (self_.cast<Alias *>()) Alias(args...);
             }, extra...);
     }
 
-    template <typename Base, typename Holder, typename Alias, typename... Extra,
-              typename std::enable_if<!std::is_same<Base, Alias>::value &&
-                                      !std::is_constructible<Base, Args...>::value, int>::type = 0>
-    void execute(pybind11::class_<Base, Holder, Alias> &class_, const Extra&... extra) const {
-        class_.def("__init__", [](Alias *self_, Args... args) { new (self_) Alias(args...); }, extra...);
+    template <typename Class, typename... Extra,
+              typename std::enable_if<Class::has_alias &&
+                                      !std::is_constructible<typename Class::type, Args...>::value, int>::type = 0>
+    void execute(Class &cl, const Extra&... extra) const {
+        using Alias = typename Class::type_alias;
+        cl.def("__init__", [](Alias *self_, Args... args) { new (self_) Alias(args...); }, extra...);
     }
 };
 

tests/CMakeLists.txt

@@ -6,6 +6,7 @@ endif()
 set(PYBIND11_TEST_FILES
   test_buffers.cpp
   test_callbacks.cpp
+  test_class_args.cpp
   test_constants_and_functions.cpp
   test_eigen.cpp
   test_enum.cpp