From 5fffe200e337f1a1bb13034fac29a63ad6eae304 Mon Sep 17 00:00:00 2001
From: Jason Rhinelander <jason@imaginary.ca>
Date: Tue, 6 Sep 2016 12:17:06 -0400
Subject: [PATCH 1/2] Allow arbitrary class_ template option ordering

The current pybind11::class_<Type, Holder, Trampoline> fixed template
ordering results in a requirement to repeat the Holder with its default
value (std::unique_ptr<Type>) argument, which is a little bit annoying:
it needs to be specified not because we want to override the default,
but rather because we need to specify the third argument.

This commit removes this limitation by making the class_ template take
the type name plus a parameter pack of options.  It then extracts the
first valid holder type and the first subclass type for holder_type and
trampoline type_alias, respectively.  (If unfound, both fall back to
their current defaults, `std::unique_ptr<type>` and `type`,
respectively).  If any unmatched template arguments are provided, a
static assertion fails.

What this means is that you can specify or omit the arguments in any
order:

    py::class_<A, PyA> c1(m, "A");
    py::class_<B, PyB, std::shared_ptr<B>> c2(m, "B");
    py::class_<C, std::shared_ptr<C>, PyB> c3(m, "C");

It also allows future class attributes (such as base types in the next
commit) to be passed as class template types rather than needing to use
a py::base<> wrapper.
---
 docs/advanced.rst                | 31 +++++++-------
 include/pybind11/cast.h          |  7 ++++
 include/pybind11/common.h        | 22 ++++++++++
 include/pybind11/operators.h     | 10 +++--
 include/pybind11/pybind11.h      | 70 +++++++++++++++++++++-----------
 tests/CMakeLists.txt             |  1 +
 tests/test_class_args.cpp        | 69 +++++++++++++++++++++++++++++++
 tests/test_class_args.py         |  7 ++++
 tests/test_issues.cpp            |  2 +-
 tests/test_virtual_functions.cpp | 20 ++++-----
 10 files changed, 187 insertions(+), 52 deletions(-)
 create mode 100644 tests/test_class_args.cpp
 create mode 100644 tests/test_class_args.py

diff --git a/docs/advanced.rst b/docs/advanced.rst
index cf588af3e..6a4a02077 100644
--- a/docs/advanced.rst
+++ b/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.
 
diff --git a/include/pybind11/cast.h b/include/pybind11/cast.h
index c8c8f77ba..e5579e1b4 100644
--- a/include/pybind11/cast.h
+++ b/include/pybind11/cast.h
@@ -798,6 +798,13 @@ protected:
     holder_type holder;
 };
 
+template <typename base, typename holder> struct is_holder_type :
+    // PYBIND11_DECLARE_HOLDER_TYPE holder types:
+    std::conditional<std::is_base_of<detail::type_caster_holder<base, holder>, detail::type_caster<holder>>::value,
+    std::true_type,
+    std::false_type>::type {};
+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"); } };
diff --git a/include/pybind11/common.h b/include/pybind11/common.h
index 3d5aeb22a..6aeeb3d7a 100644
--- a/include/pybind11/common.h
+++ b/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 void if none match.
+template <template<class> class Predicate, class... Ts> struct first_of;
+template <template<class> class Predicate> struct first_of<Predicate> {
+    using type = void;
+};
+template <template<class> class Predicate, class T, class... Ts>
+struct first_of<Predicate, T, Ts...> {
+    using type = typename std::conditional<
+        Predicate<T>::value,
+        T,
+        typename first_of<Predicate, Ts...>::type
+    >::type;
+};
+template <template<class> class Predicate, class... T> using first_of_t = typename first_of<Predicate, 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;
diff --git a/include/pybind11/operators.h b/include/pybind11/operators.h
index 39f9786a7..eda51a178 100644
--- a/include/pybind11/operators.h
+++ b/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...);
     }
 };
 
diff --git a/include/pybind11/pybind11.h b/include/pybind11/pybind11.h
index 2104671db..3f04ba54a 100644
--- a/include/pybind11/pybind11.h
+++ b/include/pybind11/pybind11.h
@@ -563,7 +563,7 @@ public:
 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:
@@ -804,10 +804,31 @@ protected:
 };
 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_valid_class_option =
+        detail::bool_constant<
+            is_holder<T>::value ||
+            is_subtype<T>::value
+        >;
+
+    using extracted_holder_t = typename detail::first_of_t<is_holder, options...>;
+
 public:
-    typedef detail::instance<type, holder_type> instance_type;
+    using type = type_;
+    using type_alias = detail::first_of_t<is_subtype, options...>;
+    constexpr static bool has_alias = !std::is_void<type_alias>::value;
+    using holder_type = typename std::conditional<
+        std::is_void<extracted_holder_t>::value,
+        std::unique_ptr<type>,
+        extracted_holder_t
+    >::type;
+    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");
 
     PYBIND11_OBJECT(class_, detail::generic_type, PyType_Check)
 
@@ -827,7 +848,7 @@ public:
 
         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 +873,25 @@ public:
 
     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;
     }
 
@@ -1071,19 +1092,21 @@ private:
 
 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
@@ -1091,11 +1114,12 @@ template <typename... Args> struct init {
             }, 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...);
     }
 };
 
diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
index 0efefaae3..6d72ad7ed 100644
--- a/tests/CMakeLists.txt
+++ b/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
diff --git a/tests/test_class_args.cpp b/tests/test_class_args.cpp
new file mode 100644
index 000000000..c1207016c
--- /dev/null
+++ b/tests/test_class_args.cpp
@@ -0,0 +1,69 @@
+/*
+    tests/test_class_args.cpp -- tests that various way of defining a class work
+
+    Copyright (c) 2016 Wenzel Jakob <wenzel.jakob@epfl.ch>
+
+    All rights reserved. Use of this source code is governed by a
+    BSD-style license that can be found in the LICENSE file.
+*/
+
+#include "pybind11_tests.h"
+
+PYBIND11_DECLARE_HOLDER_TYPE(T, std::shared_ptr<T>);
+
+template <int N> class BreaksBase {};
+template <int N> class BreaksTramp : public BreaksBase<N> {};
+// These should all compile just fine:
+typedef py::class_<BreaksBase<1>, std::unique_ptr<BreaksBase<1>>, BreaksTramp<1>> DoesntBreak1;
+typedef py::class_<BreaksBase<2>, BreaksTramp<2>, std::unique_ptr<BreaksBase<2>>> DoesntBreak2;
+typedef py::class_<BreaksBase<3>, std::unique_ptr<BreaksBase<3>>> DoesntBreak3;
+typedef py::class_<BreaksBase<4>, BreaksTramp<4>> DoesntBreak4;
+typedef py::class_<BreaksBase<5>> DoesntBreak5;
+typedef py::class_<BreaksBase<6>, std::shared_ptr<BreaksBase<6>>, BreaksTramp<6>> DoesntBreak6;
+typedef py::class_<BreaksBase<7>, BreaksTramp<7>, std::shared_ptr<BreaksBase<7>>> DoesntBreak7;
+typedef py::class_<BreaksBase<8>, std::shared_ptr<BreaksBase<8>>> DoesntBreak8;
+#define CHECK_BASE(N) static_assert(std::is_same<typename DoesntBreak##N::type, BreaksBase<N>>::value, \
+        "DoesntBreak" #N " has wrong type!")
+CHECK_BASE(1); CHECK_BASE(2); CHECK_BASE(3); CHECK_BASE(4); CHECK_BASE(5); CHECK_BASE(6); CHECK_BASE(7); CHECK_BASE(8);
+#define CHECK_ALIAS(N) static_assert(DoesntBreak##N::has_alias && std::is_same<typename DoesntBreak##N::type_alias, BreaksTramp<N>>::value, \
+        "DoesntBreak" #N " has wrong type_alias!")
+#define CHECK_NOALIAS(N) static_assert(!DoesntBreak##N::has_alias && std::is_void<typename DoesntBreak##N::type_alias>::value, \
+        "DoesntBreak" #N " has type alias, but shouldn't!")
+CHECK_ALIAS(1); CHECK_ALIAS(2); CHECK_NOALIAS(3); CHECK_ALIAS(4); CHECK_NOALIAS(5); CHECK_ALIAS(6); CHECK_ALIAS(7); CHECK_NOALIAS(8);
+#define CHECK_HOLDER(N, TYPE) static_assert(std::is_same<typename DoesntBreak##N::holder_type, std::TYPE##_ptr<BreaksBase<N>>>::value, \
+        "DoesntBreak" #N " has wrong holder_type!")
+CHECK_HOLDER(1, unique); CHECK_HOLDER(2, unique); CHECK_HOLDER(3, unique); CHECK_HOLDER(4, unique); CHECK_HOLDER(5, unique);
+CHECK_HOLDER(6, shared); CHECK_HOLDER(7, shared); CHECK_HOLDER(8, shared);
+
+// There's no nice way to test that these fail because they fail to compile; leave them here,
+// though, so that they can be manually tested by uncommenting them (and seeing that compilation
+// failures occurs).
+
+// We have to actually look into the type: the typedef alone isn't enough to instantiate the type:
+#define CHECK_BROKEN(N) static_assert(std::is_same<typename Breaks##N::type, BreaksBase<-N>>::value, \
+        "Breaks1 has wrong type!");
+
+//// Two holder classes:
+//typedef py::class_<BreaksBase<-1>, std::unique_ptr<BreaksBase<-1>>, std::unique_ptr<BreaksBase<-1>>> Breaks1;
+//CHECK_BROKEN(1);
+//// Two aliases:
+//typedef py::class_<BreaksBase<-2>, BreaksTramp<-2>, BreaksTramp<-2>> Breaks2;
+//CHECK_BROKEN(2);
+//// Holder + 2 aliases
+//typedef py::class_<BreaksBase<-3>, std::unique_ptr<BreaksBase<-3>>, BreaksTramp<-3>, BreaksTramp<-3>> Breaks3;
+//CHECK_BROKEN(3);
+//// Alias + 2 holders
+//typedef py::class_<BreaksBase<-4>, std::unique_ptr<BreaksBase<-4>>, BreaksTramp<-4>, std::shared_ptr<BreaksBase<-4>>> Breaks4;
+//CHECK_BROKEN(4);
+//// Invalid option (not a subclass or holder)
+//typedef py::class_<BreaksBase<-5>, BreaksTramp<-4>> Breaks5;
+//CHECK_BROKEN(5);
+//// Invalid option: multiple inheritance not supported:
+//template <> struct BreaksBase<-8> : BreaksBase<-6>, BreaksBase<-7> {};
+//typedef py::class_<BreaksBase<-8>, BreaksBase<-6>, BreaksBase<-7>> Breaks8;
+//CHECK_BROKEN(8);
+
+test_initializer class_args([](py::module &m) {
+    // Just test that this compiled okay
+    m.def("class_args_noop", []() {});
+});
diff --git a/tests/test_class_args.py b/tests/test_class_args.py
new file mode 100644
index 000000000..a1fc21a7d
--- /dev/null
+++ b/tests/test_class_args.py
@@ -0,0 +1,7 @@
+
+import pytest
+
+def test_class_args():
+    # There's basically nothing to test here; just make sure the code compiled and declared its definition
+    from pybind11_tests import class_args_noop
+    class_args_noop()
diff --git a/tests/test_issues.cpp b/tests/test_issues.cpp
index 16a44c74f..0a5a0b09a 100644
--- a/tests/test_issues.cpp
+++ b/tests/test_issues.cpp
@@ -51,7 +51,7 @@ void init_issues(py::module &m) {
         }
     };
 
-    py::class_<Base, std::unique_ptr<Base>, DispatchIssue>(m2, "DispatchIssue")
+    py::class_<Base, DispatchIssue>(m2, "DispatchIssue")
         .def(py::init<>())
         .def("dispatch", &Base::dispatch);
 
diff --git a/tests/test_virtual_functions.cpp b/tests/test_virtual_functions.cpp
index e591ba922..ac5b3fbbd 100644
--- a/tests/test_virtual_functions.cpp
+++ b/tests/test_virtual_functions.cpp
@@ -253,31 +253,31 @@ public:
 
 void initialize_inherited_virtuals(py::module &m) {
     // Method 1: repeat
-    py::class_<A_Repeat, std::unique_ptr<A_Repeat>, PyA_Repeat>(m, "A_Repeat")
+    py::class_<A_Repeat, PyA_Repeat>(m, "A_Repeat")
         .def(py::init<>())
         .def("unlucky_number", &A_Repeat::unlucky_number)
         .def("say_something", &A_Repeat::say_something)
         .def("say_everything", &A_Repeat::say_everything);
-    py::class_<B_Repeat, std::unique_ptr<B_Repeat>, PyB_Repeat>(m, "B_Repeat", py::base<A_Repeat>())
+    py::class_<B_Repeat, PyB_Repeat>(m, "B_Repeat", py::base<A_Repeat>())
         .def(py::init<>())
         .def("lucky_number", &B_Repeat::lucky_number);
-    py::class_<C_Repeat, std::unique_ptr<C_Repeat>, PyC_Repeat>(m, "C_Repeat", py::base<B_Repeat>())
+    py::class_<C_Repeat, PyC_Repeat>(m, "C_Repeat", py::base<B_Repeat>())
         .def(py::init<>());
-    py::class_<D_Repeat, std::unique_ptr<D_Repeat>, PyD_Repeat>(m, "D_Repeat", py::base<C_Repeat>())
+    py::class_<D_Repeat, PyD_Repeat>(m, "D_Repeat", py::base<C_Repeat>())
         .def(py::init<>());
 
     // Method 2: Templated trampolines
-    py::class_<A_Tpl, std::unique_ptr<A_Tpl>, PyA_Tpl<>>(m, "A_Tpl")
+    py::class_<A_Tpl, PyA_Tpl<>>(m, "A_Tpl")
         .def(py::init<>())
         .def("unlucky_number", &A_Tpl::unlucky_number)
         .def("say_something", &A_Tpl::say_something)
         .def("say_everything", &A_Tpl::say_everything);
-    py::class_<B_Tpl, std::unique_ptr<B_Tpl>, PyB_Tpl<>>(m, "B_Tpl", py::base<A_Tpl>())
+    py::class_<B_Tpl, PyB_Tpl<>>(m, "B_Tpl", py::base<A_Tpl>())
         .def(py::init<>())
         .def("lucky_number", &B_Tpl::lucky_number);
-    py::class_<C_Tpl, std::unique_ptr<C_Tpl>, PyB_Tpl<C_Tpl>>(m, "C_Tpl", py::base<B_Tpl>())
+    py::class_<C_Tpl, PyB_Tpl<C_Tpl>>(m, "C_Tpl", py::base<B_Tpl>())
         .def(py::init<>());
-    py::class_<D_Tpl, std::unique_ptr<D_Tpl>, PyB_Tpl<D_Tpl>>(m, "D_Tpl", py::base<C_Tpl>())
+    py::class_<D_Tpl, PyB_Tpl<D_Tpl>>(m, "D_Tpl", py::base<C_Tpl>())
         .def(py::init<>());
 
 };
@@ -287,7 +287,7 @@ test_initializer virtual_functions([](py::module &m) {
     /* Important: indicate the trampoline class PyExampleVirt using the third
        argument to py::class_. The second argument with the unique pointer
        is simply the default holder type used by pybind11. */
-    py::class_<ExampleVirt, std::unique_ptr<ExampleVirt>, PyExampleVirt>(m, "ExampleVirt")
+    py::class_<ExampleVirt, PyExampleVirt>(m, "ExampleVirt")
         .def(py::init<int>())
         /* Reference original class in function definitions */
         .def("run", &ExampleVirt::run)
@@ -301,7 +301,7 @@ test_initializer virtual_functions([](py::module &m) {
         .def(py::init<int, int>());
 
 #if !defined(__INTEL_COMPILER)
-    py::class_<NCVirt, std::unique_ptr<NCVirt>, NCVirtTrampoline>(m, "NCVirt")
+    py::class_<NCVirt, NCVirtTrampoline>(m, "NCVirt")
         .def(py::init<>())
         .def("get_noncopyable", &NCVirt::get_noncopyable)
         .def("get_movable", &NCVirt::get_movable)

From 6b52c838d78cb697f68761c75b82d2a1806aedde Mon Sep 17 00:00:00 2001
From: Jason Rhinelander <jason@imaginary.ca>
Date: Tue, 6 Sep 2016 12:27:00 -0400
Subject: [PATCH 2/2] Allow passing base types as a template parameter

This allows a slightly cleaner base type specification of:

    py::class_<Type, Base>("Type")

as an alternative to

    py::class_<Type>("Type", py::base<Base>())

As with the other template parameters, the order relative to the holder
or trampoline types doesn't matter.

This also includes a compile-time assertion failure if attempting to
specify more than one base class (but is easily extendible to support
multiple inheritance, someday, by updating the class_selector::set_bases
function to set multiple bases).
---
 docs/advanced.rst                | 13 +++++++------
 docs/classes.rst                 | 16 ++++++++++++----
 include/pybind11/cast.h          | 10 +++++-----
 include/pybind11/common.h        | 16 ++++++++--------
 include/pybind11/pybind11.h      | 33 +++++++++++++++++++++++---------
 tests/test_inheritance.cpp       |  9 +++++++++
 tests/test_inheritance.py        |  5 ++++-
 tests/test_issues.cpp            |  2 +-
 tests/test_virtual_functions.cpp | 12 ++++++------
 9 files changed, 76 insertions(+), 40 deletions(-)

diff --git a/docs/advanced.rst b/docs/advanced.rst
index 6a4a02077..748f91e2e 100644
--- a/docs/advanced.rst
+++ b/docs/advanced.rst
@@ -1227,7 +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:
 
@@ -1811,16 +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);
 
diff --git a/docs/classes.rst b/docs/classes.rst
index 5afb21edc..80f378f68 100644
--- a/docs/classes.rst
+++ b/docs/classes.rst
@@ -185,9 +185,10 @@ 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
 
@@ -195,6 +196,12 @@ construction using the ``base`` attribute:
        .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
diff --git a/include/pybind11/cast.h b/include/pybind11/cast.h
index e5579e1b4..f6d86e3cf 100644
--- a/include/pybind11/cast.h
+++ b/include/pybind11/cast.h
@@ -798,12 +798,12 @@ protected:
     holder_type holder;
 };
 
+// PYBIND11_DECLARE_HOLDER_TYPE holder types:
 template <typename base, typename holder> struct is_holder_type :
-    // PYBIND11_DECLARE_HOLDER_TYPE holder types:
-    std::conditional<std::is_base_of<detail::type_caster_holder<base, holder>, detail::type_caster<holder>>::value,
-    std::true_type,
-    std::false_type>::type {};
-template <typename base, typename deleter> struct is_holder_type<base, std::unique_ptr<base, deleter>> : std::true_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); } };
diff --git a/include/pybind11/common.h b/include/pybind11/common.h
index 6aeeb3d7a..c086e8796 100644
--- a/include/pybind11/common.h
+++ b/include/pybind11/common.h
@@ -358,20 +358,20 @@ 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 void if none match.
-template <template<class> class Predicate, class... Ts> struct first_of;
-template <template<class> class Predicate> struct first_of<Predicate> {
-    using type = void;
+// 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 T, class... Ts>
-struct first_of<Predicate, T, Ts...> {
+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, Ts...>::type
+        typename first_of<Predicate, Default, Ts...>::type
     >::type;
 };
-template <template<class> class Predicate, class... T> using first_of_t = typename first_of<Predicate, T...>::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;
diff --git a/include/pybind11/pybind11.h b/include/pybind11/pybind11.h
index 3f04ba54a..0d16c7a1f 100644
--- a/include/pybind11/pybind11.h
+++ b/include/pybind11/pybind11.h
@@ -802,34 +802,47 @@ protected:
 
     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... 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_subtype<T>::value ||
+            is_base_class<T>::value
         >;
 
-    using extracted_holder_t = typename detail::first_of_t<is_holder, options...>;
-
 public:
     using type = type_;
-    using type_alias = detail::first_of_t<is_subtype, options...>;
+    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 = typename std::conditional<
-        std::is_void<extracted_holder_t>::value,
-        std::unique_ptr<type>,
-        extracted_holder_t
-    >::type;
+    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)
 
     template <typename... Extra>
@@ -843,6 +856,8 @@ public:
         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);
 
diff --git a/tests/test_inheritance.cpp b/tests/test_inheritance.cpp
index e1aad9920..798befffc 100644
--- a/tests/test_inheritance.cpp
+++ b/tests/test_inheritance.cpp
@@ -31,6 +31,11 @@ public:
     Rabbit(const std::string &name) : Pet(name, "parrot") {}
 };
 
+class Hamster : public Pet {
+public:
+    Hamster(const std::string &name) : Pet(name, "rodent") {}
+};
+
 std::string pet_name_species(const Pet &pet) {
     return pet.name() + " is a " + pet.species();
 }
@@ -59,6 +64,10 @@ test_initializer inheritance([](py::module &m) {
     py::class_<Rabbit>(m, "Rabbit", py::base<Pet>())
         .def(py::init<std::string>());
 
+    /* And another: list parent in class template arguments */
+    py::class_<Hamster, Pet>(m, "Hamster")
+        .def(py::init<std::string>());
+
     m.def("pet_name_species", pet_name_species);
     m.def("dog_bark", dog_bark);
 
diff --git a/tests/test_inheritance.py b/tests/test_inheritance.py
index b55490cf6..d4cea8253 100644
--- a/tests/test_inheritance.py
+++ b/tests/test_inheritance.py
@@ -2,7 +2,7 @@ import pytest
 
 
 def test_inheritance(msg):
-    from pybind11_tests import Pet, Dog, Rabbit, dog_bark, pet_name_species
+    from pybind11_tests import Pet, Dog, Rabbit, Hamster, dog_bark, pet_name_species
 
     roger = Rabbit('Rabbit')
     assert roger.name() + " is a " + roger.species() == "Rabbit is a parrot"
@@ -16,6 +16,9 @@ def test_inheritance(msg):
     assert molly.name() + " is a " + molly.species() == "Molly is a dog"
     assert pet_name_species(molly) == "Molly is a dog"
 
+    fred = Hamster('Fred')
+    assert fred.name() + " is a " + fred.species() == "Fred is a rodent"
+
     assert dog_bark(molly) == "Woof!"
 
     with pytest.raises(TypeError) as excinfo:
diff --git a/tests/test_issues.cpp b/tests/test_issues.cpp
index 0a5a0b09a..55502fe18 100644
--- a/tests/test_issues.cpp
+++ b/tests/test_issues.cpp
@@ -96,7 +96,7 @@ void init_issues(py::module &m) {
 
     py::class_<ElementBase, std::shared_ptr<ElementBase>> (m2, "ElementBase");
 
-    py::class_<ElementA, std::shared_ptr<ElementA>>(m2, "ElementA", py::base<ElementBase>())
+    py::class_<ElementA, ElementBase, std::shared_ptr<ElementA>>(m2, "ElementA")
         .def(py::init<int>())
         .def("value", &ElementA::value);
 
diff --git a/tests/test_virtual_functions.cpp b/tests/test_virtual_functions.cpp
index ac5b3fbbd..381b87e0e 100644
--- a/tests/test_virtual_functions.cpp
+++ b/tests/test_virtual_functions.cpp
@@ -258,12 +258,12 @@ void initialize_inherited_virtuals(py::module &m) {
         .def("unlucky_number", &A_Repeat::unlucky_number)
         .def("say_something", &A_Repeat::say_something)
         .def("say_everything", &A_Repeat::say_everything);
-    py::class_<B_Repeat, PyB_Repeat>(m, "B_Repeat", py::base<A_Repeat>())
+    py::class_<B_Repeat, A_Repeat, PyB_Repeat>(m, "B_Repeat")
         .def(py::init<>())
         .def("lucky_number", &B_Repeat::lucky_number);
-    py::class_<C_Repeat, PyC_Repeat>(m, "C_Repeat", py::base<B_Repeat>())
+    py::class_<C_Repeat, B_Repeat, PyC_Repeat>(m, "C_Repeat")
         .def(py::init<>());
-    py::class_<D_Repeat, PyD_Repeat>(m, "D_Repeat", py::base<C_Repeat>())
+    py::class_<D_Repeat, C_Repeat, PyD_Repeat>(m, "D_Repeat")
         .def(py::init<>());
 
     // Method 2: Templated trampolines
@@ -272,12 +272,12 @@ void initialize_inherited_virtuals(py::module &m) {
         .def("unlucky_number", &A_Tpl::unlucky_number)
         .def("say_something", &A_Tpl::say_something)
         .def("say_everything", &A_Tpl::say_everything);
-    py::class_<B_Tpl, PyB_Tpl<>>(m, "B_Tpl", py::base<A_Tpl>())
+    py::class_<B_Tpl, A_Tpl, PyB_Tpl<>>(m, "B_Tpl")
         .def(py::init<>())
         .def("lucky_number", &B_Tpl::lucky_number);
-    py::class_<C_Tpl, PyB_Tpl<C_Tpl>>(m, "C_Tpl", py::base<B_Tpl>())
+    py::class_<C_Tpl, B_Tpl, PyB_Tpl<C_Tpl>>(m, "C_Tpl")
         .def(py::init<>());
-    py::class_<D_Tpl, PyB_Tpl<D_Tpl>>(m, "D_Tpl", py::base<C_Tpl>())
+    py::class_<D_Tpl, C_Tpl, PyB_Tpl<D_Tpl>>(m, "D_Tpl")
         .def(py::init<>());
 
 };