Merge pull request #403 from jagerman/alias-initialization

Implement py::init_alias<>() constructors

docs/advanced.rst

@@ -480,6 +480,36 @@ can now create a python class that inherits from ``Dog``:
     See the file :file:`tests/test_virtual_functions.cpp` for complete examples
     using both the duplication and templated trampoline approaches.
 
+Extended trampoline class functionality
+=======================================
+
+The trampoline classes described in the previous sections are, by default, only
+initialized when needed.  More specifically, they are initialized when a python
+class actually inherits from a registered type (instead of merely creating an
+instance of the registered type), or when a registered constructor is only
+valid for the trampoline class but not the registered class.  This is primarily
+for performance reasons: when the trampoline class is not needed for anything
+except virtual method dispatching, not initializing the trampoline class
+improves performance by avoiding needing to do a run-time check to see if the
+inheriting python instance has an overloaded method.
+
+Sometimes, however, it is useful to always initialize a trampoline class as an
+intermediate class that does more than just handle virtual method dispatching.
+For example, such a class might perform extra class initialization, extra
+destruction operations, and might define new members and methods to enable a
+more python-like interface to a class.
+
+In order to tell pybind11 that it should *always* initialize the trampoline
+class when creating new instances of a type, the class constructors should be
+declared using ``py::init_alias<Args, ...>()`` instead of the usual
+``py::init<Args, ...>()``.  This forces construction via the trampoline class,
+ensuring member initialization and (eventual) destruction.
+
+.. seealso::
+
+    See the file :file:`tests/test_alias_initialization.cpp` for complete examples
+    showing both normal and forced trampoline instantiation.
+
 .. _macro_notes:
 
 General notes regarding convenience macros

include/pybind11/pybind11.h

@@ -1112,17 +1112,17 @@ private:
 
 NAMESPACE_BEGIN(detail)
 template <typename... Args> struct init {
-    template <typename Class, typename... Extra, typename std::enable_if<!Class::has_alias, int>::type = 0>
-    void execute(Class &cl, const Extra&... extra) const {
+    template <typename Class, typename... Extra, enable_if_t<!Class::has_alias, int> = 0>
+    static void execute(Class &cl, const Extra&... extra) {
         using Base = typename Class::type;
         /// Function which calls a specific C++ in-place constructor
         cl.def("__init__", [](Base *self_, Args... args) { new (self_) Base(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 {
+              enable_if_t<Class::has_alias &&
+                          std::is_constructible<typename Class::type, Args...>::value, int> = 0>
+    static void execute(Class &cl, const Extra&... extra) {
         using Base = typename Class::type;
         using Alias = typename Class::type_alias;
         handle cl_type = cl;
@@ -1135,14 +1135,22 @@ template <typename... Args> struct init {
     }
 
     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 {
+              enable_if_t<Class::has_alias &&
+                          !std::is_constructible<typename Class::type, Args...>::value, int> = 0>
+    static void execute(Class &cl, const Extra&... extra) {
+        init_alias<Args...>::execute(cl, extra...);
+    }
+};
+template <typename... Args> struct init_alias {
+    template <typename Class, typename... Extra,
+              enable_if_t<Class::has_alias && std::is_constructible<typename Class::type_alias, Args...>::value, int> = 0>
+    static void execute(Class &cl, const Extra&... extra) {
         using Alias = typename Class::type_alias;
         cl.def("__init__", [](Alias *self_, Args... args) { new (self_) Alias(args...); }, extra...);
     }
 };
 
+
 inline void keep_alive_impl(handle nurse, handle patient) {
     /* Clever approach based on weak references taken from Boost.Python */
     if (!nurse || !patient)
@@ -1177,6 +1185,7 @@ struct iterator_state {
 NAMESPACE_END(detail)
 
 template <typename... Args> detail::init<Args...> init() { return detail::init<Args...>(); }
+template <typename... Args> detail::init_alias<Args...> init_alias() { return detail::init_alias<Args...>(); }
 
 template <typename Iterator,
           typename Sentinel,

tests/CMakeLists.txt

@@ -4,6 +4,7 @@ if(NOT CMAKE_BUILD_TYPE AND NOT CMAKE_CONFIGURATION_TYPES)
 endif()
 
 set(PYBIND11_TEST_FILES
+  test_alias_initialization.cpp
   test_buffers.cpp
   test_callbacks.cpp
   test_class_args.cpp

tests/test_alias_initialization.cpp

+/*
+    tests/test_alias_initialization.cpp -- test cases and example of different trampoline
+    initialization modes
+
+    Copyright (c) 2016 Wenzel Jakob <wenzel.jakob@epfl.ch>, Jason Rhinelander <jason@imaginary.ca>
+
+    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"
+
+test_initializer alias_initialization([](py::module &m) {
+    // don't invoke Python dispatch classes by default when instantiating C++ classes that were not
+    // extended on the Python side
+    struct A {
+        virtual ~A() {}
+        virtual void f() { py::print("A.f()"); }
+    };
+
+    struct PyA : A {
+        PyA() { py::print("PyA.PyA()"); }
+        ~PyA() { py::print("PyA.~PyA()"); }
+
+        void f() override {
+            py::print("PyA.f()");
+            PYBIND11_OVERLOAD(void, A, f);
+        }
+    };
+
+    auto call_f = [](A *a) { a->f(); };
+
+    py::class_<A, PyA>(m, "A")
+        .def(py::init<>())
+        .def("f", &A::f);
+
+    m.def("call_f", call_f);
+
+
+    // ... unless we explicitly request it, as in this example:
+    struct A2 {
+        virtual ~A2() {}
+        virtual void f() { py::print("A2.f()"); }
+    };
+
+    struct PyA2 : A2 {
+        PyA2() { py::print("PyA2.PyA2()"); }
+        ~PyA2() { py::print("PyA2.~PyA2()"); }
+        void f() override {
+            py::print("PyA2.f()");
+            PYBIND11_OVERLOAD(void, A2, f);
+        }
+    };
+
+    py::class_<A2, PyA2>(m, "A2")
+        .def(py::init_alias<>())
+        .def("f", &A2::f);
+
+    m.def("call_f", [](A2 *a2) { a2->f(); });
+
+});
+

tests/test_alias_initialization.py

+import pytest
+import gc
+
+def test_alias_delay_initialization(capture, msg):
+
+    # A only initializes its trampoline class when we inherit from it; if we
+    # just create and use an A instance directly, the trampoline initialization
+    # is bypassed and we only initialize an A() instead (for performance
+    # reasons)
+    from pybind11_tests import A, call_f
+
+    class B(A):
+        def __init__(self):
+            super(B, self).__init__()
+
+        def f(self):
+            print("In python f()")
+
+    # C++ version
+    with capture:
+        a = A()
+        call_f(a)
+        del a
+        gc.collect()
+    assert capture == "A.f()"
+
+    # Python version
+    with capture:
+        b = B()
+        call_f(b)
+        del b
+        gc.collect()
+    assert capture == """
+        PyA.PyA()
+        PyA.f()
+        In python f()
+        PyA.~PyA()
+    """
+
+def test_alias_delay_initialization(capture, msg):
+    from pybind11_tests import A2, call_f
+
+    # A2, unlike the above, is configured to always initialize the alias; while
+    # the extra initialization and extra class layer has small virtual dispatch
+    # performance penalty, it also allows us to do more things with the
+    # trampoline class such as defining local variables and performing
+    # construction/destruction.
+
+    class B2(A2):
+        def __init__(self):
+            super(B2, self).__init__()
+
+        def f(self):
+            print("In python B2.f()")
+
+    # No python subclass version
+    with capture:
+        a2 = A2()
+        call_f(a2)
+        del a2
+        gc.collect()
+    assert capture == """
+        PyA2.PyA2()
+        PyA2.f()
+        A2.f()
+        PyA2.~PyA2()
+    """
+
+    # Python subclass version
+    with capture:
+        b2 = B2()
+        call_f(b2)
+        del b2
+        gc.collect()
+    assert capture == """
+        PyA2.PyA2()
+        PyA2.f()
+        In python B2.f()
+        PyA2.~PyA2()
+    """

tests/test_issues.cpp

@@ -134,30 +134,6 @@ void init_issues(py::module &m) {
     m2.def("expect_float", [](float f) { return f; });
     m2.def("expect_int", [](int i) { return i; });
 
-    // (no id): don't invoke Python dispatch code when instantiating C++
-    // classes that were not extended on the Python side
-    struct A {
-        virtual ~A() {}
-        virtual void f() { py::print("A.f()"); }
-    };
-
-    struct PyA : A {
-        PyA() { py::print("PyA.PyA()"); }
-
-        void f() override {
-            py::print("PyA.f()");
-            PYBIND11_OVERLOAD(void, A, f);
-        }
-    };
-
-    auto call_f = [](A *a) { a->f(); };
-
-    pybind11::class_<A, std::unique_ptr<A>, PyA>(m2, "A")
-        .def(py::init<>())
-        .def("f", &A::f);
-
-    m2.def("call_f", call_f);
-
     try {
         py::class_<Placeholder>(m2, "Placeholder");
         throw std::logic_error("Expected an exception!");

tests/test_issues.py

@@ -79,30 +79,6 @@ def test_no_id(capture, msg):
     """
     assert expect_float(12) == 12
 
-    from pybind11_tests.issues import A, call_f
-
-    class B(A):
-        def __init__(self):
-            super(B, self).__init__()
-
-        def f(self):
-            print("In python f()")
-
-    # C++ version
-    with capture:
-        a = A()
-        call_f(a)
-    assert capture == "A.f()"
-
-    # Python version
-    with capture:
-        b = B()
-        call_f(b)
-    assert capture == """
-        PyA.PyA()
-        PyA.f()
-        In python f()
-    """
 
 
 def test_str_issue(msg):