Add example showing how to bind virtual classes, passing to overrides by...

Add example showing how to bind virtual classes, passing to overrides by reference, registering shared_ptr, and overriding factories and data classes from Python

unittest/CMakeLists.txt

@@ -62,6 +62,8 @@ if(CMAKE_CXX_STANDARD GREATER 14 AND CMAKE_CXX_STANDARD LESS 98)
   config_bind_optional(std "std::optional")
 endif()
 
+add_lib_unit_test(bind_virtual_factory)
+
 add_python_unit_test("py-matrix" "unittest/python/test_matrix.py" "unittest")
 
 add_python_unit_test("py-tensor" "unittest/python/test_tensor.py" "unittest")
@@ -118,4 +120,8 @@ set_tests_properties("py-std-vector" PROPERTIES DEPENDS ${PYWRAP})
 
 add_python_unit_test("py-user-struct" "unittest/python/test_user_struct.py"
                      "python;unittest")
-set_tests_properties("py-std-vector" PROPERTIES DEPENDS ${PYWRAP})
+set_tests_properties("py-user-struct" PROPERTIES DEPENDS ${PYWRAP})
+
+add_python_unit_test("py-bind-virtual" "unittest/python/test_bind_virtual.py"
+                     "python;unittest")
+set_tests_properties("py-bind-virtual" PROPERTIES DEPENDS ${PYWRAP})

unittest/bind_virtual_factory.cpp

+/// Copyright 2023 LAAS-CNRS, INRIA
+#include <eigenpy/eigenpy.hpp>
+
+using std::shared_ptr;
+namespace bp = boost::python;
+
+// fwd declaration
+struct MyVirtualData;
+
+/// A virtual class with two pure virtual functions taking different signatures,
+/// and a polymorphic factory function.
+struct MyVirtualClass {
+  MyVirtualClass() {}
+  virtual ~MyVirtualClass() {}
+
+  // polymorphic fn taking arg by shared_ptr
+  virtual void doSomethingPtr(shared_ptr<MyVirtualData> const &data) const = 0;
+  // polymorphic fn taking arg by reference
+  virtual void doSomethingRef(MyVirtualData &data) const = 0;
+
+  virtual shared_ptr<MyVirtualData> createData() const {
+    return std::make_shared<MyVirtualData>(*this);
+  }
+};
+
+struct MyVirtualData {
+  MyVirtualData(MyVirtualClass const&) {}
+  virtual ~MyVirtualData() {}  // virtual dtor to mark class as polymorphic
+};
+
+shared_ptr<MyVirtualData> callDoSomethingPtr(const MyVirtualClass &obj) {
+  auto d = obj.createData();
+  printf("Created MyVirtualData with address %p\n", (void *)d.get());
+  obj.doSomethingPtr(d);
+  return d;
+}
+
+shared_ptr<MyVirtualData> callDoSomethingRef(const MyVirtualClass &obj) {
+  auto d = obj.createData();
+  printf("Created MyVirtualData with address %p\n", (void *)d.get());
+  obj.doSomethingRef(*d);
+  return d;
+}
+
+void throw_virtual_not_implemented_error() {
+  throw std::runtime_error("Called C++ virtual function.");
+}
+
+/// Wrapper classes
+struct VirtualClassWrapper : MyVirtualClass, bp::wrapper<MyVirtualClass> {
+  void doSomethingPtr(shared_ptr<MyVirtualData> const &data) const override {
+    if (bp::override fo = this->get_override("doSomethingPtr")) {
+      /// shared_ptr HAS to be passed by value.
+      /// Boost.Python's argument converter has the wrong behaviour for
+      /// reference_wrapper<shared_ptr<T>>, so boost::ref(data) does not work.
+      fo(data);
+      return;
+    }
+    throw_virtual_not_implemented_error();
+  }
+
+  /// The data object is passed by mutable reference to this function,
+  /// and wrapped in a @c boost::reference_wrapper when passed to the override.
+  /// Otherwise, Boost.Python's argument converter will convert to Python by
+  /// value and create a copy.
+  void doSomethingRef(MyVirtualData &data) const override {
+    if (bp::override fo = this->get_override("doSomethingRef")) {
+      fo(boost::ref(data));
+      return;
+    }
+    throw_virtual_not_implemented_error();
+  }
+
+  shared_ptr<MyVirtualData> createData() const override {
+    if (bp::override fo = this->get_override("createData")) return fo();
+    return default_createData();
+  }
+
+  shared_ptr<MyVirtualData> default_createData() const {
+    return MyVirtualClass::createData();
+  }
+};
+
+/// This "trampoline class" does nothing but is ABSOLUTELY required to ensure
+/// downcasting works properly with non-smart ptr signatures. Otherwise,
+/// there is no handle to the original Python object ( @c PyObject *).
+/// Every single polymorphic type exposed to Python should be exposed through such a trampoline.
+/// Users can also create their own wrapper classes by taking inspiration from boost::python::wrapper<T>.
+struct DataWrapper : MyVirtualData, bp::wrapper<MyVirtualData> {
+  /// we have to use-declare non-defaulted constructors
+  /// (see https://en.cppreference.com/w/cpp/language/default_constructor)
+  /// or define them manually.
+  using MyVirtualData::MyVirtualData;
+};
+
+/// Take and return a const reference
+const MyVirtualData &iden_ref(const MyVirtualData &d) {
+  // try cast to holder
+  return d;
+}
+
+/// Take a shared_ptr (by const reference or value, doesn't matter), return by const reference
+const MyVirtualData &iden_shared(const shared_ptr<MyVirtualData> &d) {
+  // get boost.python's custom deleter
+  // boost.python hides the handle to the original object in there
+  // dter being nonzero indicates shared_ptr was wrapped by Boost.Python
+  auto *dter = std::get_deleter<bp::converter::shared_ptr_deleter>(d);
+  if (dter != 0) printf("> input shared_ptr has a deleter\n");
+  return *d;
+}
+
+/// Take and return a shared_ptr
+shared_ptr<MyVirtualData> copy_shared(const shared_ptr<MyVirtualData> &d) {
+  auto *dter = std::get_deleter<bp::converter::shared_ptr_deleter>(d);
+  if (dter != 0) printf("> input shared_ptr has a deleter\n");
+  return d;
+}
+
+BOOST_PYTHON_MODULE(bind_virtual_factory) {
+  assert(std::is_polymorphic<MyVirtualClass>::value &&
+         "MyVirtualClass should be polymorphic!");
+  assert(std::is_polymorphic<MyVirtualData>::value &&
+         "MyVirtualData should be polymorphic!");
+
+  bp::class_<VirtualClassWrapper, boost::noncopyable>(
+      "MyVirtualClass", bp::init<>(bp::args("self")))
+      .def("doSomething", bp::pure_virtual(&MyVirtualClass::doSomethingPtr),
+           bp::args("self", "data"))
+      .def("doSomethingRef", bp::pure_virtual(&MyVirtualClass::doSomethingRef),
+           bp::args("self", "data"))
+      .def("createData", &MyVirtualClass::createData,
+           &VirtualClassWrapper::default_createData, bp::args("self"));
+
+  bp::register_ptr_to_python<shared_ptr<MyVirtualData> >();
+  /// Trampoline used as 1st argument
+  /// otherwise if passed as "HeldType", we need to define
+  /// the constructor and call initializer manually.
+  bp::class_<DataWrapper, boost::noncopyable>("MyVirtualData", bp::no_init)
+      .def(bp::init<MyVirtualClass const&>(bp::args("self", "model")));
+
+  bp::def("callDoSomethingPtr", callDoSomethingPtr, bp::args("obj"));
+  bp::def("callDoSomethingRef", callDoSomethingRef, bp::args("obj"));
+
+  bp::def("iden_ref", iden_ref, bp::return_internal_reference<>());
+  bp::def("iden_shared", iden_shared, bp::return_internal_reference<>());
+  bp::def("copy_shared", copy_shared);
+}

unittest/python/test_bind_virtual.py

+import bind_virtual_factory as bvf
+
+
+class ImplClass(bvf.MyVirtualClass):
+    def __init__(self):
+        self.val = 42
+        super().__init__()
+
+    def createData(self):
+        return ImplData(self)
+
+    # override MyVirtualClass::doSomethingPtr(shared_ptr data)
+    def doSomethingPtr(self, data):
+        print("Hello from doSomething!")
+        assert isinstance(data, ImplData)
+        print("Data value:", data.value)
+        data.value += 1
+
+    # override MyVirtualClass::doSomethingPtr(data&)
+    def doSomethingRef(self, data):
+        print("Hello from doSomethingRef!")
+        print(type(data))
+        assert isinstance(data, ImplData)
+        print("Data value:", data.value)
+        data.value += 1
+
+
+class ImplData(bvf.MyVirtualData):
+    def __init__(self, c):
+        super().__init__(c)  # parent virtual class requires arg
+        self.value = c.val
+
+
+def test_instantiate_child():
+    obj = ImplClass()
+    data = obj.createData()
+    print(data)
+
+
+def test_call_do_something_ptr():
+    obj = ImplClass()
+    print("Calling doSomething (by ptr)")
+    d1 = bvf.callDoSomethingPtr(obj)
+    print("Output data.value:", d1.value)
+
+
+def test_call_do_something_ref():
+    obj = ImplClass()
+    print("Ref variant:")
+    d2 = bvf.callDoSomethingRef(obj)
+    print(d2.value)
+    print("-----")
+
+
+def test_iden_fns():
+    obj = ImplClass()
+    d = obj.createData()
+    print(d, type(d))
+
+    # take and return const T&
+    d1 = bvf.iden_ref(d)
+    print(d1, type(d1))
+    assert isinstance(d1, ImplData)
+
+    # take a shared_ptr, return const T&
+    d2 = bvf.iden_shared(d)
+    assert isinstance(d2, ImplData)
+    print(d2, type(d2))
+
+    print("copy shared ptr -> py -> cpp")
+    d3 = bvf.copy_shared(d)
+    assert isinstance(d3, ImplData)
+    print(d3, type(d3))
+
+
+test_instantiate_child()
+test_call_do_something_ptr()
+test_call_do_something_ref()
+test_iden_fns()