Redesigned virtual call mechanism and user-facing syntax (breaking change!)

Sergey Lyskov pointed out that the trampoline mechanism used to override
virtual methods from within Python caused unnecessary overheads when
instantiating the original (i.e. non-extended) class.

This commit removes this inefficiency, but some syntax changes were
needed to achieve this. Projects using this features will need to make a
few changes:

In particular, the example below shows the old syntax to instantiate a
class with a trampoline:

class_<TrampolineClass>("MyClass")
    .alias<MyClass>()
    ....

This is what should be used now:

class_<MyClass, std::unique_ptr<MyClass, TrampolineClass>("MyClass")
    ....

Importantly, the trampoline class is now specified as the *third*
argument to the class_ template, and the alias<..>() call is gone. The
second argument with the unique pointer is simply the default holder
type used by pybind11.

example/example12.cpp

@@ -82,15 +82,11 @@ void runExample12Virtual(Example12 *ex) {
 }
 
 void init_ex12(py::module &m) {
-    /* Important: use the wrapper type as a template
-       argument to class_<>, but use the original name
-       to denote the type */
-    py::class_<PyExample12>(m, "Example12")
-        /* Declare that 'PyExample12' is really an alias for the original type 'Example12' */
-        .alias<Example12>()
+    /* Important: indicate the trampoline class PyExample12 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_<Example12, std::unique_ptr<Example12>, PyExample12>(m, "Example12")
         .def(py::init<int>())
-        /* Copy constructor (not needed in this case, but should generally be declared in this way) */
-        .def(py::init<const PyExample12 &>())
         /* Reference original class in function definitions */
         .def("run", &Example12::run)
         .def("run_bool", &Example12::run_bool)

example/issues.cpp

@@ -42,8 +42,7 @@ void init_issues(py::module &m) {
         }
     };
 
-    py::class_<DispatchIssue> base(m2, "DispatchIssue");
-    base.alias<Base>()
+    py::class_<Base, std::unique_ptr<Base>, DispatchIssue>(m2, "DispatchIssue")
         .def(py::init<>())
         .def("dispatch", &Base::dispatch);
 
@@ -108,4 +107,28 @@ void init_issues(py::module &m) {
     // (no id): don't cast doubles to ints
     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() { std::cout << "A.f()" << std::endl; }
+    };
+
+    struct PyA : A {
+        PyA() { std::cout << "PyA.PyA()" << std::endl; }
+
+        void f() override {
+            std::cout << "PyA.f()" << std::endl;
+            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);
 }

example/issues.py

@@ -9,6 +9,7 @@ from example.issues import Placeholder, return_vec_of_reference_wrapper
 from example.issues import iterator_passthrough
 from example.issues import ElementList, ElementA, print_element
 from example.issues import expect_float, expect_int
+from example.issues import A, call_f
 import gc
 
 print_cchar("const char *")
@@ -55,3 +56,19 @@ except Exception as e:
     print("Failed as expected: " + str(e))
 
 print(expect_float(12))
+
+class B(A):
+    def __init__(self):
+        super(B, self).__init__()
+
+    def f(self):
+        print("In python f()")
+
+print("C++ version")
+a = A()
+call_f(a)
+
+print("Python version")
+b = B()
+call_f(b)
+

example/issues.ref

@@ -12,3 +12,9 @@ Failed as expected: Incompatible function arguments. The following argument type
     1. (int) -> int
     Invoked with: 5.2
 12.0
+C++ version
+A.f()
+Python version
+PyA.PyA()
+PyA.f()
+In python f()