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.
diff --git a/example/issues.cpp b/example/issues.cpp
index d1a1941..e164708 100644
--- a/example/issues.cpp
+++ b/example/issues.cpp
@@ -42,8 +42,7 @@
}
};
- 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 @@
// (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);
}