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Problem description
When using PYBIND11_OVERRIDE_PURE inside of class with multiple inheritance, e.g.
class A { virtual ~A() {} };
class B { virtual std::string fn() const = 0; virtual ~B() {} }
class PyClass : public A, public B {
public:
std::string fn() const override {
PYBIND11_OVERRIDE_PURE(QString, B, fn, );
}
};
// A is not exposed in Python
py::class_<B, PyB>(m, "B", py::multiple_inheritance())
.def(py::init<>()) // should this be init_alias?
.def("function", &B::fn);This functions throws a "Tried to call pure virtual function ..." exception when constructing a new B in Python.
Removing this static_cast:
https://github.com/pybind/pybind11/blob/master/include/pybind11/pybind11.h#L2729
...solves the issue.
Is there any reason to have this static_cast here? Is there something I'm missing?
Reproducible example code
#include <iostream>
#include <pybind11/embed.h>
#include <pybind11/pybind11.h>
class A {
public:
virtual ~A() {}
};
class B {
public:
virtual std::string fn() const = 0;
virtual ~B() {}
};
constexpr const char* fmt_s = "{:20}: {}\n";
// switch A and B here fixes the issue since the function is from B
class PyB : public A, public B {
public:
std::string fn() const override {
std::cout << "fn(), this : " << (void*)this << "\n";
std::cout << "fn(), static_cast<> : " << (void*)static_cast<const B*>(this)
<< "\n";
std::cout << "fn(), dynamic_cast<> : " << (void*)dynamic_cast<const B*>(this)
<< "\n";
PYBIND11_OVERRIDE_PURE(std::string, B, fn, );
}
};
namespace py = pybind11;
PYBIND11_EMBEDDED_MODULE(cpp_module, m) {
py::class_<B, PyB>(m, "B", py::multiple_inheritance())
.def(py::init<>())
.def("fn", &B::fn);
}
int main() {
py::scoped_interpreter guard{};
py::exec(R"(
from cpp_module import B
class MyB(B):
def fn(self): return "MyB"
b = MyB()
)");
}