Question on py::multiple_inheritance with virtual inheritance

[leakec]

I ran into an issue the other day and py::multiple_inheritance solved it. However, based on the documentation I could find, it doesn't look like it should be needed in this scenario (since there is only a single chain of inheritance). However, adding it definitely solves the problem.

I think one of three things is going on:

1. I have misread or misunderstood the documentation.
2. This is case where it is needed, and the documentation should be updated.
3. This shouldn't be needed, but there is a bug somewhere else, and this is fixing it.

If it's 1, please let me know and I can just close this. If it's 2 or 3, I'm happy to help dig in and figure out what is going on and/or submit PRs to help update documentation. I just want to make sure it's not 1 before I spend any time on 2 or 3.

The only documentation I could find on this is here in the changelog, which states it should only be needed "when C++ bases are hidden from pybind11." Maybe virtual inheritance counts as being hidden from pybind11, but at least to me, that's not obvious, as the virtual base class is included when creating the derived class in pybind11.

I've whittled this down to a simplest working case. Here are the relevant files:

test.h

struct Base {
public:
    int x;
    virtual ~Base();
};

struct VirtualBase : virtual Base {
public:
    int y;
    VirtualBase(int y);
    void printX();
    void printY();
};

test.cc

#include <iostream>
#include "test.h"

Base::~Base() {
};

VirtualBase::VirtualBase(int z) {
    y = z;
    x = z*2;
}

void VirtualBase::printX() {
    std::cout << x << std::endl;
}

void VirtualBase::printY() {
    std::cout << y << std::endl;
}

test_Py.cc

#include <pybind11/pybind11.h>
#include <pybind11/numpy.h>
#include <pybind11/stl.h>
#include "test.h"

namespace py = pybind11;

PYBIND11_MODULE(test_Py, m) {

    py::class_<Base>(m, "Base")
        .def_readwrite("x", &Base::x);

    py::class_<VirtualBase, Base>(m, "VirtualBase")
        .def(py::init<int>())
        .def_readwrite("y", &VirtualBase::y)
        .def("printX", &VirtualBase::printX)
        .def("printY", &VirtualBase::printY);
}

test.py

from test_Py import VirtualBase

vb = VirtualBase(3)

# From member access
print("From member access:")
print(f"x: {vb.x}")
print(f"y: {vb.y}")

print("From methods:")
print("x: ",end="", flush=True)
vb.printX()
print("y: ",end="", flush=True)
vb.printY()

CMakeLists.txt

cmake_minimum_required(VERSION 3.25)

project(example)

# TODO: Change for release
add_compile_options(-Wall -Werror)

# Contorl whether we build with shared libraries or static libraries
option(BUILD_SHARED_LIBS "Build using shared libraries" OFF)

# If not building with shared libs, set POSITION_INDEPENDENT_CODE 
# so that -fPIC gets used. This will
# allow linking the static libraries into the shared pybind11 libraries.
if(NOT BUILD_SHARED_LIBS)
    set(CMAKE_POSITION_INDEPENDENT_CODE ON)
endif()

# Use C++ 20
set(CMAKE_CXX_STANDARD 20)

# Turn on generation of compile_commands.json for LSP
set(CMAKE_EXPORT_COMPILE_COMMANDS ON)

# Turn on colored diagnostics
set(CMAKE_COLOR_DIAGNOSTICS ON)

# Find Python in the system
find_package(Python3 REQUIRED COMPONENTS Interpreter Development)

# Find pybind11 in the system
# This is needed for CMake < 3.27. 
# After Cmake 3.27+, can remove setting PYBIND11_FINDPYTHON.
set(PYBIND11_FINDPYTHON ON)
find_package(pybind11 3.0 REQUIRED CONFIG)

# Create the bf library
add_library(test test.cc)

# Create the BF.py Python file
pybind11_add_module(test_Py test_Py.cc)
target_link_libraries(test_Py PRIVATE test)

install(TARGETS test test_Py DESTINATION .)

I'm building and installing like this:

cmake -B build -G Ninja -DCMAKE_INSTALL_PREFIX="." .
cmake --build build
cmake --install build

After building and installing, if I run python test.py I see the following output:

From member access:
x: 3
y: 3
From methods:
x: 6
y: 3

which is incorrect. I should see x=6 and y=6 in both cases. However, if I add py::multiple_inheritance() to the VirtualBase in test_Py.cc so the file now reads

test_Py.cc

#include <pybind11/pybind11.h>
#include <pybind11/numpy.h>
#include <pybind11/stl.h>
#include "test.h"

namespace py = pybind11;

PYBIND11_MODULE(test_Py, m) {

    py::class_<Base>(m, "Base")
        .def_readwrite("x", &Base::x);

    py::class_<VirtualBase, Base>(m, "VirtualBase", py::multiple_inheritance())
        .def(py::init<int>())
        .def_readwrite("y", &VirtualBase::y)
        .def("printX", &VirtualBase::printX)
        .def("printY", &VirtualBase::printY);
}

The output of test.py becomes

From member access:
x: 6
y: 3
From methods:
x: 6
y: 3

as expected. Similarly, if I make it just normal inheritance rather than virtual inheritance, I can remove the py::multiple_inheritance() and everything works as expected.

I realize the example at a glance looks strange--why would anyone ever do virtual inheritance with only a single base class and no derived classes--but that is only because I've boiled it down to this simplest working case from a much more complex example.