[BUG]: Pass by reference does copy

[ManuelSchneid3r]

Required prerequisites

• Make sure you've read the documentation. Your issue may be addressed there.
• Search the issue tracker and Discussions to verify that this hasn't already been reported. +1 or comment there if it has.
• Consider asking first in the Gitter chat room or in a Discussion.

What version (or hash if on master) of pybind11 are you using?

2.10.2

Problem description

The docs state that passing arguments as reference and pointer should work, but actually, while pointers works well, references dont. They end up beeing copied, which is a blocker if the type is not copy constructible.

class PyPlugin
{
public:
    virtual void takeThisAndModifyR(StandardItem &item) {};
    virtual void takeThisAndModifyP(StandardItem *item) {};
};
class PyPluginTrampoline : PyPlugin
{
public:
    using PyPlugin::PyPlugin;  // Inherit the constructors
    void takeThisAndModifyR(StandardItem &item) override  { PYBIND11_OVERRIDE(void, PyPlugin, takeThisAndModifyR, item); }
    void takeThisAndModifyP(StandardItem *item) override { PYBIND11_OVERRIDE(void, PyPlugin, takeThisAndModifyP, item); }
};

Then in a pyhton plugin I do:

class Plugin(Plugin):
  …
    def takeThisAndModifyR(self, item):
        item.id = "takeThisAndModifyR";

    def takeThisAndModifyP(self, item):
        item.id = "takeThisAndModifyP";
…

In the app i do

auto item = StandardItem("id");
INFO << item.id();
p->takeThisAndModifyP(&item);
INFO << item.id();
p->takeThisAndModifyR(item);
INFO << item.id();

Finally this prints:

10:47:43 [info:python] id
10:47:43 [info:python] takeThisAndModifyP
10:47:43 [warn:albert] [17ms] Failed loading 'find': return_value_policy = copy, but type albert::StandardItem is non-copyable!

Of course I exposed the item class. Left it out for readability. Obviously pybind does not like references. Is this a bug or a known limitation and I should rather look for a workaround?

Related:

• How to pass arguments by reference from c++ to python with pybind11? #3991
• https://stackoverflow.com/questions/72500615/how-to-pass-arguments-by-reference-when-call-python-in-c-with-pybind11
• https://pybind11.readthedocs.io/en/stable/faq.html#limitations-involving-reference-arguments
• std::function<void(Foo &)> reports error when Foo is non-copyable and non-movable #1123

Reproducible example code

No response

Is this a regression? Put the last known working version here if it is.

Not a regression

[Iximiel]

Hello, I have a problem that i think it is related to the topic of this issue:

I want to have a two way communication to and from python and what I come out is using a helper class in C++ with its python interface that should do that.

So I tried to pass by reference to a python function loaded in the embedded interpreter, and what python sees its a copy of my C++ object, so calling methods in python that should modify it do not affect the C++ instatiated object.

   mylib::SimpleIO mytest;
   auto fn = py_module.attr(funcName);
   //This will NOT modify the data in mytest
   fn(mytest, setValue).cast<py::tuple>();

But if i cast the pointer of the C++ object with py::cast(&myobject) i can pass it (the object, not the casted pointer) to the python function and the methods in python affect my C++ object

   mylib::SimpleIO mytest;
   auto fn = py_module.attr(funcName);
   auto obj = py::cast(&mytest);
   //This will modify the data in mytest
   fn(mytest, setValue).cast<py::tuple>();

I know I'm not clear with words: I have set up a small gist with the example of this. I'm using pybind v2.10.4, all the details are within the CMakeLists.txt in the gist

My question is: is this a wanted behaviour?

[ManuelSchneid3r]

People having to get stuff done could redirect to a function casting the reference to a pointer. works for me.