undefined symbol: _Z9ConstantsRN8nanobind7module_E

[nyckmaia]

// constants.h

#ifndef CONSTANTS_H
#define CONSTANTS_H

#include <array>
#include <map>
#include <string>
#include <vector>

extern const float a;
extern const float b;
extern const std::vector<float> c;
extern const std::map<int, std::pair<std::string, std::string>> d;

enum class MyEnumClass {
    A,
    B,
    C
};

// constants.cpp

#include "myproject/constants.h"

const float a = 1.0f;
const float b = 2.0f;
const std::vector<float> c = 3.0f;
const std::map<int, std::pair<std::string, std::string>> d = ...;

Nanobind wrapper

// py_constants.cpp

#include <nanobind/nanobind.h>

#include "maiacore/constants.h"

namespace nb = nanobind;

void Constants(const nb::module_ &m) {
    nb::enum_<MyEnum>(m, "MyEnum")
        .value("A", MyEnum::A)
        .value("B", MyEnum::B);
}

// myproject.cpp
namespace nb = nanobind;

void Constants(nb::module_ &);
void OtherClass(nb::module_ &);

NB_MODULE(myproject, m) {
    m.doc() = "...";

    Constants(m);
    OtherClass(m);
    ...
}

CMakLists.txt

nanobind_add_module(${PROJECT_NAME}
        STABLE_ABI
        NB_STATIC
        ${myproject_src} ${nanobind_wrapper_src}
    )

It compiles ok without any error or warning, including the constants.cpp file and py_constants.cpp file (wrapper).
So, I enter inside the mymodule.so directory and tried to import it.

Python Error

Python 3.8.10 (default, Jun 22 2022, 20:18:18)
[GCC 9.4.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import myproject as my
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ImportError: /mnt/c/Users/nyck/Desktop/maialib/build/Linux/module/myproject.cpython-38-x86_64-linux-gnu.so: undefined symbol: _Z9ConstantsRN8nanobind7module_E
>>>

Help

Can you help me to fix this Python import?

My System

• Ubuntu 22 LTS
• C++ Compiler: gcc 9.3.0
• C++ 17
• nanobind 2.0.0

[nyckmaia]

UPDATE 01

I found a mistake.
In the file py_constants.cpp I use the keyword const in the line: void Constants(const nb::module_ &m) {.
But in the file myproject.cpp I don't use const, as you can see: void Constants(nb::module_ &);

So, I remove the const keyword.
It compiles without any error or warning and genarated the Python module file.
Now, when I tried to import it, I got a different error: bad_cast

Env: Linux Ubuntu + GCC 9.3 + Python 3.8.10

Python 3.8.10 (default, Jun 22 2022, 20:18:18)
[GCC 9.4.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import maiacore as mc
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ImportError: std::bad_cast

Env: Windows 11 + Clang 18.3 + Python 3.10.6

Python 3.10.6 (tags/v3.10.6:9c7b4bd, Aug  1 2022, 21:53:49) [MSC v.1932 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> import maiacore as mc
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ImportError: bad cast

Help

1. How can I debug this bad_cast runtime import error to discovery the bug in my code?
2. I have some C++ methods that uses nlohmann::json type. Is there any problem with that? Do I have to convert the nlohmann::json return type below to a nb::dict too?
   Exemple:

cls.def(
    "myMethod",
    [](A& self, const nb::dict& config) -> nlohmann::json {
        // Python dict type to Json as std::string
        std::string jsonString = nb::cast<std::string>(nb::str(config));

        // JSON as string to nlohmann::json type
        nlohmann::json jsonObj = nlohmann::json::parse(jsonString);

        return self.myMethod(jsonObj);
    },
    nb::arg("config") = nb::dict());

3. Is there a better way to convert nb::dict to nlohmann::json and vice versa?

[wjakob]

1. Set a breakpoint on __cxa_throw (this only works on Linux/macOS, not sure how to do it in Windows)
2. AFAIK there aren't yet nanobind bindings for this json library. You'll have to make your own.
3. See point 2. I suggest that you carefully study the nlohmann_json pybind11 bindings and convert them to nanobind. Such a conversoin may be useful to others as well. I don't really use JSON and therefore haven't had the time to look into this myself.