将 nlohmann::json 与 cppyy 一起使用？

Question

Is it possible to pass a python dict into a function expecting a nlohmann::json ( nlohmann/json ) object via cppyy ? This question has to have come up by now, but I wasn't able to find anything on it.

Minimal example to reproduce (without regard to performance/safety, pls forgive):

test-json.h

#include <iostream>
#include <nlohmann/json.hpp>

using nlohmann::json;

void print_name_and_age(json j) {
    std::cout << j["name"] << "\n"
              << j["age"] << "\n";
} 

test-cppyy.py

import cppyy
cppyy.include('test-json.h')

from cppyy.gbl import print_name_and_age

some_dict = {
    "name": "alfred",
    "age": 25
}

print_name_and_age(some_dict)

runs into

    print_name_and_age(some_dict)
NotImplementedError: void ::print_name_and_age(nlohmann::json j) =>
    NotImplementedError: could not convert argument 1 (this method cannot (yet) be called)

I would like to be able to pass a python dict into the C++ function, and receive it as a nlohmann::json object. I presume I would need to write some custom converter for this?

Design requirement/background (optional)

I have a reinforcement learning environment class (written in C++) that needs to accept some configuration to initialize it (in its constructor). Everything's all fine passing a nlohmann::json object into the constructor while in the C++ domain, but I have a Python wrapper around the class too, written with cppyy that provides similar functionality to the C++ interface.Uptill now, because of the aforementioned issue, I've been forced to receive a const std::map<std::string, float>& in the constructor instead of a nlohmann::json , which is what a python dict containing only str -> float mappings easily gets converted to by cppyy . But this obviously limits my input json files to only contain float s as values (my usecase requires having strings as keys but string s, int s, float s and bool s as values in the JSON file). I can ofcourse write some pre-processing code to encode my heterogenous python dict into a homogenous str->float mapping on the python front (and do the same for C++) but I'd like a cleaner solution, if possible.

Could anyone please help me achieve passing a python dict into the cppyy -imported C++ function and have it converted into a nlohmann::json object in the C++ function? If this requires forking cppyy to add extra converter code/too much trouble I presume I would need to use a std::map<std::string, std::any / variant> alternative? I haven't worked alot with std::any/variant , would like to ask if this would even be possible - python dict to map<string, any> - if this is the best alternative to a custom converter - in terms of performance / clean elegant code.

Environment:

Python 3.9.13

C++17 ( I believe cppyy-2.4.0 doesn't support C++20 yet, I don't have any constraint on the C++ standard)

cppyy==2.3.1
cppyy-backend==1.14.8
cppyy-cling==6.25.3
cppyythonizations==1.2.1

MacOS Monterey, Apple M1

Answer 1

This has been answered atGitHub: Conversion from python dict into nlohmann::json