Json object from string from a TCP socket

[marcociara379]

Hello everyone,
I am trying to pass some information from a Python script to a C++ program. The communication occurs with zmq via TCP. The Python publisher publishes the following info:

topic = "vPad_pos_body_frame"
position_orientation_dict = dict(x_NED=str(0.0),
                                    y_NED=str(1.0),
                                    z_NED=str(2.0),
                                    pose_w=str(3.0),
                                    pose_x=str(4.0),
                                    pose_y=str(5.0),
                                    pose_z=str(6.0) )
send_str_and_json(socket_manager, topic, json.dumps(position_orientation_dict))

where the function send_str_and_json() looks like:

def send_str_and_json(socket, string, json_dict, flags=0):
    """send a dictionary as json with the topic string"""
    socket.send_string(string, zmq.SNDMORE)
    return socket.send_json(json_dict, flags)

On the C++ program, I am able to receive the dumped json string, but as soon as I try to access the keys:values after parsing, an error rises: terminate called after throwing an instance of 'nlohmann::detail::type_error' what(): [json.exception.type_error.305] cannot use operator[] with a string argument with string Aborted (core dumped).
As far as I can tell, the string is not parsed to create an object, but it remains a string. (confirmed by nlohmann::json::is_object() that returns 0. The string is correctly received by the software, and i can print it ("{\"x_NED\": \"0.0\", \"y_NED\": \"1.0\", \"z_NED\": \"2.0\", \"pose_w\": \"3.0\", \"pose_x\": \"4.0\", \"pose_y\": \"5.0\", \"pose_z\": \"6.0\"}")
The code of the subscriber program in C++ is the following:

#include <iostream>
#include <string>
#include <zmq.hpp>
#include <json.hpp>
#include <unistd.h>

/* test program to listen to the track_vPad.py output (pad position in body frame) sent on port 5556 */

int main()
{
    std::string topic = "vPad_pos_body_frame";
    zmq::context_t context(1);
    zmq::socket_t subscriber(context, ZMQ_SUB);
    // subscriber.setsockopt(ZMQ_SUBSCRIBE, "vPad_pos_body_frame", 1);
    subscriber.connect("tcp://127.0.0.1:6543");
    subscriber.setsockopt(ZMQ_SUBSCRIBE, topic.c_str(), topic.length());
    std::cout << "Connected. "<< std::endl;
    nlohmann::json j;

    sleep(1);

    while(true)
    {
        zmq::message_t msg;
        subscriber.recv(&msg);
        std::string topic_str = std::string(static_cast<char*>(msg.data()), msg.size());
        std::cout << "Received topic " << topic_str << " " << std::endl;
        if( std::string(static_cast<char*>(msg.data())) == topic)
        {

            zmq::message_t json_msg;
            subscriber.recv(&json_msg);
            std::string json_str = std::string(static_cast<char*>(json_msg.data()), json_msg.size());
            // std::stringstream(json_str) >> j;
            j = nlohmann::json::parse(json_str);
            std::cout << j.is_object() << std::endl;
            std::string x_NED; 
            x_NED = j["x_NED"];
            std::cout << "Received JSON: \n" <<  x_NED << " \n \n " << std::endl;
        }
    }
    return 0;
}

Do you have any clue on how to create a json object from the string passed through TCP and then access its keys:values?

Thanks a lot for your help,

Marco

PS:

• the json library tests during compilation report no errors.
• I use Ubuntu 18.04

[pboettch]

What do you get if you print out json_str? Is it what you pasted in your post - the one which espaces the quotes?

For performance reasons you should consider using cbor (with cbor2 in python) for data-sending. In my use-case I reduced the cpu-load by 15% when switching to cbor instead of json-strings.

Also, there is a json-parse (as well as for cbor) function which accepts a const uint8_t *-pointer, This saves you from copying the string to std::string.

[marcociara379]

Hi @pboettch,
thanks a lot for your precious hints.
Yes, if I print json_str the output is:
"{\"x_NED\": \"0.0\", \"y_NED\": \"1.0\", \"z_NED\": \"2.0\", \"pose_w\": \"3.0\", \"pose_x\": \"4.0\", \"pose_y\": \"5.0\", \"pose_z\": \"6.0\"}".

Marco

[pboettch]

This means that the string you are sending from python has been double-dumped as json. If you print json_dict in your send_str_and_json python function, what does it show?

[marcociara379]

{"x_NED": "0.0", "y_NED": "1.0", "z_NED": "2.0", "pose_w": "3.0", "pose_x": "4.0", "pose_y": "5.0", "pose_z": "6.0"}

[marcociara379]

yes, I dumped twice, apparently the zmq send_json already "dumps" it.
Thanks for your help, you have been extremely useful.
By the way, if you have a couple of minutes, would you mind explain a bit more about the json-parse function, so that I do not have to copy to a std::string the sent json?

Thanks a lot again

[pboettch]

Just check the possible ::parse-function of nlohmann::json in the documentation. You will find one which is parse(const uint8_t *, size_t) or similar.

[marcociara379]

awesome, I close the issue since you solved it.

have a nice day.