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Am not so sure what you want to do, but I can tell you C++ and Json... hummm

you can use a lib called rapid json and generate the json you need appending key-values as it were a dictionary

here an example:

#include <iostream>
#include <string>

#include <rapidjson\document.h>
#include <rapidjson\writer.h>
#include <rapidjson\stringbuffer.h>

void output(const rapidjson::Document& document)
{

    std::string strOp = document["op"].GetString();
    std::string ret = "{\"op\":\"" + strOp;

    std::string strTopic = document["topic"].GetString();
    ret += "\", \"topic\":\"" + strTopic;

    std::string strType = document["type"].GetString();
    ret += "\", \"type\":\"" + strType + "\"}";


    std::cout << "json" << std::endl;
    std::cout << ret << std::endl;
    //{ "op": "subscribe", "topic" : "/cmd_vel", "type" : "geometry_msgs/Twist" }
}

int main(int argC, char** argV)
{
    std::cout << "ROS" << std::endl;
    rapidjson::Document fromScratch;

    fromScratch.SetObject();
    rapidjson::Document::AllocatorType& allocator = fromScratch.GetAllocator();

    fromScratch.AddMember("op", "subscribe", allocator);
    fromScratch.AddMember("topic", "/cmd_vel", allocator);
    fromScratch.AddMember("type", "geometry_msgs/Twist", allocator);

    output(fromScratch);

    getchar();
    return 0;
}

The output will look like: