How to call a variable from another file?

I will try to be as succinct as possible. I Have the following files in my:

Given below DBAccess2.h

#ifndef DBAccess2_HH
#define DBAccess2_HH

#include <iostream> //NOTE-I have deleted headers for sake of readability

using namespace std;

//=========================//
struct DeviceStatus
{
      string DevStat_Addr;
      string DevStat_EstbCD;
      string DevStat_CPUID;

      friend inline std::ostream &operator<<(std::ostream &out, DeviceStatus const& Dev)
      {
          return out << "\n("<< Dev.DevStat_Addr << "," << Dev.DevStat_EstbCD << "," << Dev.DevStat_CPUID << ")\n\n";
      }
};
//=========================//

class sqliteDB {

    using Records = std::list<DeviceStatus>;
    Records Device_Status_list;

    public:
       list<DeviceStatus> GET_ALL_Device_Status();
       Records const& get() const { return Device_Status_list; }
       void writeJson(std::ostream& os) const;
       void printList() const;
};
#endif

Below is DBAccess22.cpp

#include <iostream> //NOTE-I have deleted headers for sake of readability
#include "DBAccess2.h"
//====XX===//
list<DeviceStatus> sqliteDB::GET_ALL_Device_Status()
{
    //==SOME CODE TO ACCESS SQLITE DATABASE===//

    return Device_Status_list;
}
//====XX===//
void sqliteDB::writeJson(std::ostream& os) const {
    using namespace boost::property_tree;
    ptree pt;

    for (auto &entry : Device_Status_list) 
    {       
        pt.add("Addr", entry.DevStat_Addr);
        pt.add("EstbCD", entry.DevStat_EstbCD);
        pt.add("CPUID", entry.DevStat_CPUID);
    }
    write_json(os, pt, false);
}
//====XX===//

Below is POST1.h file:

#ifndef POST1_HH
#define POST1_HH
#include <iostream> //NOTE-I have deleted headers for sake of readability
#include "DBAccess2.h"

class POST1
{

    public:
      string TokenNo;
      string CommandStatus;
      string JSON_Cmnd_String;

      void POST_Device_Status();
};

#endif

Below is POST1.cpp:

#include <iostream> //NOTE-I have deleted headers for sake of readability
#include "DBAccess2.h"
#include "POST1.h"

using namespace std ;

void POST1::POST_Device_Status()
{
    POST1 POST_OBJ; 

    POST_OBJ.TokenNo = "1122";
    POST_OBJ.CommandStatus = "0";
    POST_OBJ.JSON_Cmnd_String = "??????"; //I Want os  into this

    cout << "Token is:" << POST_OBJ.TokenNo << endl;
    cout << "CommandStatus is:" << POST_OBJ.CommandStatus << endl;
    cout << "JSON_Cmnd_String is:" << POST_OBJ.JSON_Cmnd_String << endl;
}

And below is my main.cpp:

#include "DBAccess2.h"
#include "POST1.h"
using namespace std ;

template <typename List>
static void printList(List const& list) {
    for (auto& r : list) std::cout << r << "";
}

void dump(sqliteDB const& db)
{
    printList(db.get());
    db.writeJson(std::cout);
}

int main()
{
    sqliteDB object1;
    POST1 object2;

    dump(object1);
    object1.GET_ALL_Device_Status();

    dump(object1);
    object2.POST_Device_Status();

    return 0;
}

In file DBAccess22.cpp variable os contains some data. I am trying call os into POST1.cpp (see commented) but I get various errors. how can I do that ?

Note: I am new to c++ (2 weeks old).