If you access the structure in the vector, it will fall off.

#include<stdio.h>
# include "ObdCapture.h"

int main() {

ObdCapture*oc=new ObdCapture();

oc->init();
oc->set_addr((char*) "00:00:00:08:A5:C6"); 

inta[] = {1,2,3,4,5};
if(oc->setMonitored(a,sizeof(a)/sizeof(a[0]))||oc->connectElm()){
    oc->setFile("log");
    if(oc->f_con_write()){
        std::cout<<"end"<<std::endl;
    }
}

// Memory release
deleteoc;
}

#ifndef_OBDCAP_H_
#define_OBDCAP_H_
# include <vector>
# include <string>
# include<sys/types.h>
# include<sys/socket.h>
# include<bluetooth/bluetooth.h>
# include<bluetooth/rfcomm.h>
# include<boot/property_tree/ptree.hpp>
# include <fstream>

typeedef structure elmCom {
            char*code;
            int num;
            int bit;
            int mode;
}elmCom;

classObdCapture
{
public:
// Constructor
    ObdCapture();
// Destructor
    ~ObdCapture();
    void init();
 // OBD code to monitor
    int setMonitored(inta[], intl_length);
// Obtaining List of OBD Codes Being Monitored
    std::vector<elmCom>getMonitored();
// Verifying Connectivity
    int connectElm();
    void set_addr(char*addr);
    std::string get_addr();
    std::string getFile();
    void setFile(std:string fname);
    // Read and write to file
    intf_con_write();

private:
    std::string fname;// filename
    time_tt;
    std::vector<elmCom>cap_register;
    std::vector<elmCom>elcom;//obdPID list (read from xml)   
    char can_buf [1025];
    sockaddr_rcaddr;
    char buf [1025]; // Receive Buffers
    ints, status, server;
    char dest [18];
    std::string path;// file path 
    std::ifstream fin;// input stream
    std::ofstream fout;// output stream
    std::string file_path;
    unsigned int flags;// flag indicating the correct order of processing
    boot::property_tree::ptreept;//xml tree
};

#endif//_OBDCAP_H_

#include<stdio.h>
# include <string.h>
# include <unistd.h>
# include<sys/socket.h>
# include<bluetooth/bluetooth.h>
# include<bluetooth/rfcomm.h>
# include <iostream>
# include <fstream>
# include <vector>
# include <time.h>
# include<boot/foreach.hpp>
# include<boot/property_tree/xml_parser.hpp>
# include<boot/property_tree/ptree.hpp>
# include<boot/optional.hpp>
# include "ObdCapture.h"

using namespace std; 
using namespace boot::property_tree;

// FILE OPERATION AND BLUETOOTH COMMUNICATION
#define BIT(num)((unsigned int)1<<(num))

#define AVA_BLUETOOTH BIT(0)// Bluetooth connection established
#define AVA_FILE BIT(1)//Preparing Write Files
# Define AVA_OBD_CODE BIT(2) // ObD-II Retrieval Commands
#define AVA_CAP_OBD BIT(3)//OBD-II Capture
#define AVA_BT_ADDR BIT(4) // Prepare Acquisition Address
#define ERR_BLUE_NOT_READ BIT (5)


ObdCapture::ObdCapture():elcom(138), cap_register(138)
{   
    elmSwitch=0;
    addr=(sockaddr_rc){0};//sockaddr_rcaddr
    flags = 0;
    t = time (NULL);
    memset(buf, '\0', sizeof(buf)); // buf initialization
}

ObdCapture::~ObdCapture()
{
    cout<<"Destructor Call\n";
    if(fname.empty()){
        fout.close();
    }
    if(flags&AVA_BLUETOOTH){
         close(s);
    }
}

voidObdCapture::init(){
ptree ptt;
read_xml ("obd_config.xml", ptt); // Boost's xml library
std::string str;
int num;
int bit;
int mode;
inti = 0;
BOOST_FOREACH(constree::value_type&pct, ptt.get_child("pids")){                     
        str = pct.second.get<std::string>("code");
        num = pct.second.get<int>("num");
        bit = pct.second.get<int>("bit");
        mode = pct.second.get<int>("mode");
        char*code = new char [str.size()+1]; // Free memory
        std::strcpy(code,str.c_str());// copy
        elmComelc={code, num, bit, mode};
        cout<<"elcis"<<elc.code<<endl;
        elcom.push_back(elc);
        cout<elcom.size()<endl;
        cout<<i++<<endl;
// re-subs
}
if((flags&AVA_OBD_CODE)==0){
    flags + = AVA_OBD_CODE;
    cout<<"AVA_OBD_CODE"<<endl;
}
//cout<<elcom.at(0).code<<endl;<- This causes an error.  
}
/* Hereinafter abbreviated */