How to get not matched records?

pqr,1234PQ,1234MN
xyz,7890AA,7890BB
mno,4567XX,4567YY
zzz,1111AA,2222BB
first field is city second field is x & third is y.

Contents of b.txt are as follows

abc,8899,TT
abc,8898,TT
abc,8897,TT
pqr,7766,RR
pqr,7765,RR
mno,6654,SS
xyz,5523,PP
xyz,5522,PP
first field is city second field is poco & third is adm.if city in b.txt matches with a.txt I want print the output as follows

pqr,@7766,@7765,1234PQ,1234MN,RR
mno,@6654,4567XX,4567YY,SS
xyz,@5523,@5522,7890AA,7890BB,PP

If city in b.txt does not exist in a.txt I want to write those records in seperate file.How to do this in c++?

The coding is as follows

#include <iostream>
#include <sstream>
#include <fstream>
#include <list>
#include <map>
#include <algorithm>

struct CityParms {
        std::string x,y,adm;
        std::list<std::string> poco_list;
        CityParms(const std::string& x = "",const std::string &y = "") : x(x),y(y) {}
};
typedef std::map<std::string,CityParms> CityParmsMap;

void process_a(CityParmsMap& map),process_b(CityParmsMap& map),report(const CityParmsMap& map);

int main()
{
        CityParmsMap map;
        process_a(map);
        process_b(map);
        report(map);
}

void process_a(CityParmsMap& map)
{
        std::ifstream f("a.txt");
        std::string str;
        while (getline(f,str)) {
                std::istringstream istr(str);
                std::string city,x,y;
                if (getline(istr,city,',') && getline(istr,x,',') && getline(istr,y,','))
                        map[city] = CityParms(x,y);
        }
}

void process_b(CityParmsMap& Map)
{
        std::ifstream f("b.txt");
        std::string str;
        while (getline(f,str)) {
                std::istringstream istr(str);
                std::string city,poco,adm;
                if (getline(istr,city,',') && getline(istr,poco,',') && getline(istr,adm,',')) {
                        CityParmsMap::const_iterator Citr = Map.find(city);
                        if (Citr != Map.end()) {
                                CityParms& cityParms = Map[city];
                                cityParms.poco_list.push_back("@"+poco);
                                cityParms.adm = adm;
                        } // Here what should be the condition if city is not found...
                }
        }
}

void report(const CityParmsMap& Map)
{
        std::ofstream f("report.csv");
        typedef CityParmsMap::const_iterator Citr;
        for (Citr itr = Map.begin();itr != Map.end();++itr) {
                const CityParms& cityParms = itr->second;
                const std::list<std::string> &poco_list = cityParms.poco_list;
                if (!poco_list.size())
                        continue;
                f << itr->first << ',';
                copy (poco_list.begin(),poco_list.end(),std::ostream_iterator<std::string>(f,","));
                f << cityParms.x << ',';
                f << cityParms.y << ',';
                f << cityParms.adm /*<< ','*/;
                f << '\n';
        }
}

Can anyone help me?