array subscript operator overloading in c++

int &HTable::operator[](const string &key){
    int hashVal = hash(key);
    list<City> citylist = _pht->at(hashVal);
    std::list<City>::iterator it = citylist.begin();
    for (; it != citylist.end(); it++){
        std::cout << "came_inside" << endl;
        City ob = (*it);
        if (ob.city == key){
            return ob.population;
        }
    }
    City newcity(key,0);
    citylist.push_front(newcity);
    _pht->erase(_pht->begin() + hashVal);
    _pht->insert(_pht->begin() + hashVal, citylist);
    return newcity.population;

}

My class is

class HTable { public: HTable( int );

    int &operator[ ]( const string & );
    void print ( ) const;
    int  size  ( ) const;

private:
    int _size;
    vector< list< City > > *_pht;

    int hash( const string & ) const;

};

My problem is when I try to use this

HTable fl_cities( n );                      // hash table with n lists


fl_cities["abcd"] = 1000;
fl_cities["abc"] = 111;
fl_cities["abcdefdf"] = 111;

cout << fl_cities["abc"] << endl;    // return 0
cout << fl_cities["abcdefdf"] << endl; // return 0
cout << fl_cities["abcd"] << endl;  // return 0

I don't get expected value, it shows 0 because I am assigning 0 then returning the value. It is suppose to return the pointer and then when I assign the value it should go there but it doesn't work.

I have tried this operator with simple int array and it worked perfect in that case. But in this problem, list inside a vector it doesn't work.

Any help is appreciated !