sorting strings in a file

I need a solution for sorting of unix pwd file using C++ based on the last name. The format of the file is username, password, uid, gid, name, homedir, shell. All are seperated by colon delimiters. The name field contains first name follwed by last name both seperated by space I am able to sort the values using map and i am posting my code. Can some one suggest me improvements that I can do to my code please. Also I am unable to see the sorted lines in my file.

string line,item;
fstream myfile("pwd.txt");
vector<string> lines;
map<string,int> lastNames;
map<string,int>::iterator it;

if(myfile.is_open())
 {    
  char delim =':';
  int count =0;
  while(!myfile.eof())
  {
    count++;
    vector<string> tokens;
    getline(myfile,line);
    istringstream iss(line);

    lines.push_back(line);

    while(getline(iss,item,delim))
    {
        tokens.push_back(item);
    }
    cout<<tokens.size()<<endl;;
    size_t i =tokens[4].find(" ");
    string temp = tokens[4].substr(i,(tokens[4].size()-i));
    cout<<temp<<endl;

    lastNames.insert(pair<string,int>(temp,count));
    tokens.clear();

  }

    myfile.seekg(0,ios::beg);

    for(it=lastNames.begin();it!=lastNames.end();it++)
  {
         cout << (*it).first << " => " << (*it).second << endl;
         int value=lastNames[(*it).first ];
         myfile<<lines[value-1]<<endl;
         cout<<lines[value-1]<<endl;
         cout<<value<<endl;
  }

}

Also I am having problem writing to the file I am unable to see the sorted results.

my problem:

    Can someone please explain me why I am unable to see the written results in the file!

Thanks & Regards,

Mousey.