Output operator overloading problem

[t1dUs]

First of all hello to everyone.

I am trying to create a small program that does the following: stores a series of aircraft types (class acfttype in acfttype.cpp) inside a list (class listacfttype in listacfttype.cpp).

Acfttype works flawlessly, I have tested it with no problem. The problem is when I have created the listacfttype overriding "operator<<". The program compiles and links fine, but when I try to execute it I get a "Segmentation fault" error.

Here's the code for acfttype:

#ifndef ACFTTYPE
#define ACFTTYPE

#include <string>
#include <iostream>
#include <algorithm>

using namespace std;

class acfttype {
 private:
  string model;
 public:
  acfttype() {}
  acfttype(string s) { model=s; }
  acfttype(const acfttype &at) { model=at.gettype(); }
  ~acfttype() {}

  void settype(string s) { model = s; }
  string gettype() const { return model; }

  friend ostream& operator<<(ostream &out, acfttype &at) {
    out << at.model << endl;
    return out;
  }
  friend istream& operator>>(istream &in, acfttype &at) {
    string temp;
    cout << "Type model: ";
    getline(in, temp);
    transform(temp.begin(), temp.end(), temp.begin(), ::toupper);
    at.model = temp;
    return in;
  }

  friend bool operator==(const acfttype &at1, const acfttype &at2) { return at1 == at2; }
  friend bool operator!=(const acfttype &at1, const acfttype &at2) { return at1 != at2; }
  friend bool operator<(const acfttype &at1, const acfttype &at2) { return at1 < at2; }
  friend bool operator>(const acfttype &at1, const acfttype &at2) { return at1 > at2; }
  friend bool operator<=(const acfttype &at1, const acfttype &at2) { return at1 <= at2; }
  friend bool operator>=(const acfttype &at1, const acfttype &at2) { return at1 >= at2; }
};

#endif 

And here's the code for listacfttype:

#ifndef LISTACFTTYPE
#define LISTACFTTYPE

#include <iostream>
#include <string>
#include <list>
#include <cctype>
#include <stdlib.h>

#include "acfttype.cpp"

using namespace std;

class listacfttype {  
private:
  list<acfttype> lst;
public:
  listacfttype() {}
  ~listacfttype() {}
  
  void addtype (string &s) {
    lst.push_back(s);
  }
  
  void orderlistacfttype() { lst.sort(); }
  void removeduplicates() { lst.unique(); }

  friend ostream& operator<<(ostream &out, listacfttype &lt) {
    int i=0;
    i++;
    return out;
  }
  friend istream& operator>>(istream &in, listacfttype &lt);
};

#endif 

The class listacfttype works fine, but as soon as I add the "operator<<" begins to fail. As you can see for now the only thing I'm doing inside "operator<<" is declaring an int and ++, but I get the "Segmentation fault" error, can anyone enlighten me?

Just in case: I'm programming in linux, but I assume this to be more of a general programming problem, it it's not the case please forgive me.

[binarybob350]

I just tried the following in a test app in windows and it did not crash.

string s = "ABC";

listacfttype lt;
lt.addtype( s );

cout << lt << endl;