Question about ambiguous overloading operator >> in STL.

Hi, everyone!

Please look at the following codes. I think I overload the
operator clearly. But compiler still say that it is ambiguous.
Here are the source codes and related error message.

Source:

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#include <iostream>
#include <map>
#include <set>
#include <string>

using namespace std;

class employee_data {
public:
employee_data() : name (""), skill(0), salary(0) {}
employee_data(string n, int s, long sa) :
name (n), skill (s), salary (sa) {}

string name;
int skill;
long salary;

friend ostream& operator<< (ostream& os, const employee_data& e);
};

ostream& operator<< (ostream& os, const employee_data& e) {
os << "employee: " << e.name << " " << e.skill << " " << e.salary;
return os;
}

class employee {
public:
employee (int i, employee_data e) :
identification_code (i), description (e) {}

int identification_code; // key expression to identify an employee
employee_data description;

bool operator< (const employee& e) const {
return identification_code < e.identification_code; }
};

int main()
{
set <employee, less<employee> > employee_set;

multiset <employee, less<employee> > employee_multiset;

map <int, employee_data, less<int> > employee_map;
multimap <int, employee_data, less<int> > employee_multimap;

employee_data ed1 ("john", 1, 5000);
employee_data ed2 ("tom", 5, 2000);
employee_data ed3 ("mary", 2, 3000);

employee e1 (1010, ed1);
employee e2 (2020, ed2);
employee e3 (3030, ed3);

pair<const int, employee_data> a[2] = { make_pair (2020, ed2),
make_pair (3030, ed3) };
employee_map.insert (&a[0], &a[2]);

employee_data d = employee_map[2020];
cout << d << endl;

return 1;
}
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Error message:
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C:\Program Files\Microsoft Visual Studio\MyProjects\testMap1\testMap1.cpp(86) : error C2593: 'operator <<' is ambiguous
--------

Thanks in advance,
George