Go4Expert - associative container data structure in c++

how i stated:
Code:
#include <iostream>

#include <string>

#include <map>

using namespace std;



class Student

{

public:

  //--- Constructor

  Student (int id = 0, double gpa = 0);

  //--- Output operator

  friend ostream & operator<<(ostream & out, const Student & s);

  //--- Input operator

  friend istream & operator>>(istream & in, Student & s);

  //--- Less-than operator

  friend bool operator<(const Student & a, const Student & b);

private:

  //--- Data members

  int myId;

  double myGPA;

};  // end of class Student



//--- Definition of constructor

inline Student::Student(int id, double gpa)

  : myId(id), myGPA(gpa)

{ }



//--- Definition of input operator

inline istream & operator>>(istream & in, Student & s)

{

  in >> s.myId >> s.myGPA;

  

}



//--- Definition of output operator

inline ostream & operator<<(ostream & out, const Student & s)

{ out << "id = " << s.myId << "  GPA = " << s.myGPA;

 return out;

}



//--- Definition of less-than operator

inline bool operator<(const Student & a, const Student & b)

{ return a.myId < b.myId; }

//================================================================



int main()

{

        //typedef multimap<string, Student>grades

  typedef multimap<string, Student, less<string>> grades1;

  typedef multimap<Student, string, less<Student>> grades2;



  grades1 a1;   // associative array of Student, index type is string

  grades2 a2;   // associative array of string, index type is Student



  Student s;

  s = Student(12345, 3.3);  a1["Fred"] = s;   a2[s] = "Fred";

  s = Student(32322, 3.9);  a1["Al"] = s;     a2[s] = "Al";

  s = Student(13131, 2.5);  a1["Joan"] = s;   a2[s] = "Joan";

  s = Student(22121, 4.0);  a1["Barb"] = s;   a2[s] = "Barb";

  s = Student(28888, 2.9);  a1["George"] = s; a2[s] = "George";

  s = Student(19981, 3.0);  a1["Dot"] = s;    a2[s] = "Dot";

  s = Student(20012, 2.9);  a1["Sue"] = s;    a2[s] = "Sue";

  string name;

  cout << "Enter a name: ";

  cin >> name;

  grades1::iterator it1 = a1.find(name);

  cout << name << " has ";

  if (it1 == a1.end())

    cout << "no info";

  else

    cout << a1[name];

  cout << endl;



  Student aStudent;

  cout << "Enter a Student's id and GPA: ";

  cin >> aStudent;

  grades2::iterator it2 = a2.find(aStudent);

  cout << "Student " << aStudent << " is ";



  if (it2 == a2.end())

    cout << "no info";

  else

    cout << a2[aStudent];

  cout << endl;



}