C++输入/输出运算符重载

#include <iostream>

using namespace std;

class distance {
   private:
      int feet;             // 0 to infinite
      int inches;           // 0 to 12

   public:
      // required constructors
      distance() {
         feet = 0;
         inches = 0;
      }
      distance(int f, int i) {
         feet = f;
         inches = i;
      }
      frIEnd ostream &operator<<( ostream &output, const distance &D ) { 
         output << "F : " << D.feet << " I : " << D.inches;
         return output;            
      }

      frIEnd istream &operator>>( istream  &input, distance &D ) { 
         input >> D.feet >> D.inches;
         return input;            
      }
};

int main() {
   distance D1(11, 10), D2(5, 11), D3;

   cout << "Enter the value of object : " << endl;
   cin >> D3;
   cout << "First distance : " << D1 << endl;
   cout << "Second distance :" << D2 << endl;
   cout << "Third distance :" << D3 << endl;

   return 0;
}