重载小于（<）运算符以几种方式对对象进行排序

Question

If a class has overloaded an operator to facilitate sorting its objects by a particular attribute, is there a way to overload the operator again to sort by another attribute?

For example, the class below has overloaded operator< to compare its data member minutes, is there a way to do the same for its data member hours or would I just be better off creating a binary predicate for each sorting criteria? Thanks in advance.

class PhoneCall {
  friend ostream& operator<<( ostream&,const PhoneCall& );
  private:
      int minutes;
      int hours;

  public:
      PhoneCall(int = 0);
      bool operator<(const PhoneCall&);
};

ostream& operator<<(ostream& out, const PhoneCall& p) {
   out << "Phone call lasted " << p.minutes << " minutes" << endl;
   return out;
}

PhoneCall::PhoneCall(int ct) {
    minutes = ct;
}

bool PhoneCall::operator<(const PhoneCall& p) {
    bool less = (minutes < p.minutes)? true: false;
    return less;
}

Answer 1

You can also present extra friend functions that provide alternative sorting methods, and in c++11 you can define them inplace:

class PhoneCall {
 friend ostream& operator<<( ostream&,const PhoneCall& );
 private:
  int minutes;
  int hours;

 public:
  PhoneCall(int = 0);
  bool operator<(const PhoneCall&);
  friend bool LessTime(const PhoneCall& L, const PhoneCall& R)
  {  return L.minutes+L.hours*60 < R.minutes+R.hours*60; }

};

std::vector<PhoneCall> calls;
std::sort(calls.begin(), calls.end(), LessTime);

Also, you can overload the comparator on ordered containers such as set:

std::multiset<PhoneCall, LessTime> timeSet;

The use of an inline friend here, compared to a public static method is of minimal benefit. The real benefit of inline friend is when doing operator overloads, such as operator <<

Answer 2

No, what you want is not possible. Instead, std::sort provides two overloads. One that uses operator< by default and an overload which takes a functor for the comparison:

template< class RandomIt, class Compare >
void sort( RandomIt first, RandomIt last, Compare comp );

This can be used as such:

std::vector<PhoneCall> calls;
std::sort(calls.begin(), calls.end(), [](const PhoneCall& lhs, const PhoneCall& rhs) {
    return lhs.hours < rhs.hours;
});

Notice the lambda function as the third parameter does compare by hours.