free function or member function for overloading += operator?

Hi,

I have the following two programs. One is to overload += as a free
function, the other is to overload += as a member function. Both of
them works fine.

I'm wondering which one I should choose in practice. What are the pros
and cons for these two implementations?

Thanks,
Peng

$ cat main_free.cc
#include <iostream>

class Integer {
  public:
    Integer(int i) : _i(i) {}
    friend Integer &operator+=(Integer &, int);
    friend std::ostream &operator<<(std::ostream &, const Integer &);
  private:
    int _i;
};

Integer & operator+=(Integer & I, int i) {
  I._i += i;
  return I;
}

std::ostream &operator<<(std::ostream &os, const Integer &I) {
  os << "Integer: " << I._i;
  return os;
}

int main ()
{
  Integer I(0);
  std::cout << I << std::endl;
  I += 10;
  std::cout << I << std::endl;
}

$ cat main_member.cc
#include <iostream>

class Integer {
  public:
    Integer(int i) : _i(i) {}
    Integer &operator+=(int i) {
      _i += i;
      return *this;
    }
    friend std::ostream &operator<<(std::ostream &, const Integer &);
  private:
    int _i;
};

std::ostream &operator<<(std::ostream &os, const Integer &I) {
  os << "Integer: " << I._i;
}

int main ()
{
  Integer I(0);
  std::cout << I << std::endl;
  I += 10;
  std::cout << I << std::endl;
}

$ ./main_free.exe
Integer: 0
Integer: 10
$ ./main_member.exe
Integer: 0
Integer: 10