c++ operator重载

#include <iostream>
using namespace std;
class A
{
public:
 A(double _data = 0.0) :
  data(_data)
 {

 }
 A& operator =(const A& rhs)
 {
  data = rhs.data;
  return *this;
 }
 friend A operator +(const A& lhs, const A& rhs);
 friend A operator -(const A& lhs, const A& rhs);
 friend A operator *(const A& lhs, const A& rhs);
 friend A operator +(const A& lhs, double rhs);
 friend A operator +(double lhs, const A& rhs);
 friend A operator *(const A& lhs, double rhs);
 friend A operator *(double lhs, const A& rhs);
 friend A operator -(const A& lhs, double rhs);
 friend A operator -(double lhs, const A& rhs);
 friend ostream& operator <<(ostream& fout, A& a);
 //  A& operator += (const A& rhs);
 //  A& operator -= (const A& rhs);
 //  A& operator *= (const A& rhs);
private:
 double data;
};
A operator +(const A& lhs, const A& rhs)
{
 A res(0);
 res.data = lhs.data + rhs.data;
 return res;
}
A operator -(const A& lhs, const A& rhs)
{
 A res(0);
 res.data = lhs.data - rhs.data;
 return res;
}
A operator *(const A& lhs, const A& rhs)
{
 A res(0);
 res.data = lhs.data * rhs.data;
 return res;
}
A operator +(const A& lhs, double rhs)
{
 A res(0);
 res.data = lhs.data + rhs;
 return res;
}
A operator +(double lhs, const A& rhs)
{
 A res(0);
 res.data = lhs + rhs.data;
 return res;
}
A operator *(const A& lhs, double rhs)
{
 A res(0);
 res.data = lhs.data * rhs;
 return res;
}
A operator *(double lhs, const A& rhs)
{
 A res(0);
 res.data = lhs * rhs.data;
 return res;
}
A operator -(const A& lhs, double rhs)
{
 A res(0);
 res.data = lhs.data - rhs;
 return res;
}
A operator -(double lhs, const A& rhs)
{
 A res(0);
 res.data = lhs - rhs.data;
 return res;
}
ostream& operator <<(ostream& fout, A& a)
{
 fout << a.data;
 return fout;
}
int main(int argc, char* argv[])
{
 A a(2.3);
 A b(1.2);
 A d(3.4);
 A c;
 c = a + b + d;
 c = a + b;
 c = a + 1.0;
 c = a - b;
 c = a - 1.0;
 c = a * b;
 c = a * 1.0;
 cout << c << endl;
 c = 1.0 + 2.0 * a * a - 3.0 * a * b;
 cout << c << endl;
 return 0;
}

 

#include <string>
#include <fstream>
#include <iostream>
using namespace std;
class Person
{
public:
 Person(string name, string sex, string age) :
  _name(name), _sex(sex), _age(age)
 {
 }
 string getName() const
 {
  return _name;
 }
 string getSex() const
 {
  return _sex;
 }
 string getAge() const
 {
  return _age;
 }
private:
 string _name;
 string _sex;
 string _age;
};

ostream& operator<<(ostream &c, const Person& p) //写成iostream会出错的
{
return c << p.getName() << "\t" << p.getSex() << "\t" << p.getAge() << "\n";
}

int main()
{
 ofstream outfile("out.txt");
 if (!outfile)
 {
  cout << "create or open file error" << endl;
  exit(-1);
 }
 Person p(string("aa"), string("Man"), string("13"));
 outfile << p;
 return 0;
}