c++ operator重载的例子

#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;
}