C++ Operator overloaded

Operator overloaded是C++獨特的功能，能夠把operator重新定義，符合class的需求，但是在定義的時候要注意是否違反class原本的意義。

• Rule of thumb, if a class needs a destructor, it will also need the assignment  operator and a copy constructor. C++ Primer P.485
• If the operator often does the same work, the common work should be put in private utility functions.

1. #include <iostream>
2. #include <cstdlib>
3. using namespace std;
4. //////////////////////////////////////////////////////
5. class Point{
6. public:
7.     //constructor
8.     Point(double xval=0.0, double yval=0.0):
9.         x(xval), y(yval){}
10.  
11.     //copy constructor
12.     Point(const Point &rhs):
13.         x(rhs.x), y(rhs.y){}
14.  
15.     //destructor
16.     ~Point(){}
17.  
18.     //must return a new object
19.     friend Point
20.     operator+ (const Point &lhs, const Point &rhs){
21.         Point ret(lhs);
22.         //call overloaded operator +=
23.         ret += rhs;
24.         return ret;
25.     }
26.  
27.     //must return reference
28.     Point& operator+= (const Point &rhs){
29.         this->x += rhs.x;
30.         this->y += rhs.y;
31.         return *this;
32.     }
33.  
34.     //prefix operator,return a reference
35.     Point& operator++ (){
36.         this->x += 1.0;
37.         this->y += 1.0;
38.         return *this;
39.     }
40.  
41.     //posfix operator++, return a new object
42.     Point operator++(int){
43.         Point ret(*this);
44.         ++*this;
45.         return ret;
46.     }
47.  
48.     friend inline bool
49.     operator== (const Point &lhs, const Point &rhs){
50.         return lhs.x == rhs.x && lhs.y == rhs.y;
51.     }
52.  
53.     friend inline bool
54.     operator!= (const Point &lhs, const Point &rhs){
55.         return !(lhs == rhs);
56.     }
57.  
58.     //input operators must deal with errors and EOF
59.     friend istream&
60.     operator>> (istream &in, Point &obj){
61.         in>>obj.x>>obj.y;
62.         if(!in)
63.             obj = Point();
64.         return in;
65.     }
66.  
67.     //IO operators must be nomember functions
68.     //should not print a newline
69.     friend ostream&
70.     operator<< (ostream &os, const Point &obj){
71.         os<<"("<<obj.x<<", "<<obj.y<<")";
72.         return os;
73.     }
74.  
75. private:
76.     double x, y;
77. };
78. //////////////////////////////////////////////////////
79.