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.  

C++ Class基本架構

常常在使用C++ Class，但是常常會忘記架構，以下使用個簡單的範例來展示class的基本結構。

1. #include <iostream>
2. #include <string>
3. #include <cstdlib>
4. using namespace std;
5. ////////////////////////////////////////////////////
6. class Base
7. {
8. public:
9.     //constructor
10.     Base(int num=0,string str=""):
11.         base_id(num), base_name(str){
12.         cout<<"Base construction\n";
13.     }
14.  
15.     //Destructor
16.     //若Base class的destructor沒有設定為virtual
17.     //則derived clss的destructor不會被呼叫，無法運作
18.     virtual ~Base(){
19.         cout<<"Base destroyed\n";
20.     }
21.  
22.     //overloading
23.     virtual void print(){
24.         cout<<"base id = "<<base_id
25.             <<" name = "<<base_name<<endl;
26.     }
27.  
28.     //abstract method, 必須是virtual，否則無法compile
29.     virtual void printall() = 0;
30.  
31. protected:
32.     int base_id;
33.     string base_name;
34. };
35. ////////////////////////////////////////////////////
36. class Derived : public Base
37. {
38. public:
39.     //constructor
40.     Derived(int num=0, string str="", int val=0):
41.         Base(num, str), derived_id(val){
42.         cout<<"Derived construction"<<endl;
43.     }
44.  
45.     //Destructor
46.     virtual ~Derived(){
47.         cout<<"Derived destroyed\n";
48.     }
49.  
50.     virtual void print(){
51.         cout<<"derived id = "<<derived_id<<endl;
52.     }
53.  
54.     //Derived class必須實作來自Base class的abstract method
55.     //否則在程式中class無法被implementation
56.     void printall(){
57.         cout<<"hello world"<<endl;
58.     }
59.  
60. protected:
61.     int derived_id;
62. };
63. ////////////////////////////////////////////////////
64. int main()
65. {
66.     Base *ptr = new Derived(1, "derived", 2);
67.  
68.     ptr->print();    //call Derived print()
69.     ptr->printall();
70.     delete ptr;
71.  
72.     return EXIT_SUCCESS;
73. }

• 在建構Derived class時，會先呼叫Base class的constructor，再呼叫Derived class的constructor，而destroy object時是相反的方向。
• 當Base class的method全部為abstract method時，可將Base class視為Java的interface來使用。