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  1. #include <iostream>
  2.  
  3. using namespace std;
  4.  
  5. class base
  6. {
  7. protected:
  8.     int x; /*position*/
  9. public:
  10.     base();
  11.     int getX() const; /*get position*/
  12.     void setX(int); /*set position*/
  13.     friend ostream &operator<<(ostream &, base);
  14. };
  15.  
  16. class A: public base
  17. {
  18.     /*memebers and memeber functions*/
  19. };
  20.  
  21. class B: public base
  22. {
  23. private:
  24.     int (A::*fp_getA_X)() const = &A::getX; /*pointer to memeber function */
  25.     A const * target = nullptr;
  26. public:
  27.     B(const A*);
  28.     void setA(const A*);
  29.     int getA_X() const;
  30.     friend ostream &operator<<(ostream &, B);
  31. };
  32.  
  33. B::B(const A* a): target(a){};
  34.  
  35. void B::setA(const A* a){ target = a; }
  36.  
  37. int B::getA_X() const 
  38. {
  39.     return target->getX();
  40.     /* return (target->*fp_getA_X)(); // alternative */
  41. }
  42. ostream &operator<<(ostream &os, B b)
  43. {
  44.     os << "a's position (" << b.getA_X() << ")";
  45.     return os;
  46. }
  47.  
  48. base::base():x(0){}
  49. int base::getX() const { return x; }
  50. void base::setX(int n) { x = n; }
  51. ostream &operator<<(ostream &os, base obj)
  52. {
  53.     os << "(" << obj.getX() << ")";
  54.     return os;
  55. }
  56.  
  57. int main()
  58. {
  59.     // #: 可以代表任一數字
  60.     /*
  61.     b#們全部一開始得到a1的參考，
  62.     a1.x改變b#們都可以知道，
  63.     而b#也有自己的x (用static_cast<base>(b1)或b1.getX()查看)。
  64.     最後b3變成參考a2，
  65.     a2.x改變b3就可以知道。
  66.     */
  67.     A a1, a2;
  68.     B b1(&a1),b2(&a1),b3(&a1);
  69.     cout << "a1 = " << a1 << endl;
  70.     cout << "b1 : " << b1 << endl;
  71.     cout << "b1 = " << "(" << b1.getX() << ")" << endl;
  72.     cout << "b1 = " << (static_cast<base>(b1)) << endl;
  73.     cout << endl;
  74.  
  75.     a1.setX(2);
  76.     b1.setX(4);
  77.     b2.setX(5);
  78.     b3.setX(6);
  79.     cout << "a1 = " << a1 << endl;
  80.     cout << "b1 : " << b1 << endl;
  81.     cout << "b1 = " << (static_cast<base>(b1)) << endl;
  82.     cout << "b2 = " << (static_cast<base>(b2)) << endl;
  83.     cout << "b3 = " << (static_cast<base>(b3)) << endl;
  84.     cout << endl;
  85.  
  86.     a2.setX(22);
  87.     b3.setA(&a2);
  88.     cout << "a1 = " << a1 << endl;
  89.     cout << "a2 = " << a2 << endl;
  90.     cout << "b1 : " << b1 << endl;
  91.     cout << "b1 = " << (static_cast<base>(b1)) << endl;
  92.     cout << "b2 = " << (static_cast<base>(b2)) << endl;
  93.     cout << "b3 = " << (static_cast<base>(b3)) << endl;
  94.     cout << "b3 : " << b3 << endl;
  95.     cout << endl;
  96.  
  97.     a2.setX(34);
  98.     b3.setX(-2);
  99.     cout << "a2 = " << a2 << endl;
  100.     cout << "b3 : " << b3 << endl;
  101.     cout << "b3 = " << (static_cast<base>(b3)) << endl;
  102.  
  103.     return 0;
  104. }
Success #stdin #stdout 0s 15240KB
comments (?)
stdin 
Standard input is empty
stdout 
a1 = (0)
b1 : a's position (0)
b1 = (0)
b1 = (0)

a1 = (2)
b1 : a's position (2)
b1 = (4)
b2 = (5)
b3 = (6)

a1 = (2)
a2 = (22)
b1 : a's position (2)
b1 = (4)
b2 = (5)
b3 = (6)
b3 : a's position (22)

a2 = (34)
b3 : a's position (34)
b3 = (-2)
https://ideone.com/M7qqoJ
language:
C++ (gcc 8.3)
created:
6 years ago
visibility:
public

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