C++类中静态数据成员的未定义引用问题

longdeqidao

1. #include <iostream>
   
2. #include <vector>
   
3. #include <string>
   
4. #include <algorithm>
   
5. #include <functional>
   
6. 
   
7. 
   
8. using namespace std;
   
9. 
   
10. class num_sequence {
    
11. public:
    
12.     virtual ~num_sequence() {}
    
13. 
    
14.     virtual int elem(int pos) const = 0;
    
15.     virtual const char *what_am_i() const = 0;
    
16.     static int max_elems() { return _max_elems; }
    
17.     virtual ostream &print(ostream &os = cout) const = 0;
    
18. 
    
19. protected:
    
20.     virtual void gen_elems(int pos) const = 0;
    
21.     bool check_integrity(int pos, int size)  const;
    
22. 
    
23.     const static int _max_elems = 1024;
    
24. };
    
25. 
    
26. bool num_sequence::
    
27. check_integrity(int pos, int size) const
    
28. {
    
29.     if (pos <= 0 || pos > _max_elems)
    
30.     {
    
31.         cerr << "!! invalid position: " << pos
    
32.              << "Cannot honor request\n";
    
33.         return false;
    
34.     }
    
35.     if (pos > size)
    
36.         gen_elems(pos);
    
37. 
    
38.     return true;
    
39. }
    
40. 
    
41. ostream& operator << (ostream &os, const num_sequence &ns)
    
42. {
    
43.     return ns.print(os);
    
44. }
    
45. 
    
46. //class Fabonacci
    
47. 
    
48. class Fibonacci: public num_sequence {
    
49. public:
    
50.     Fibonacci(int len = 1, int beg_pos = 1):
    
51.         _length(len), _beg_pos(beg_pos) {}
    
52. 
    
53.     virtual int elem(int pos) const;
    
54.     virtual const char *what_am_i() const { return "Fibonacci"; }
    
55.     virtual ostream &print(ostream &os = cout) const;
    
56.     int length() const { return _length; }
    
57.     int beg_pos() const { return _beg_pos; }
    
58. 
    
59. protected:
    
60.     virtual void gen_elems(int pos) const;
    
61.     int _length;
    
62.     int _beg_pos;
    
63.     static vector<int> _elems;            //-----HERE-------
    
64. };
    
65. 
    
66. 
    
67. int Fibonacci::
    
68. elem(int pos) const
    
69. {
    
70.     if (!check_integrity(pos, _elems.size()))
    
71.         return 0;
    
72.     if (pos > _elems.size())
    
73.         Fibonacci::gen_elems(pos);
    
74. 
    
75.     return _elems[pos - 1];
    
76. }
    
77. 
    
78. void Fibonacci::
    
79. gen_elems(int pos) const
    
80. {
    
81.     if (_elems.empty()) {
    
82.         _elems.push_back(1);
    
83.         _elems.push_back(1);
    
84.     }
    
85. 
    
86.     if (_elems.size() <= pos) {
    
87.         int ix = _elems.size();
    
88.         int n1 = _elems[ix - 1];
    
89.         int n2 = _elems[ix - 2];
    
90. 
    
91.         while (ix <= pos) {
    
92.             int elem = n1 + n2;
    
93.             _elems.push_back(elem);
    
94.             n2 = n1;
    
95.             n1 = elem;
    
96.             ix++;
    
97.         }
    
98.     }
    
99. }
    
100. 
     
101. ostream &Fibonacci::
     
102. print(ostream &os) const
     
103. {
     
104.     int elem_pos = _beg_pos - 1;
     
105.     int end_pos = elem_pos + _length;
     
106. 
     
107.     if (end_pos > _elems.size())
     
108.         Fibonacci::gen_elems(end_pos);
     
109. 
     
110.     while (elem_pos < end_pos)
     
111.         os << _elems[elem_pos++] << ' ';
     
112. }
     
113. 
     
114. int main()
     
115. {
     
116.     Fibonacci fib;
     
117. 
     
118.     cout << "fib: beginning at element 1 for 1 element: "
     
119.          << fib << endl;
     
120. 
     
121.     Fibonacci fib2(16);
     
122.     cout << "fib2: beginning at elemet 1 for 16 elements: "
     
123.          << fib2 << endl;
     
124. 
     
125.     Fibonacci fib3;
     
126.     cout << "fib3: beginning at element 12 for 8 elements: "
     
127.          << fib3 << endl;
     
128. 
     
129.     return 0;
     
130. }
     

复制代码

错误提示的：undefined reference to `Fibonacci::_elems'
这是书上的代码。静态成员应该会自动初始化呀。
就算我在外部定义vector<int> Fibonacci::_elems(0);
编译通过，不过有段错误！