利用std::allocator实现自定义的vector类

Posted 2020-09-24

std::allocator即空间配置器，用于内存分配。更多的细节建议大家研究相关源码。

这里仅是利用std::allocator来实现简单的自定义vector类，如有问题欢迎指正。

 1 #include <iostream>
 2 #include <memory>
 3 using std::cout;
 4 using std::endl;
 5 
 6 template <typename Tp>
 7 class Vector
 8 {
 9 public:
10     Vector()
11     : _elems(NULL)
12     , _first_free(NULL)
13     , _end(NULL)
14     {}
15 
16     ~Vector()
17     {
18         if(_elems)
19         {
20             while(_elems != _first_free)
21                 _alloc.destroy(--_first_free);
22             _alloc.deallocate(_elems,capacity()); 
23         }
24     }
25     
26     void push_back(const Tp & value)
27     {
28         if(size() == capacity())
29         {
30             reallocate();
31         }
32         _alloc.construct(_first_free++,value);
33     }
34 
35     void pop_back()
36     {
37         if(size() > 0)
38         {
39             _alloc.destory(--_first_free); 
40         }
41     }
42 
43     size_t size() const
44     {
45         return _first_free - _elems;
46     }
47     size_t capacity() const
48     {
49         return _end - _elems;
50     }
51 
52     Tp & operator[](size_t idx)
53     {
54         return _elems[idx];
55     }
56 
57 private:
58     void reallocate()
59     {
60         size_t oldCapacity = capacity();
61         size_t newCapacity = oldCapacity == 0 ? 1 : oldCapacity * 2;
62 
63         Tp * newElems = _alloc.allocate(newCapacity);
64         if(_elems)
65         {
66             std::uninitialized_copy(_elems,_first_free,newElems);
67             while(_elems != _first_free)
68                 _alloc.destroy(--_first_free);
69             _alloc.deallocate(_elems,oldCapacity);
70         }
71         _elems = newElems;
72         _first_free = _elems + oldCapacity;
73         _end = _elems + newCapacity;
74     }
75 private:
76     static std::allocator<Tp> _alloc;
77     Tp * _elems;
78     Tp * _first_free;
79     Tp * _end;
80 };
81 
82 template <typename Tp>
83 std::allocator<Tp> Vector<Tp>::_alloc;
84 
85 void display(Vector<int> & vec)
86 {
87     cout << "vec‘s size = " << vec.size() << endl;
88     cout << "vec‘s capacity = " << vec.capacity() << endl;
89 }

以下是测试代码：

 1 int test（）
 2 {
 3     Vector<int> vecInt;
 4     display(vecInt);
 5 
 6     vecInt.push_back(1);
 7     display(vecInt);
 8     vecInt.push_back(2);
 9     display(vecInt);
10     vecInt.push_back(3);
11     display(vecInt);
12     vecInt.push_back(4);
13     display(vecInt);
14     vecInt.push_back(5);
15     display(vecInt);
16     vecInt.push_back(6);
17     display(vecInt);
18     vecInt.push_back(7);
19     display(vecInt);
20 
21     for(size_t idx = 0; idx != vecInt.size(); ++idx)
22     {
23         cout << vecInt[idx] << " ";
24     }
25     cout << endl;
26     return 0;
27 }

测试结果：

 1 vec‘s size = 0
 2 vec‘s capacity = 0
 3 vec‘s size = 1
 4 vec‘s capacity = 1
 5 vec‘s size = 2
 6 vec‘s capacity = 2
 7 vec‘s size = 3
 8 vec‘s capacity = 4
 9 vec‘s size = 4
10 vec‘s capacity = 4
11 vec‘s size = 5
12 vec‘s capacity = 8
13 vec‘s size = 6
14 vec‘s capacity = 8
15 vec‘s size = 7
16 vec‘s capacity = 8
17 1 2 3 4 5 6 7