目录
昨天面试,面试官问到一个问题,vector怎么扩容的,我大概记得C++ premier有那么一句扩两倍。我说好像是扩两倍,面试官也没继续问也没反馈对错。甚是好奇对错,索性自己试验一把,并看下源码。
试验
代码
#include<iostream> #include<vector> using namespace std; int main(int argc, char *argv[]) { vector<int> vInt; for(int i = 0; i < 100; i++ ) { cout << "size:" << vInt.size() << "\t" << "capacity:" << vInt.capacity() << endl; vInt.push_back(1); } return 0; }
输出
size:0 capacity:0 size:1 capacity:1 size:2 capacity:2 size:3 capacity:4 size:4 capacity:4 size:5 capacity:8 size:6 capacity:8 size:7 capacity:8 size:8 capacity:8 size:9 capacity:16 size:10 capacity:16 size:11 capacity:16 size:12 capacity:16 size:13 capacity:16 size:14 capacity:16 size:15 capacity:16 size:16 capacity:16 size:17 capacity:32 size:18 capacity:32 ......
源码
在这只把扩容相关代码贴出来罢了。
//SGI STL v3.3 void push_back(const _Tp& __x) { if (_M_finish != _M_end_of_storage) { construct(_M_finish, __x); ++_M_finish; } else _M_insert_aux(end(), __x); } iterator insert(iterator __position, const _Tp& __x) { size_type __n = __position - begin(); if (_M_finish != _M_end_of_storage && __position == end()) { construct(_M_finish, __x); ++_M_finish; } else _M_insert_aux(__position, __x); return begin() + __n; } template <class _Tp, class _Alloc> void vector<_Tp, _Alloc>::_M_insert_aux(iterator __position, const _Tp& __x) { if (_M_finish != _M_end_of_storage) { construct(_M_finish, *(_M_finish - 1)); ++_M_finish; _Tp __x_copy = __x; copy_backward(__position, _M_finish - 2, _M_finish - 1); *__position = __x_copy; } else { const size_type __old_size = size(); //原容量为0扩为1,否则扩为原2倍 const size_type __len = __old_size != 0 ? 2 * __old_size : 1; iterator __new_start = _M_allocate(__len); iterator __new_finish = __new_start; __STL_TRY { __new_finish = uninitialized_copy(_M_start, __position, __new_start); construct(__new_finish, __x); ++__new_finish; __new_finish = uninitialized_copy(__position, _M_finish, __new_finish); } __STL_UNWIND((destroy(__new_start,__new_finish), _M_deallocate(__new_start,__len))); destroy(begin(), end()); _M_deallocate(_M_start, _M_end_of_storage - _M_start); _M_start = __new_start; _M_finish = __new_finish; _M_end_of_storage = __new_start + __len; } }
在push_back或者inster的时候,vetor容量不够扩容为原长度两倍。
这个增长因子和各版本STL实现有关,有的版本用1.5
为什么是扩为原长度2倍
因为每次扩容都涉及到原来元素的copy。空间和时间的权衡。
简单来说, 空间分配的多,平摊时间复杂度低,但浪费空间也多。
具体参见算法导论中,平摊分析那一章关于动态表扩张的分析。
STL里面的有序容器的扩展都是一个模子,vector如此,hashmap也是如此。
生成了新的空间,然后再挨个插入过去。