vc6.0 下关于vector的建议（reserve（））

在看STL标准库时，作者曾说，STL中vector几乎以一种哲学的方式管理其容量的大小，所以在谈到分期摊坏的复杂度时，几乎是可以不考虑的。但在vc6.0中则不然（vc6.0对STL不完美支持是总所周知的）。代码如下:

int main(){std::vector<Person> coll;//coll.reserve(100);coll.push_back(Person("test"));coll.push_back(Person("word"));coll.push_back(Person("rule"));coll.push_back(Person("map"));coll.push_back(Person("set"));coll.push_back(Person("vector"));coll.push_back(Person("list"));coll.push_back(Person("deque"));coll.push_back(Person("michael"));return 0;  }

在Person类中，构造函数调用输出字符“2”，拷贝构造调用输出字符“123”，析构输出“~person”和参数。终端结果如下：

2
123
~person
test
2
123
123
~person
test
~person
word
2
123
123
123
~person
test
~person
word
~person
rule
2
123
~person
map
2
123
123
123
123
123
~person
test
~person
word
~person
rule
~person
map
~person
set
2
123
~person
vector
2
123
~person
list
2
123
~person
deque
2
123
123
123
123
123
123
123
123
123
~person
test
~person
word
~person
rule
~person
map
~person
set
~person
vector
~person
list
~person
deque
~person
michael
2
123
~person
many
148 ms
~person
test
~person
word
~person
rule
~person
map
~person
set
~person
vector
~person
list
~person
deque
~person
michael
~person
many
Press any key to continue

从结果可以看出，每push_back一次，vector的中内容就需重新拷贝一次，vetor所能容纳的元素数量编译器没做优化。

将代码中的第二行屏蔽取消，结果如下：

2
123
~person
test
2
123
~person
word
2
123
~person
rule
2
123
~person
map
2
123
~person
set
2
123
~person
vector
2
123
~person
list
2
123
~person
deque
2
123
~person
michael
2
123
~person
many
48 ms
~person
test
~person
word
~person
rule
~person
map
~person
set
~person
vector
~person
list
~person
deque
~person
michael
~person
many
Press any key to continue

此时对vector容纳元素能力进行设定，没有出现大量的拷贝，故vc6.0下建议vetor声明后即调用reserve();设置，否则插入元素的代价是巨大的。