vector 的内存管理以及顺序容器的选择

1. vector  size 相关的接口：

与容器大小相关的函数有以下几个：

size()   //返回当前元素个数

max_size()    // 当前能存储元素的最多个数

capacity()     // 容器在重新获得更多存储空间时，可以存储的元素总个数

reserve()     //指定vector 预留多少个元素的存储空间 ，当reserve ()< capacity() 时没有动作

shrink_to_fit()   //C++11新增接口，释放掉当前多余的capacity, 调用后 size() = capacity()

empty()   // 判断当前vector 是否为空，等价于size() = 0;

2. vector 内存管理方式方式

2.1 vector容器为了获取快速的随机访问，元素必须在内存中连续存储。

2.2 为了避免每次增加或删除元素时都需要重新开辟存储空间， vector 实际分配的内存比当前需要的内存会多一些，即capacity()  一般会大于size()

2.3 vector 新增元素时， 若size 没有超出capacity, 则不需要重新分配空间 

2.4 当size 超过capacity 时，  capacity 一般会自动扩展为当前的2倍，或者也可以通过reserve() 来人工指定vector预留多少空间

2.5 默认空的vector capacity 为0，然后增加到1，然后增加的规律是2倍；  若有同感reserve() 制定初始大小，则在初始大小的基础上double

2.6 max_size() 反应了在特定机器上vector 对象理论上能存储元素个数的上限，maxsize: 1073741823

3. 当调用pop_back() 接口释放掉存储的元素时，std::vector 不会重新分配内存，减少capacity()

    网上有的说法是，增加是double ,减少时每次减1/4， std::vector 是不会减少的

以下示例程序比较清晰的揭示了这个过程：

typedef vector<int> vectors;

void printVectInfo(vectors& ivec)

{

   cout<<"size: "<<ivec.size()<<endl;

   cout<<"maxsize: "<<ivec.max_size()<<endl;

   cout<<"capacity: "<<ivec.capacity()<<endl;

   cout<<endl;

}

int main(int argc, char* argv[])

{

   vector<int> ivec;

   printVectInfo(ivec);

   for(vectors::size_type i= 0 ; i!= 50 ; ++i)

   {

       ivec.push_back(i);

       printVectInfo(ivec);

   }

   return 0;

}

程序执行结果：

size: 0

maxsize: 1073741823

capacity: 0

size: 1

maxsize: 1073741823

capacity: 1

size: 2

maxsize: 1073741823

capacity: 2

size: 3

maxsize: 1073741823

capacity: 4

size: 4

maxsize: 1073741823

capacity: 4

size: 5

maxsize: 1073741823

capacity: 8

size: 6

maxsize: 1073741823

capacity: 8

size: 7

maxsize: 1073741823

capacity: 8

size: 8

maxsize: 1073741823

capacity: 8

size: 9

maxsize: 1073741823

capacity: 16

size: 10

maxsize: 1073741823

capacity: 16

size: 11

maxsize: 1073741823

capacity: 16

size: 12

maxsize: 1073741823

capacity: 16

size: 13

maxsize: 1073741823

capacity: 16

size: 14

maxsize: 1073741823

capacity: 16

size: 15

maxsize: 1073741823

capacity: 16

size: 16

maxsize: 1073741823

capacity: 16

size: 17

maxsize: 1073741823

capacity: 32

size: 18

maxsize: 1073741823

capacity: 32

size: 19

maxsize: 1073741823

capacity: 32

size: 20

maxsize: 1073741823

capacity: 32

size: 21

maxsize: 1073741823

capacity: 32

size: 22

maxsize: 1073741823

capacity: 32

size: 23

maxsize: 1073741823

capacity: 32

size: 24

maxsize: 1073741823

capacity: 32

size: 25

maxsize: 1073741823

capacity: 32

size: 26

maxsize: 1073741823

capacity: 32

size: 27

maxsize: 1073741823

capacity: 32

size: 28

maxsize: 1073741823

capacity: 32

size: 29

maxsize: 1073741823

capacity: 32

size: 30

maxsize: 1073741823

capacity: 32

size: 31

maxsize: 1073741823

capacity: 32

size: 32

maxsize: 1073741823

capacity: 32

size: 33

maxsize: 1073741823

capacity: 64

size: 34

maxsize: 1073741823

capacity: 64

size: 35

maxsize: 1073741823

capacity: 64

size: 36

maxsize: 1073741823

capacity: 64

size: 37

maxsize: 1073741823

capacity: 64

size: 38

maxsize: 1073741823

capacity: 64

size: 39

maxsize: 1073741823

capacity: 64

size: 40

maxsize: 1073741823

capacity: 64

size: 41

maxsize: 1073741823

capacity: 64

size: 42

maxsize: 1073741823

capacity: 64

size: 43

maxsize: 1073741823

capacity: 64

size: 44

maxsize: 1073741823

capacity: 64

size: 45

maxsize: 1073741823

capacity: 64

size: 46

maxsize: 1073741823

capacity: 64

size: 47

maxsize: 1073741823

capacity: 64

size: 48

maxsize: 1073741823

capacity: 64

size: 49

maxsize: 1073741823

capacity: 64

size: 50

maxsize: 1073741823

capacity: 64

正常使用vector 过程中，vector 的capacity() 是按照2的次幂来增加的

===================================================================

在上面的代码中加入人工干预的reserve() :

int main(int argc, char* argv[])

{

   vector<int> ivec;

   printVectInfo(ivec);

   ivec.reserve(10);

   for(vectors::size_type i= 0 ; i!= 50 ; ++i)

   {

       ivec.push_back(i);

       printVectInfo(ivec);

   }

   return 0;

}

运行结果：

size: 0

maxsize: 1073741823

capacity: 0

size: 1

maxsize: 1073741823

capacity: 10

size: 2

maxsize: 1073741823

capacity: 10

size: 3

maxsize: 1073741823

capacity: 10

size: 4

maxsize: 1073741823

capacity: 10

size: 5

maxsize: 1073741823

capacity: 10

size: 6

maxsize: 1073741823

capacity: 10

size: 7

maxsize: 1073741823

capacity: 10

size: 8

maxsize: 1073741823

capacity: 10

size: 9

maxsize: 1073741823

capacity: 10

size: 10

maxsize: 1073741823

capacity: 10

size: 11

maxsize: 1073741823

capacity: 20

size: 12

maxsize: 1073741823

capacity: 20

size: 13

maxsize: 1073741823

capacity: 20

size: 14

maxsize: 1073741823

capacity: 20

size: 15

maxsize: 1073741823

capacity: 20

size: 16

maxsize: 1073741823

capacity: 20

size: 17

maxsize: 1073741823

capacity: 20

size: 18

maxsize: 1073741823

capacity: 20

size: 19

maxsize: 1073741823

capacity: 20

size: 20

maxsize: 1073741823

capacity: 20

size: 21

maxsize: 1073741823

capacity: 40

size: 22

maxsiz
4000
e: 1073741823

capacity: 40

size: 23

maxsize: 1073741823

capacity: 40

size: 24

maxsize: 1073741823

capacity: 40

size: 25

maxsize: 1073741823

capacity: 40

size: 26

maxsize: 1073741823

capacity: 40

size: 27

maxsize: 1073741823

capacity: 40

size: 28

maxsize: 1073741823

capacity: 40

size: 29

maxsize: 1073741823

capacity: 40

size: 30

maxsize: 1073741823

capacity: 40

size: 31

maxsize: 1073741823

capacity: 40

size: 32

maxsize: 1073741823

capacity: 40

size: 33

maxsize: 1073741823

capacity: 40

size: 34

maxsize: 1073741823

capacity: 40

size: 35

maxsize: 1073741823

capacity: 40

size: 36

maxsize: 1073741823

capacity: 40

size: 37

maxsize: 1073741823

capacity: 40

size: 38

maxsize: 1073741823

capacity: 40

size: 39

maxsize: 1073741823

capacity: 40

size: 40

maxsize: 1073741823

capacity: 40

size: 41

maxsize: 1073741823

capacity: 80

size: 42

maxsize: 1073741823

capacity: 80

size: 43

maxsize: 1073741823

capacity: 80

size: 44

maxsize: 1073741823

capacity: 80

size: 45

maxsize: 1073741823

capacity: 80

size: 46

maxsize: 1073741823

capacity: 80

size: 47

maxsize: 1073741823

capacity: 80

size: 48

maxsize: 1073741823

capacity: 80

size: 49

maxsize: 1073741823

capacity: 80

size: 50

maxsize: 1073741823

capacity: 80

reserve() 会打乱vector本身2的n次方增长规律，但reserve后也是每次增加为先前的2倍大小

======================================================

2.顺序容器选择

容器元素在内存中是否连续存储会显著影响：

1.  在容器中间添加或删除元素的代价

2.  随机访问元素的代价

连续存储时， 可以进行快速的随机访问，但删除或添加元素的代价很高（重新分配空间，元素拷贝）；

离散存储时，可以添加或删除元素的代价很低，随机访问代价很高（需要遍历整个链表）；

三种顺序容器特点：

vector: 连续存储， 支持元素的快速随机访问

list: 离散存储， 快速的添加删除元素

deque: vector 和list 综合，支持元素随机访问，以及高效的在两端进行插入或删除

顺序容器选择：

1.  若程序要求随机访问元素， vector 或deque

2.  要求容器中间位置添加或删除元素，list

3.  首尾添加或删除元素，deque

4. 输入时在中间插入元素，然后随机访问元素，则输入时使用list ,然后对list排序， 再将list 复制到vector(使用assign实现）

PS . 将上面打印size(),capacity() 的函数形参改为非引用类型时，发现size() 和capacity() 一直一样，因为拷贝形参副本时，不会预留空间

void printVectInfo(vectors  ivec)

{

   cout<<"size: "<<ivec.size()<<endl;

   cout<<"maxsize: "<<ivec.max_size()<<endl;

   cout<<"capacity: "<<ivec.capacity()<<endl;

   cout<<endl;

}