vector的capacity增长方式

vector的capacity增长方式

 

vector的capacity()调用返回vector中最大能够存储的元素个数，也即在下一次需要扩充容量之前能容纳的元素个数。reserve会使容器在必要的时候增长，以便容纳制指定数目的元素。

 

例程：

#include <iostream>

#include <vector>

 

using namespace std;

 

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

{

 vector<int> v, u;

 

 v.reserve(5);

 cout << v.capacity() << "/n";   //5 

 v.resize(10);

 cout << v.capacity() << "/n";   //10

 

 cout << "Initial capaciry of u: " << u.capacity() << "/n";

 for(int i = 0; i < 40; ++i) {

    u.push_back(i);

    cout << "Now capacity is :" << u.capacity() << "/n";

 }

 

 return 0;

}

 

上面的代码在Dev-C++和VS2005中分别打印如下的结果：

 

Dev-C++

5

10

Initial capaciry of u: 0

Now capacity is :1

Now capacity is :2

Now capacity is :4

Now capacity is :4

Now capacity is :8

Now capacity is :8

Now capacity is :8

Now capacity is :8

Now capacity is :16

Now capacity is :16

Now capacity is :16

Now capacity is :16

Now capacity is :16

Now capacity is :16

Now capacity is :16

Now capacity is :16

Now capacity is :32

Now capacity is :32

Now capacity is :32

Now capacity is :32

Now capacity is :32

Now capacity is :32

Now capacity is :32

Now capacity is :32

Now capacity is :32

Now capacity is :32

Now capacity is :32

Now capacity is :32

Now capacity is :32

Now capacity is :32

Now capacity is :32

Now capacity is :32

Now capacity is :64

Now capacity is :64

Now capacity is :64

Now capacity is :64

Now capacity is :64

Now capacity is :64

Now capacity is :64

Now capacity is :64

 

VS2005

5

10

Initial capaciry of u: 0

Now capacity is :1

Now capacity is :2

Now capacity is :3

Now capacity is :4

Now capacity is :6

Now capacity is :6

Now capacity is :9

Now capacity is :9

Now capacity is :9

Now capacity is :13

Now capacity is :13

Now capacity is :13

Now capacity is :13

Now capacity is :19

Now capacity is :19

Now capacity is :19

Now capacity is :19

Now capacity is :19

Now capacity is :19

Now capacity is :28

Now capacity is :28

Now capacity is :28

Now capacity is :28

Now capacity is :28

Now capacity is :28

Now capacity is :28

Now capacity is :28

Now capacity is :28

Now capacity is :42

Now capacity is :42

Now capacity is :42

Now capacity is :42

Now capacity is :42

Now capacity is :42

Now capacity is :42

Now capacity is :42

Now capacity is :42

Now capacity is :42

Now capacity is :42

Now capacity is :42

 

从执行结果看到，对于reserve调用（包括resize），两个编译器都是将capacity扩大到恰好容纳指定的元素。Exceptional C++ Style第一条中说对reserve的调用产生的容量可能大于指定的值，这应该是因为标准只规定了至少要满足指定的值，至于具体的值则由实现自己决定。

 

但对于普通的添加元素而导致的增长，两者的方式就不一样了。Dev-C++（GCC）采用的是2^n这么个公式，即按1,2,4,8,16,32,64这样的规律增长；而VS2005应该也是按某种规律增长，具体公式没看出来！VS2005的方式在某些情况下可能稍微节省一点内存，但相应的可能会需要更多次地扩充内存，也许效率稍差一点。