vector的capacity增長方式
vector的capacity()調用返回vector中最大能夠存儲的元素個數，也即在下一次需要擴充容量之前能容納的元素個數。reserve會使容器在必要的時候增長，以便容納制指定數目的元素。
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;
}
上面的代碼在Dev-C++和VS2005中分別打印如下的結果：
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
從執行結果看到，對於reserve調用（包括resize），兩個編譯器都是將capacity擴大到恰好容納指定的元素。Exceptional C++ Style第一條中說對reserve的調用產生的容量可能大於指定的值，這應該是因爲標準只規定了至少要滿足指定的值，至於具體的值則由實現自己決定。
但對於普通的添加元素而導致的增長，兩者的方式就不一樣了。Dev-C++（GCC）採用的是2^n這麼個公式，即按1,2,4,8,16,32,64這樣的規律增長；而VS2005應該也是按某種規律增長，具體公式沒看出來！VS2005的方式在某些情況下可能稍微節省一點內存，但相應的可能會需要更多次地擴充內存，也許效率稍差一點。