1：vetor<int> a;

a.reserve(100);

代表預留空間是100，capacity()的返回值也是就100；

vector功能和陣列相似，capacity類似於我們通常預設定的陣列的最大元素個數，如a[10]，代表的就是a陣列最多10個元素；

程式碼如下：

// vector::reserve
#include <iostream>
#include <vector>

int main ()
{
  std::vector<int>::size_type sz;

  std::vector<int> foo;
  sz = foo.capacity();
  std::cout << "making foo grow:\n"
 
;
  for (int i=0; i<100; ++i) {
    foo.push_back(i);
    if (sz!=foo.capacity()) {
      sz = foo.capacity();
      std::cout << "capacity changed: " << sz << '\n';
    }
  }

  std::vector<int> bar;
  sz = bar.capacity();
  bar.reserve(100);   // this is the only difference with foo above
 

  std::cout << "making bar grow:\n";
  for (int i=0; i<100; ++i) {
    bar.push_back(i);
    if (sz!=bar.capacity()) {
      sz = bar.capacity();
      std::cout << "capacity changed: " << sz << '\n';
    }
  }
  return 0;
}

making foo grow:

capacity changed: 1

capacity changed: 2

capacity changed: 4

capacity changed: 8

capacity changed: 16

capacity changed: 32

capacity changed: 64

capacity changed: 128

making bar grow:

capacity changed: 100

2：a.resize(n,m) or a.resize(n)

n代表的就是重新設定vector元素的多少

n大於你之前的size()時，會加上 n-size() 個元素值為m的元素（若m沒有，就設定為預設值0)；

n小於你之前的size()時，會刪除 下標為 n~size()-1 的元素。