当增加新元素（s）时，如果超过当时的容量，则容量会扩充至两倍。如果两倍容量仍不足，，就扩张至足够大的容量。
以一个例子来说明：
#include<iostream>#include<vector>#include<algorithm>using namespace std;int main(){int i;vector<int> iv(2,9);cout<<"size = "<<iv.size()<<endl;cout<<"capacity = "<<iv.capacity()<<endl;iv.push_back(1);cout<<"size = "<<iv.size()<<endl;cout<<"capacity = "<<iv.capacity()<<endl;iv.push_back(2);cout<<"size = "<<iv.size()<<endl;cout<<"capacity = "<<iv.capacity()<<endl;iv.push_back(3);cout<<"size = "<<iv.size()<<endl;cout<<"capacity = "<<iv.capacity()<<endl;iv.push_back(4);cout<<"size = "<<iv.size()<<endl;cout<<"capacity = "<<iv.capacity()<<endl;for(i = 0;i < iv.size();i++)cout<<iv[i]<<" ";cout<<endl;iv.push_back(5);cout<<"size = "<<iv.size()<<endl;cout<<"capacity = "<<iv.capacity()<<endl;for(i = 0;i < iv.size();i++)cout<<iv[i]<<" ";cout<<endl;iv.pop_back();iv.pop_back();cout<<"size = "<<iv.size()<<endl;cout<<"capacity = "<<iv.capacity()<<endl;iv.pop_back();cout<<"size = "<<iv.size()<<endl;cout<<"capacity = "<<iv.capacity()<<endl;vector<int>::iterator ivite = find(iv.begin(),iv.end(),1);if(ivite != iv.end())iv.erase(ivite);cout<<"size = "<<iv.size()<<endl;cout<<"capacity = "<<iv.capacity()<<endl;for(i = 0;i < iv.size();i++)cout<<iv[i]<<" ";cout<<endl;ivite = find(iv.begin(),iv.end(),2);if(ivite != iv.end())iv.insert(ivite,3,7);cout<<"size = "<<iv.size()<<endl;cout<<"capacity = "<<iv.capacity()<<endl;for(i = 0;i < iv.size();i++)cout<<iv[i]<<" ";cout<<endl;iv.clear();cout<<"size = "<<iv.size()<<endl;cout<<"capacity = "<<iv.capacity()<<endl;return 0;}
读书破万卷，下笔如有神。
