vector<int> ivec;
cout << "大小：" << ivec.size() << ",容量" << ivec.capacity() << endl;//0,0
ivec.push_back(10);
cout << "大小：" << ivec.size() << ",容量" << ivec.capacity() << endl;//1,1
ivec.push_back(20);
cout << "大小：" << ivec.size() << ",容量"
 
 << ivec.capacity() << endl;//2,2
ivec.push_back(30);
cout << "大小：" << ivec.size() << ",容量"<<ivec.capacity() << endl;//3,3
ivec.push_back(40);
cout << "大小：" << ivec.size() << ",容量" << ivec.capacity() << endl;//4,4
ivec.push_back(50);
cout << "大小："
 
 << ivec.size() << ",容量" << ivec.capacity() << endl;//5,5
ivec.push_back(60);
cout << "大小：" << ivec.size() << ",容量" << ivec.capacity() << endl;//6,6
ivec.push_back(70);
cout << "大小：" << ivec.size() << ",容量" << ivec.capacity() << endl;//7,9
 

ivec.push_back(80);
cout << "大小：" << ivec.size() << ",容量" << ivec.capacity() << endl;//8,9
ivec.push_back(40);
cout << "大小：" << ivec.size() << ",容量" << ivec.capacity() << endl;//9,9
ivec.push_back(40);
cout << "大小：" << ivec.size() << ",容量" << ivec.capacity() << endl;//10,13
ivec.push_back(1);
ivec.push_back(1);
ivec.push_back(1);
cout << "大小：" << ivec.size() << ",容量" << ivec.capacity() << endl;//13,13
ivec.push_back(1);
cout << "大小：" << ivec.size() << ",容量" << ivec.capacity() << endl;//14,19

capacity() 和size()區別：
size()為容器實際元素的個數
capacity()為容器目前所能容納的最大元素個數，一旦容量不夠了，就會重新分配一段連續的比原有空間大的記憶體大小，將原來的元素拷貝到新的容器空間內，繼續新增，每一次容量的增長50%

reserve(n)
按需申請n個大小的容量，此時容量capacity就變為n