sring对象为vector元素

//4 warning(s) why??#include<iostream>#include<string>#include<vector>#include<algorithm>using namespace std;int main()//int main(int argc,char* argv[])做什么?{vector<string> V;V.push_back("Jack");V.push_back("Mike");cout<<V[0]<<endl;cout<<V[1]<<endl;cout<<V[0][0]<<endl;cout<<V[1].length()<<endl;return 0;}