关于auto的一些使用方法 ：

1 数组

int a[] = {1,2,3,4,5,6};

for(auto item : a)
    cout << item << endl;

等价于

for(int i = 0; i < 6; i++)
    cout << a[i] << endl;

2、字符串

    string str = "hello world";

    for(auto ch : str)
        cout << ch <<endl;

等价于

    for(int i = 0; i <str.size(); i++)
        cout << str[i] <<endl;

3、vector

    vector<int> m_v = {1, 2, 3, 4};
    for(auto e : m_v)
        cout << e <<endl;

等价于

    for(int i = 0; i < m_v.size(); i++)
            cout <<v[i]<<endl;

4、map

    map<int,string> m = {{1, "abc"}, {2, "bca"}, {3, "cab"}};
    for(auto e : m)
            cout <<e.first<<"  "<< e.second<<endl;

等价于

    for(map<int, string>::iterator it = m.begin();it != m.end(); it++)
        cout <<it->first<<"  "<<it->second<<endl;