c++ - 自动，错误 : map iterator has no member named ‘first`

Question

map<string, int> M;
for (auto E: M) 
{ 
    cout << E.first << ": " << E.second << endl; 
    F << E.first << ": " << E.second << endl; 
}; 

我正在学习 c++，但我对 auto 感到困惑。我正在尝试将上面的代码转换成下面的代码(上面的自动代码可以正常工作)

map<string, int> M;
for (map<string, int> :: iterator p = begin(M); p != end(M); p ++ )
{
    cout << p.first << ": " << p.second << endl;
    F << p.first << ": " << p.second << endl;
}

我收到以下错误:

 error: ‘std::map<std::basic_string<char>, int>::iterator’ has no member named ‘first’
     cout << p.first << ": " << p.second << endl;
 error: ‘std::map<std::basic_string<char>, int>::iterator’ has no member named ‘second’
     cout << p.first << ": " << p.second << endl;
 error: ‘std::map<std::basic_string<char>, int>::iterator’ has no member named ‘first’
     F << p.first << ": " << p.second << endl;
 error: ‘std::map<std::basic_string<char>, int>::iterator’ has no member named ‘second’
     F << p.first << ": " << p.second << endl;

为什么它不起作用？

Answer 1

迭代器就像指针一样，必须取消引用才能使用:

cout << p->first << ": " << p->second << endl;

ranged-for 循环(带有 auto 的示例)为您做了这个。