C ++迭代映射

Question

As known the following code is used to iterate map in C++

for (std::map<char,int>::iterator it=mymap.begin(); it!=mymap.end(); ++it)
{
    std::cout << itr->first << " => " << itr->second << '\n';
}

Where itr is declared as std::map::iterator . The members first and second are declared neither in std::map nor in std::iterator . Then how is it available for access?

Answer 1

The elements of an std::map are std::pair<key_type, mapped_type> , so de-referencing a map iterator gives you a reference to one of these.

It is the std::pair class template which has first and second members.

Answer 2

The basic idea behind iterators is that they are "magical" objects used to access data, that behave like pointers do on an array - ie you use arithmetic operators (eg ++ and -- ) to move around and you dereference (using * and -> ) to access the data.

So, itr is "like" a pointer to an std::pair<char, int> , so you can access the data dereferencing it via the * operator (which yields the key/value pair ) or with the -> operator, as in your example.