C++ Primer, Fifth Edition

11.3.1. Associative Container Iterators

When we dereference an iterator, we get a reference to a value of the container’s value_type. In the case of map, the value_type is a pair in which first holds the const key and second holds the value:

// get an iterator to an element in word_countauto map_it = word_count.begin();// *map_it is a reference to a pair<const string, size_t> objectcout << map_it->first;          // prints the key for this elementcout << " " << map_it->second;  // prints the value of the elementmap_it->first = "new key";      // error: key is const++map_it->second;     // ok: we can change the value through an iterator

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