Question

I’m reading “Ivor Horton’s Beginning Programming Visual C++ 2010”, and I’m at Chapter 10-The Standard Template Library. My problem is with the map container map<Person, string> mapname. The book showed me a lot of ways of adding elements to it, such as with the pair<K, T> and using the make_pair() function later, and mapname.insert(pair). But suddenly he introduced an element adding technique used in the following code:

int main()
{
    std::map<string, int> words
    cout << "Enter some text and press Enter followed by Ctrl+Z then Enter to end:"
        << endl << endl;

    std::istream_iterator<string> begin(cin);
    std::istream_iterator<string> end;

    while(being != end)   // iterate over words in the stream
        //PROBLEM WITH THIS LINE:
        words[*begin++]++;  // Increment and store a word count

    //there are still more but irrelevant to this question)
}

The indicated line is my problem. I understand that words is the map, but I’ve never seen such initialization. And what’s going on in that thing with its increment. I believe Ivor Horton failed to elaborate this further, or at least he should’ve given introductions big enough not to suprise noobs like me.

Answer 1

You have such a map:

sts::map<std::string, int> m;

The access operator [key] gives you a reference to the element stored with that key, or inserts one if it doesn’t exist. So for an empty map, this

m["hello"];

inserts an entry in the map, with key “Hello” and value 0. It also returns a reference to the value. So you can increment it directly:

m["Bye"]++;

would insert a value 0 under key “Bye” and increment it by one, or increment an existing value by 1.

As for the stuff happening inside the [] operator,

*begin++

is a means to incrementing the istream_iterator and dereferencing the value before the increment:

begin++;

increments begin and returns the value before the increment

*someIterator

dereferences the iterator.