Question

I add the constructor and two functions to the class of my previous linked question C++ iterate through a template Map and I need help at this points:

• What do you reckon this constructor does?
• Adding one value at the beginning of map?
• I see though in the respective key only an address as value after initializing in main. What is wrong?

The operator [] is supposed to get the values for a specific key. However I cannot use it so as to get the elements of the map in the output. Any hint?

template<class K, class V>
class template_map{
public:
    template_map( V const& val) {
        m_map.insert(my_map.begin(),std::make_pair(std::numeric_limits<K>::min(),val));
    };    

    typedef typename std::map<K,V> TMap;

    TMap my_map;
    typedef typename TMap::const_iterator const_iterator;
    const_iterator begin() const { return my_map.begin(); }
    const_iterator end() const   { return my_map.end(); }
    V const& operator[]( K const& key ) const {
        return ( --my_map.upper_bound(key) )->second;
    }

    ...
};

int main()
{
    interval_map<int,int> Map1 (10);
    //Show the elements of the map?
}

Consider also that it should be a function that inserts values to the map.

Answer 1

What do you reckon this constructor does? Adding one value at the beginning of map?

It initialises the map so that map[x] == v for any x. The map associates intervals with values, internally storing a normal map keyed by the start of each interval; it’s initialised so that the entire range of the key type maps to the initial value.

I see though in the respective key only an address as value after initializing in main. What is wrong? The operator [] is supposed to get the values for a specific key. However I cannot use it so as to get the elements of the map in the output. Any hint?

I’ve no idea what you’re asking there. If you try, for example, cout << Map1[42] << '\n';, then your program should output 10, since that is the initial value assigned to the entire range of integers.

Consider also that it should be a function that inserts values to the map.

Since the internal map is publicly exposed, you can add a new interval to the map with

Map1.my_map.insert(std::make_pair(interval_start, value));

It might be more polite to make my_map private, and provide an insert() function to do that. You could also add a non-const overload of operator[] that inserts a new range and returns a reference to its value, something like

V & operator[](K const & key) {
    V const & old_value = (--my_map.upper_bound(key))->second;
    return *my_map.insert(std::make_pair(key, old_value)).first;
}

although this might not be a great idea, as you’d have to be careful that you don’t accidentally insert many ranges when you only want to read the values.

My problem is how to iterate through the map to get all its elements and print them in main. It shows me an address with a value of the object initialization.

Remembering that an iterator over a map refers to a key/value pair (of type std::pair<K,V>), you should be able to iterator over the map like this:

for (auto it = Map1.begin(); it != Map1.end(); ++it) {
    std::cout << it->first << " maps to " << it->second << '\n';
}

(in C++03, you’ll need to write template_map<int,int>::const_iterator rather than auto).