Question

I have code like this:

class MapIndex
{
private:
    typedef std::map<std::string, MapIndex*> Container;
    Container mapM;

public:
    void add(std::list<std::string>& values)
    {
        if (values.empty()) // sanity check
            return;

        std::string s(*(values.begin()));
        values.erase(values.begin());
        if (values.empty())
            return;

        MapIndex *&mi = mapM[s];  // <- question about this line
        if (!mi)
            mi = new MapIndex();
        mi->add(values);
    }
}

The main concern I have is whether the mapM[s] expression would return reference to NULL pointer if new item is added to the map?

The SGI docs say this: data_type& operator[](const key_type& k)
Returns a reference to the object that is associated with a particular key. If the map does not already contain such an object, operator[] inserts the default object data_type().

So, my question is whether the insertion of default object data_type() will create a NULL pointer, or it could create an invalid pointer pointing somewhere in the memory?

Answer 1

It’ll create a NULL (0) pointer, which is an invalid pointer anyway 🙂