Overloading [] in C++ to return lvalue

Question

I'm writing a simple hash map class:

template <class K, class V> class HashMap;

The implementation is very orthodox: I have a heap array which doubles in size when it grows large. The array holds small vectors of key/value pairs.

Vector<Pair<K, V> > *buckets;

I would like to overload the subscript operator in such a way that code like this will work:

HashMap<int, int> m;
m[0] = 10; m[0] = 20;
m[2] = m[1] = m[0];

In particular,

• For m[k] = v where m does not contain k, I'd like a new entry to be added.
• For m[k] = v where m does contain k, I'd like the old value to be replaced.
• In both of these cases, I'd like the assignment to return v.

Presumably the code will look something like

V& operator[](K &key)
{
    if (contains(key))
    {
        // the easy case
        // return a reference to the associated value
    }
    else
    {
        Vector<Pair<K, V> > *buck = buckets + hash(k) % num_buckets;
        // unfinished
    }
}

How should I handle the case where the key is not found? I would prefer to avoid copying values to the heap if I can.

I suppose I could make a helper class which overloads both the assignment operator and a cast to V, but surely there is a simpler solution?

Edit: I didn't realize that std::map required that the value type have a zero argument constructor. I guess I will just default-construct a value as well.

Answer 1

How should I handle the case where the key is not found?

Insert a new element with that key and return a reference to the value of that new element. Effectively, your pseudocode becomes something equivalent to:

if (!contains(key))
    insert(Pair<K, V>(key, V()));
return reference_to_the_element_with_that_key;

This is exactly what your requirement is, too. You said "For m[k] = v where m does not contain k, I'd like a new entry to be added."

Answer 2

How should I handle the case where the key is not found?

std::map creates a new object, and inserts it into the map, and returns its reference. You can also do the same.

Alternatively, you can throw an exception KeyNotFoundException like the way .NET map throws. Of course, you've to define KeyNotFoundException yourself, possibly deriving from std::runtime_exception.

By the way, as a general rule, always implement operator[] in pair as:

V &operator[](const K &key);
const V &operator[](const K &key) const;

Just for the sake for const-correctness. However, if you decide to create a new object and insert it into the map, when the key is not found, then this rule is not applicable here, as const version wouldn't make sense in this situation.

See this FAQ:

• What's the deal with "const-overloading"?