returning constant references from class methods

[celavek via Digitalmars-d-learn]

Hi,

I'm trying the following code:
class counter
{
public:

    final ulong[char] nucleotide_counts () const
    {
        return cached_counts;
    }

private:
    ulong[char] cached_counts;
}

void main()
{
}

I get the following error from the compiler:

   Error: cannot implicitly convert expression 
(this.cached_counts) of type const(ulong[char]) to ulong[char]

I tried making the class variable constant, I tried the const in 
the return type but I can't seem to get it right.

I would like to return a constant reference to the internal 
associative array - much like in C++. How would I do that in D?