Question

Possible Duplicate:
Can inner classes access private variables?

So I’m trying to use a priority queue, and in the context of this queue I want to define an integer i to be “less” than another integer j if D[i] < D[j]. How can I do this? (D is a data member of an object)

So far I have

/* This function gets the k nearest neighbors for a user in feature
 * space.  These neighbors are stored in a priority queue and then
 * transferred to the array N. */
void kNN::getNN() {
    int r;
    priority_queue<int, vector<int>, CompareDist> NN;

    /* Initialize priority queue */
    for (r = 0; r < k; r++) {
        NN.push(r);
    }

    /* Look at the furthest of the k users.  If current user is closer,
     * replace the furthest with the current user. */
    for (r = k; r < NUM_USERS; r++) {
        if (NN.top() > r) {
            NN.pop();
            NN.push(r);
        }
    }

    /* Transfer neighbors to an array. */
    for (r = 0; r < k; r++) {
        N[r] = NN.top();
        NN.pop();
    }
}

And in kNN.hh:

class kNN {

private:
    struct CompareDist {
        bool operator()(int u1, int u2) {
            if (D[u1] < D[u2])
                return true;
            else
                return false;
        }
    };
...

However, this is giving me the error

kNN.hh: In member function ‘bool kNN::CompareDist::operator()(int, int)’:
kNN.hh:29: error: invalid use of nonstatic data member ‘kNN::D’

What can I do about this? It seems that C++ doesn’t like it if I refer to specific objects in the comparator, but I have no idea how to solve this without referring to D.

Thanks!

Answer 1

You may pass a reference to the D object into the constructor of the CompareDist object, and then use that D object in operator().

In this sample, I store a pointer to D. Depending upon the type of D, you may want to store a copy of D. (If D is a raw array, the syntax in my sample can be simplified.)

struct CompareDist {
    const DType* pD;
    CompareDist(const DType& D) : pd(&D) {}
    bool operator()(int u1, int u2) {
        return (*pD)[u1] < (*pD)[u2];
    }
};

priority_queue<int, vector<int>, CompareDist> NN(CompareDist(D));