Comparators in C++ and priority queues

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!

This question isn't actually about priority queues and comparators, but about how inner classes can access data members of enclosing classes: http://stackoverflow.com/questions/486099/can-inner-classes-access-private-variables, http://stackoverflow.com/questions/1604853/nested-class-access-to-enclosing-class-private-data-members — jogojapan, Nov 28 '12 at 03:44

Robᵩ · Accepted Answer · 2012-11-28T03:41:42.400

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));

Jack · Answer 2 · 2012-11-28T03:48:32.993

-1

Ok, now I've read your question better and I can update the answer: the problem is that D is an object that is contained inside an instance of kNN, hence it is not static and can't be accessed from a static context.

You can solve the problem by using a static reference to D inside the class, something like

// .h
class kNN {
  static Type* current;
  ..

    struct CompareDist {
      bool operator()(int u1, int u2) {
        if ((*current)[u1] < (*current)[u2])
          return true;
        else
          return false;
      }
    };
}

// .cpp
Type *kNN::current = NULL;

void kNN::method()
{
  kNN::current = D; // beforing using the comparator
  ..
}

In addition signature should use elements by const reference, eg

bool operator()(const int &u1, const int &u2)

edited Nov 28 '12 at 03:48

answered Nov 28 '12 at 03:25

Jack

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Sorry, I'm really confused about this. When am I referring to a specific instance of a collection? Also why do I need to pass a reference to an integer if it's just an integer and not an object? – i love stackoverflow Nov 28 '12 at 03:32
I updated the answer to reflect your specific situation, you need to pass by reference to avoid copying the value. – Jack Nov 28 '12 at 03:49
using static variables like this is not a good practice – Timo Nov 28 '12 at 04:54

Comparators in C++ and priority queues

2 Answers2