Question

I have been stuck on this problem for many hours, but I have a feeling there is a trivial solution that I am not seeing. I am trying to use a priority_queue to create a MinHeap of pointers to a struct I defined. My issue lies in having to overload the > operator for the pointer to this struct (which I know is not possible) in order to match the priority_queue template. This is my last resort attempt to use the stl priority_queue before I decided to ditch it and write my own heap code.

The relevant snippets of my code are: (i) part the struct definition:

typedef struct Node Node;
struct Node
{
    int frequency;
    bool operator>( const Node& other ) const{
        return frequency > other.frequency;
    }
};

(ii) priority queue initialization:

priority_queue<Node*,vector<Node*>,greater<Node*> > q;

And (iii) the for loop constructing the initial heap (assume int_array has been initialized):

for (int i = 0; i < SIZEOFINTARRAY; i++)
{
    Node *n = new Node;
    n->frequency = int_array[i];
    q.push(n);
}

Currently, the elements in this “heap” are returned FIFO, and no sorting happens. I assume this is because the priority comparison checks the pointer and elements earlier in an array are located lower in memory. Please give me any tips on how I could accomplish this.

PS. Sorry if this post does not adhere to stackoverflow standards (I tried my best to follow the rules but it is my first post). I welcome all criticism so I can never make the same mistakes again.

Answer 1

a generic way would be to implement a sort of deref_greater which is just like std::greater but dereferences the input args first.

template<class T>
struct deref_greater : public std::binary_function<T, T, bool>
{
  bool operator()( const T& _Left, const T& _Right ) const
  {
    return *_Left > *_Right;
  }
};