指针类型的priority_queue元素的排序

Question

Let's say we have a priority_queue that holds a bunch of ListNode objects declared as below:

class ListNode {
  int val;
  ListNode *next;
public:
  explicit ListNode(int v) : val(v), next(NULL) {}
  inline bool operator<(const ListNode& rhs) const {
    return val < rhs.val;
  }
};

std::priority_queue<ListNode> pq;

By overriding operator< method or providing a sorting functor we can have the priority_queue hold the ListNode objects in val's ascending order.

My question is if the priority_queue holds the pointers to ListNode class instead can I have the pointers sorted so that the val's pointed are in ascending order. How do I do that?

std::priority_queue<ListNode *> pq1;

Thanks!

Answer 1

As you said, std::priority_queue accepts as third template parameter a comparison functor that it has to use to perform the comparisons.

Just write your own that dereferences the items before comparing them:

template<typename T>
struct PtrLess
{
    bool operator()(const T* left, const T* right)
    {
        return *left < *right;
    }
};


std::priority_queue<ListNode *, std::vector< ListNode * >, PtrLess< ListNode > > pq1;

Answer 2

A pointer to ListNode is like an everyday pointer. You cannot overload an operator between two pointers.

However, you can override the comparison operator for the purpose of the priority_queue . It would go something like this:

struct ListNodePtrLess {
    bool operator()(const ListNode* a, const ListNode* b) {
        return a->val < b->val;
    }
};

typedef std::priority_queue<ListNode*, std::vector<ListNode*>, ListNodePtrLess> MyPriorityQueue;

(also: you will need to make ListNodePtrLess a friend of ListNode , or let it access the val field in some different way)