leetcode692，这个优先队列到底是怎么用的？

class Solution {
public:
    vector<string> topKFrequent(vector<string>& words, int k) {
        vector<string> res(k);
        unordered_map<string, int> freq;
        auto cmp = [](pair<string, int>& a, pair<string, int>& b) {
            return a.second > b.second || (a.second == b.second && a.first < b.first);
        };
        //这里，我在csdn中找的别的例子一般也就是把函数单独写出来然后放着里。为什么会用到decltype呢？为什么q后面也要写(cmp)呢？C++reference中也没找到相关的例子
        priority_queue<pair<string, int>, vector<pair<string, int>>, decltype(cmp) > q(cmp);
        for (auto word : words) ++freq[word];
        for (auto f : freq) {
            q.push(f);
            if (q.size() > k) q.pop();
        }
        for (int i = res.size() - 1; i >= 0; --i) {
            res[i] = q.top().first; q.pop();
        }
        return res;
    }
};