priority_queues in C++

I'm a bit of a noob to iterators. I'm trying to create a priority_queue, sorted by vector length. (I.e., I want to pop off the longest vectors in order.)

This is the resource that I've been using:

http://www.cplusplus.com/reference/stl/priority_queue/priority_queue/

I tried this code, and it seems to do what I want:

// testing to make sure that a priority queue will always give me the longest vector
priority_queue< vector<int> > q;

vector<int> f;
f.push_back(1);

vector<int> g;
g.push_back(19);
g.push_back(80);

vector<int> y;
y.push_back(62);
y.push_back(10);
y.push_back(11);

q.push(f);
q.push(g);
q.push(y);

vector<int> out = q.top();

for (unsigned int i = 0; i < out.size(); i++) {
    cout << out[i] << endl;
}

My questions: 1. Will this always give me the longest vector? This seems to be the case. 2. 开发者_运维技巧If not, what else should I do? The iterator syntax on the reference page is like... o_O

Thanks!!

---

No, the code doesn't do what you expect. It compares the vectors lexicographically rather than by length. To compare by length use a custom comparator:

struct LengthCompare {
    bool operator() (const vector<int>& a, const vector<int>& b) {
        return a.size() < b.size();
    }
};

priority_queue<vector<int>, vector<vector<int> >, LengthCompare> q;

Also note that your queue stores copies of the vectors, which might be not so efficient because it may copy them when it builds the heap. Store (smart) pointers instead.

---

priority_queues in C++ use a Comparison object to determine what is the greatest element. By default, this is the < (less-than) operator over the objects held in the priority_queue - so you need to know what < means over vectors. This page http://www.cplusplus.com/reference/stl/vector/operators/ has some information about that.