priority_queue emplace gives segmentation fault

Question

The following code gives segmentation fault, could someone enlighten me? All I wanted to achieve is to have a priority queue sorted with ascending order of tv.t or descending order of tv.m.

struct tv {
    int m;
    int c;
    int t;
    tv(int mm, int cc, int tt): m(mm), c(cc), t(tt) {}
};


bool comp(struct tv & t1 , struct tv & t2) {
    if (t1.t == t2.t) {
        return t1.m < t2.m;
    }
    return t1.t > t2.t;
}


int main(int argc, char** argv) {
    priority_queue<struct tv, vector<struct tv>, decltype(&comp)> q;
    q.emplace(0, 0, 0);
    q.emplace(1, 0, 0);
    q.emplace(1, 1, 1);
    q.emplace(1, 2, 0);
    return 0;
}

Answer 1

You gave your priority queue a comparator type in the template argument list, but you didn't give it an actual comparator in the constructor.

priority_queue<tv, vector<tv>, decltype(&comp)> q(comp);

As you have it, it is calling a null function pointer. This is one nice thing about using function objects instead of function pointers. They can be default constructed and do the right thing.

struct comp_t {
    bool operator()(const tv & t1 , const tv & t2) const {
        if (t1.t == t2.t) {
            return t1.m < t2.m;
        }
        return t1.t > t2.t;
    }
};

priority_queue<tv, vector<tv>, comp_t> q;