Access violation in custom comparator in priority queue

Question

I have an issue were a program of mine throws an access violation. Through debugging I found that for some reason the comparator-function is null. However I am not sure on why that is or how to fix it.

I created a minimum working example to reproduce the issue:

#include "Tryout.h"
#include <queue>

struct Event {
    uint64_t mv_timeout;
};

bool CompareEvents(const Event& a, const Event& b) {
    return a.mv_timeout < b.mv_timeout;
}

int main() {
    std::priority_queue<Event, std::vector<Event>, decltype(&CompareEvents)> mt_eventQueue;
    Event lo_event1{
        .mv_timeout = 1,
    };
    Event lo_event2{
        .mv_timeout = 2,
    };
    mt_eventQueue.push(lo_event1);
    mt_eventQueue.push(lo_event2);
    return 0;
}

When executing it crashes when adding the second event.

Exception thrown at 0x0000000000000000 in Tryout.exe: 0xC0000005: Access violation executing location 0x0000000000000000.

As mentioned it seems the comparing function is null, even tough I passed it via template.
What is going wrong here?

Answer 1

You need to specify the comparison function explicitly in the constructor

std::priority_queue<Event, std::vector<Event>, decltype(&CompareEvents)> mt_eventQueue( CompareEvents );

Otherwise the default argument will be used for the comparison function that will yield a null pointer.

The used constructor has the following declaration with default arguments

explicit priority_queue(const Compare& x = Compare(), Container&& = Container());