C++ priority_queue size() issue

Question

I tried to get the size of an empty priority_queue. Something strange happened. Could anybody explain why this happened? Thanks a lot.

#include <iostream>
#include <queue>
using namespace std;

int main()
{
    priority_queue<int, vector<int>, less<int> > asc_queue;
    cout << asc_queue.size() << " " << asc_queue.size() - 1 << endl;
}

Output:

0 18446744073709551615

Answer 1

std::priority_queue::size() returns the size of the container as a std::size_t (technically the size_type of the underlying container of the priority queue) which is essentially an unsigned int - therefore trying to minus 1 from an empty container size gives you the unsigned decimal representation of 0xffffffffffffffffL which is why you get the large value you see.