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

Because it is an unsigned value. There is no -1 for unsigned values. — Sam Varshavchik
– Sam Varshavchik, Commented Nov 20, 2016 at 1:28

sjrowlinson · Accepted Answer · 2016-11-20 01:32:25Z

3

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.

edited Nov 20, 2016 at 1:32

answered Nov 20, 2016 at 1:27

sjrowlinson

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1 Comment

user207421 Over a year ago

It gives you the unsigned decimal representation of 0xffffffffffffffffL. No overflow here.

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