ERROR: AddressSanitizer: heap-use-after-free without ever using free()?

Question

I am learning c++ right now, trying to solve 239. Sliding Window Maximum on Leetcode. Keep encountering ERROR: AddressSanitizer: heap-use-after-free even though I have never used free() anywhere in my code. What could be the problem here?

My code:

#include <vector>
#include <deque>
class Solution {
public:
    vector<int> maxSlidingWindow(vector<int>& nums, int k) {
        if (k == 1) {
            return nums;
        }

        deque<int> q = {nums[k-1]};
        deque<int> qi = {k-1};
        for (int i = k-2; i >= 0; i--) {
            if (nums[i] > q.front()) {
                q.push_front(nums[i]);
                qi.push_front(i);
            }
        }
        vector<int> output = {q.front()};

        for (int i = k; i < nums.size(); i++) {
            if (qi.front() < i - k + 1) {
                q.pop_front();
                qi.pop_front();
            }
            if (q.size() > 0) {
                while (q.back() <= nums[i]) {
                    q.pop_back();
                    qi.pop_back();
                }
            }
            q.push_back(nums[i]);
            qi.push_back(i);

            output.push_back(q.front());
        }
        return output;
    }
};

Full error message:

=================================================================
==23==ERROR: AddressSanitizer: heap-use-after-free on address 0x6150000009fc at pc 0x000000356a92 bp 0x7ffd9138ecb0 sp 0x7ffd9138eca8
READ of size 4 at 0x6150000009fc thread T0
    #2 0x7f1796504082  (/lib/x86_64-linux-gnu/libc.so.6+0x24082)
0x6150000009fc is located 508 bytes inside of 512-byte region [0x615000000800,0x615000000a00)
freed by thread T0 here:
    #4 0x7f1796504082  (/lib/x86_64-linux-gnu/libc.so.6+0x24082)
previously allocated by thread T0 here:
    #4 0x7f1796504082  (/lib/x86_64-linux-gnu/libc.so.6+0x24082)
Shadow bytes around the buggy address:
  0x0c2a7fff80e0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c2a7fff80f0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c2a7fff8100: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd
  0x0c2a7fff8110: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd
  0x0c2a7fff8120: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd
=>0x0c2a7fff8130: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd[fd]
  0x0c2a7fff8140: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c2a7fff8150: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd
  0x0c2a7fff8160: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd
  0x0c2a7fff8170: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd
  0x0c2a7fff8180: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd
Shadow byte legend (one shadow byte represents 8 application bytes):
  Addressable:           00
  Partially addressable: 01 02 03 04 05 06 07 
  Heap left redzone:       fa
  Freed heap region:       fd
  Stack left redzone:      f1
  Stack mid redzone:       f2
  Stack right redzone:     f3
  Stack after return:      f5
  Stack use after scope:   f8
  Global redzone:          f9
  Global init order:       f6
  Poisoned by user:        f7
  Container overflow:      fc
  Array cookie:            ac
  Intra object redzone:    bb
  ASan internal:           fe
  Left alloca redzone:     ca
  Right alloca redzone:    cb
  Shadow gap:              cc
==23==ABORTING

Answer 1

So the error occurs when q.back() tries to access the last element of q when q is empty. After I changed this part of the code:

            if (q.size() > 0) {
                while (q.back() <= nums[i]) {
                    q.pop_back();
                    qi.pop_back();
                }
            }

to this:

            while (!q.empty()) {
                if (q.back() <= nums[i]) {
                    q.pop_back();
                    qi.pop_back();
                } else {
                    break;
                }
            }

the problem is resolved.