size_t is unsigned, so counting down past zero wraps around

Fri, 03 Jul 2026 00:00:00 GMT

I had a loop that walked a vector backwards and it never terminated. The bug is that size_t is unsigned, so it can never be negative — subtracting past zero wraps to a huge positive number instead.

// Broken: when i == 0, --i wraps to SIZE_MAX, so i >= 0 is always true.
for (size_t i = v.size() - 1; i >= 0; --i) {
    use(v[i]);
}

The same trap hides in any v.size() - 1 when the vector is empty: 0u - 1 is SIZE_MAX, not -1, so a "last index" computed that way points way off the end.

One fix is to offset the index by one and count the other variable down, so the loop variable stays in valid unsigned territory:

// Fixed: i is the count remaining; the real index is i - 1.
for (size_t i = v.size(); i > 0; --i) {
    use(v[i - 1]);
}

The general rule: if a value can go negative, don't store it in an unsigned type. Reach for a signed index (or C++20's std::ssize) when you need to count below zero.

Shreyash Rai — #gotchas

size_t is unsigned, so counting down past zero wraps around