C++ std::sort custom comparator runs indefinitely when the comparator returns true

Question

So I encountered this very weird behavior on an edge case when sorting a vector using a custom comparator.

When running this code, it will not halt, and goes forever:

int main() {
    auto comp = [](int lhs, int rhs) {
        return true;
    };
    
    vector<int> vec{0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0};
    sort(vec.begin(), vec.end(), comp);
    
    for (int num : vec)
        cout << num;
    
    return 0;
}

however, when I change the true to false, it works perfectly.

auto comp = [](int lhs, int rhs) {
    return false;
};

What's weirder, when I decrease the number of 0's to be sorted, it also works perfectly. (It works with 16 or less 0's, when I add one more 0 to make it 17, the program will not halt again. (Will g++ switch to another sorting algorithm if the length exceeds 16?)

vector<int> vec{0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0};

Why is this the case? Am I missing some important concepts in C++'s sort() function?

Answer 1

This comparator:

auto comp = [](int lhs, int rhs) {
   return true;
};

violates the requirement of std::sort that the comparator must establish a strict-weak-ordering. Note that this function returns true regardless of the order of arguments, implying that 2 elements can both be less than the other, which doesn't really make sense. Violating this requirement of std::sort invokes undefined behavior (this is sufficient to explain the differing behavior you see with different vector sizes).

---

On the other hand, this comparator:

auto comp = [](int lhs, int rhs) {
    return false;
};

is perfectly fine. It basically says that no elements compare less than any other (i.e. they are all equivalent). This satisfies strict-weak-ordering, so std::sort will work just fine with it.

Of course, std::sort won't do anything useful with the second comparator, since all the elements are already "sorted". This might reorder the elements though; but if you use std::stable_sort then the original range is guaranteed to be unchanged.