What conditions must the 'less' comparator meet for the correct operation of the std::sort algorithm?

The "less-than" comparator used in std::sort must satisfy the following conditions for the sorting to be correct:

1. Transitivity: If a < b and b < c, then a < c.
2. Antisymmetry: If a < b is true, then b < a should be false.
3. Irreflexivity: For no a should a < a be true.

In other words, the comparator should define a strict weak ordering. This guarantees that the sorting algorithm can correctly order the elements.

Example of a comparator for sorting a vector of integers in ascending order:

bool less(int a, int b) {
    return a < b;
}

std::sort(vec.begin(), vec.end(), less);

If the comparator violates these properties, the behavior of std::sort is undefined.