Why does std::sort work when the comparison function uses greater-than (>), but not greater-than-or-equal (>=)?

Question

On WIN32, Visual Studio 2022. When I define a vector<int> containing one hundred 0s and sort it with the code below, an exception "invalid comparator" throws.

vector<int> v(100, 0);
sort(v.begin(), v.end(), [](const int& a, const int& b)
    {
        return a >= b;
    });

However, if I use return a > b, it will execute well. Why is that?

Answer 1

This is just how it is required to work. You need strict weak ordering.

For the rationale, I believe that the sufficient explanation is that this enables you to determine whether those elements are equal (useful for e.g. std::sets). <= or >= can't do that.

<= or >= can also do that, but it seems like it was just decided to use < instead of any other relation. With this decision in mind, standard library facilities are implemented and they heavily rely on it.