Question

What is the behavior of std::sort when used with ints that are equal is it going to keep them in the same order or just do some unpredictable stuff?

Answer 1

std::sort doesn’t preserve the order of the equivalent elements, std::stable_sort does. However, in case of int‘s you will not notice the difference unless you use some non-trivial ordering as in the following example:

struct half_less
{
    bool operator()(int a, int b) const { return (a / 2) < (b / 2); }
};

std::sort(begin, end, half_less());

Here is another example when std::stable_sort is a more suitable candidate than std::sort