How can I make an unordered set of pairs of integers in C++?

There is no standard way of computing a hash on a pair. Add this definition to your file:

struct pair_hash {
    inline std::size_t operator()(const std::pair<int,int> & v) const {
        return v.first*31+v.second;
    }
};

Now you can use it like this:

std::unordered_set< std::pair<int, int>,  pair_hash> u_edge_;

This works, because pair<T1,T2> defines equality. For custom classes that do not provide a way to test equality you may need to provide a separate function to test if two instances are equal to each other.

Of course this solution is limited to a pair of two integers. Here is a link to an answer that helps you define a more general way of making hash for multiple objects.