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

Question

The following program does not compile an unordered set of pairs of integers, but it does for integers. Can unordered_set and its member functions be used on us

Answer 1

The problem is that std::unordered_set is using std::hash template to compute hashes for its entries and there is no std::hash specialization for pairs. So you will have to do two things:

1. Decide what hash function you want to use.
2. Specialize std::hash for your key type (std::pair) using that function.

Here is a simple example:

#include 

namespace std {
template <> struct hash> {
    inline size_t operator()(const std::pair &v) const {
        std::hash int_hasher;
        return int_hasher(v.first) ^ int_hasher(v.second);
    }
};

}

int main()
{
    std::unordered_set< std::pair > edge;
}