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

You need to provide a specialization for std::hash<> that works with std::pair. Here is a very simple example of how you could define the specialization:

#include 
#include 

namespace std
{
    template<>
    struct hash>
    {
        size_t operator () (std::pair const& p)
        {
            // A bad example of computing the hash, 
            // rather replace with something more clever
            return (std::hash()(p.first) + std::hash()(p.second));
        }
    };
}

class A
{
private:
    // This won't give you problems anymore
    std::unordered_set< std::pair > u_edge_;
};