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 user-defined types, and how can I define it?

#include <unordered_set>
...

class A{
...
private: 
    std::unordered_set< std::pair<int, int> > u_edge_;
};

Compiler error:

error: no matching function for call to 'std::unordered_set >::unordered_set()'

Answer 1

Your code compiles on VS2010 SP1 (VC10), but it fails to compile with GCC g++ 4.7.2.

However, you may want to consider boost::hash from Boost.Functional to hash a std::pair (with this addition, your code compiles also with g++).

#include <unordered_set>
#include <boost/functional/hash.hpp>

class A
{
private: 
    std::unordered_set< 
        std::pair<int, int>, 
        boost::hash< std::pair<int, int> > 
    > u_edge_;
};