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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<std::pair<unsigned int, unsigned int> >::unordered_set()'

Answer

The other answers here all suggest building a hash function that somehow combines your two integers.

This will work, but produces non-unique hashes. Though this is fine for your use of unordered_set, for some applications it may be unacceptable. In your case, if you happen to choose a bad hash function, it may lead to many unnecessary collisions.

But you can produce unique hashes!

int is usually 4 bytes. You could make this explicit by using int32_t.

The hash's datatype is std::size_t. On most machines, this is 8 bytes. You can check this upon compilation.

Since a pair consists of two int32_t types, you can put both numbers into an std::size_t to make a unique hash.

That looks like this (I can't recall offhandedly how to force the compiler to treat a signed value as though it were unsigned for bit-manipulation, so I've written the following for uint32_t.):

#include <cassert>
#include <cstdint>
#include <unordered_set>
#include <utility>
struct IntPairHash {
  std::size_t operator()(const std::pair<uint32_t, uint32_t> &p) const {
    assert(sizeof(std::size_t)>=8);  //Ensure that std::size_t, the type of the hash, is large enough
    //Shift first integer over to make room for the second integer. The two are
    //then packed side by side.
    return (((uint64_t)p.first)<<32) | ((uint64_t)p.second);
  }
};
int main(){
  std::unordered_set< std::pair<uint32_t, uint32_t>, IntPairHash> uset;
  uset.emplace(10,20);
  uset.emplace(20,30);
  uset.emplace(10,20);
  assert(uset.size()==2);
}