pair<int,int> pair as key of unordered_map issue

Question

My code:

 typedef pair<int,int> Pair
  tr1::unordered_map<Pair,bool> h;
  h.insert(make_pair(Pair(0,0),true));

Erorr

 undefined reference to `std::tr1::hash<std::pair<int, int> >::operator()(std::pair<int, int>) const'

Something I need to fix?

thanks

Answer 1

This happens because there is no specialization for std::tr1::hash<Key> with Key = std::pair<int, int>. You must to specialize std::tr1::hash<Key> with Key = std::pair<int, int> before declaring tr1::unordered_map<Pair,bool> h;. This happens because std don't know how to hash a pair<int, int>.

Following there is a example of how to specialize std::tr1::hash<>

template <>
struct std::tr1::hash<std::pair<int, int> > {
public:
        size_t operator()(std::pair<int, int> x) const throw() {
             size_t h = SOMETHING;//something with x   
             return h;
        }
};

Answer 2

Unordered Map does not contain a hash function for a pair, So if we want to hash a pair then we have to explicitly provide it with a hash function that can hash a pair.

If we want to use pair as a key to unordered_map, there are 2 ways to do it :

1. Define specializaion for std::hash

typedef std::pair<std::string,std::string> pair;

struct pair_hash
{
    template <class T1, class T2>
    std::size_t operator() (const std::pair<T1, T2> &pair) const
    {
        return std::hash<T1>()(pair.first) ^ std::hash<T2>()(pair.second);
    }
};

int main()
{
    std::unordered_map<pair,int,pair_hash> unordered_map =
    {
        {{"C++", "C++11"}, 2011},
        {{"C++", "C++14"}, 2014},
        {{"C++", "C++17"}, 2017},
        {{"Java", "Java 7"}, 2011},
        {{"Java", "Java 8"}, 2014},
        {{"Java", "Java 9"}, 2017}
    };

    for (auto const &entry: unordered_map)
    {
        auto key_pair = entry.first;
        std::cout << "{" << key_pair.first << "," << key_pair.second << "}, "
                  << entry.second << '\n';
    }

    return 0;
}

2. Using Boost Library Another good way is to use boost::hash from Boost.functional which can be used to hash integers,floats,pointers,strings,arrays,pairs and theh STL containers.

#include <iostream>
#include <boost/functional/hash.hpp>
#include <unordered_map>
#include <utility>

typedef std::pair<std::string,std::string> pair;

int main()
{
    std::unordered_map<pair,int,boost::hash<pair>> unordered_map =
    {
        {{"C++", "C++11"}, 2011},
        {{"C++", "C++14"}, 2014},
        {{"C++", "C++17"}, 2017},
        {{"Java", "Java 7"}, 2011},
        {{"Java", "Java 8"}, 2014},
        {{"Java", "Java 9"}, 2017}
    };

    for (auto const &entry: unordered_map)
    {
        auto key_pair = entry.first;
        std::cout << "{" << key_pair.first << "," << key_pair.second << "}, "
                  << entry.second << '\n';
    }

    return 0;
}