Use std::pair as key to std::unordered_map in C++

This post will discuss how to use std::pair as a key to std::unordered_map in C++.

The std::map uses a predicate for key comparison, whose type is specified as the third template parameter of std::map. The default value for this parameter is std::less, which will delegate to operator<. Since operator< is defined for pairs, the following declaration works in C++:

1	std::map<std::pair<std::string, std::string>, int> m;

On the other hand, std::unordered_map throws a compilation error when std::pair is used as a key.

1	std::unordered_map<std::pair<std::string, std::string>, int> m;

This is because std::unordered_map uses std::hash for computing hash value for its keys and there is no specialization of std::hash for std::pair in the C++ standard library. If we want to use a pair as key to std::unordered_map, we can follow any of the following approaches:

1. Define specialization for `std::hash` function

Here, the idea is to define our own specialization for std::hash that works with std::pair.

#include <iostream>

#include <unordered_map>

#include <utility>

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 << std::endl;

}

return 0;

}

Download Run Code

Output:

{Java,Java 8}, 2014
{Java,Java 7}, 2011
{Java,Java 9}, 2017
{C++,C++17}, 2017
{C++,C++14}, 2014
{C++,C++11}, 2011

The above code uses XOR as a hash combination function for simplicity. XOR should be avoided as XOR maps identical values and symmetric pairs to 0 (x ^ x == y ^ y == 0, x ^ y == y ^ x). This would end up with far too many collisions in the real world. For production code, we should shift/rotate one of the hashes before XORing.

2. Using boost Library

Another good alternative is to use the boost::hash from Boost.Functional, which can be used to hash integers, floats, pointers, strings, arrays, pairs, and the 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 << std::endl;

}

return 0;

}

Download Code

Output:

{Java,Java 8}, 2014
{Java,Java 9}, 2017
{Java,Java 7}, 2011
{C++,C++17}, 2017
{C++,C++14}, 2014
{C++,C++11}, 2011

That’s all about using std::pair as key to std::unordered_map in C++.

Also See:

3 ways to use std::pair as a key to std::map in C++

Use std::pair as key to std::unordered_map in C++

1. Define specialization for std::hash function

2. Using boost Library

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1. Define specialization for `std::hash` function