Best implement to use pair as key to std::unordered_map in C++

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Why XOR is a weak way to hash C++ pairs, and a reusable hash_combine template for unordered_map, unordered_set, tuples, and custom types.

Reference: C++ Standard Library: A tutorial and reference, Second version Chapter 7.9.2: Creating and Controlling unordered Container

All solutions I found in Google use XOR to generate hashcode of pair, which is totally bad. see why-is-xor-the-default-way-to-combine-hashes.

A crude crossing chute drops several paired shapes into the same bin while a layered mixing mill distributes other pairs cleanly across separate compartments

However, the book has given us the best solution, using hash_combine, which is taken from Boost. The solution is much better than XOR when I tested it in Online Judge(Atcoder). I organized the code as a template as follow. You can copy and paste it as much as you can. And it is convenient to change it to fit any custom struct/class.

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#include <functional>
// from boost (functional/hash):
// see http://www.boost.org/doc/libs/1_35_0/doc/html/hash/combine.html template
template <typename T>
inline void hash_combine(std::size_t &seed, const T &val) {
    seed ^= std::hash<T>()(val) + 0x9e3779b9 + (seed << 6) + (seed >> 2);
}
// auxiliary generic functions to create a hash value using a seed
template <typename T> inline void hash_val(std::size_t &seed, const T &val) {
    hash_combine(seed, val);
}
template <typename T, typename... Types>
inline void hash_val(std::size_t &seed, const T &val, const Types &... args) {
    hash_combine(seed, val);
    hash_val(seed, args...);
}

template <typename... Types>
inline std::size_t hash_val(const Types &... args) {
    std::size_t seed = 0;
    hash_val(seed, args...);
    return seed;
}

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

#include <bits/stdc++.h>
using namespace std;
using ll = long long;

int main() {
    unordered_map<pair<ll, ll>, ll, pair_hash> slopeCount;
    unordered_set<pair<ll, ll>, pair_hash> seen;
    return 0;
}

There is a hash implementation for Tuple. I updated the answer inStackOverflow。Please go there if you need hash tuple.