std::hash<std::pair<int, int> >
阿新 • • 發佈：2022-03-27
標題是搞笑的
! 這個問題只需要 since C++11
問題：怎麼讓 unordered_map 支援使用 pair 作為 key？
如果你能把兩個東西壓到一個基本型別裡那麼就不用解決這個問題了 .
我們需要手寫一個 Hash 函式吧 .
如果你用 xor 會被輕易卡掉
注意 unordered_map 如果雜湊衝突了是單次線性的 .
事實證明一個有效的 Hash 函式是能加快程式執行速度的 .
#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); } }; // unordered<pair<string, string>, int, pair_hash>
Bonus.
Tuple
#include <functional> namespace hash_tuple { template <typename TT> struct hash { size_t operator()(TT const &tt) const { return std::hash<TT>()(tt); } }; // from boost (functional/hash): // see http://www.boost.org/doc/libs/1_35_0/doc/html/hash/combine.html template template <class T> inline void hash_combine(std::size_t &seed, T const &v) { seed ^= hash_tuple::hash<T>()(v) + 0x9e3779b9 + (seed << 6) + (seed >> 2); } // Recursive template code derived from Matthieu M. template <class Tuple, size_t Index = std::tuple_size<Tuple>::value - 1> struct HashValueImpl { void operator()(size_t &seed, Tuple const &tuple) const { HashValueImpl<Tuple, Index - 1>{}(seed, tuple); hash_combine(seed, std::get<Index>(tuple)); } }; template <class Tuple> struct HashValueImpl<Tuple, 0> { void operator()(size_t &seed, Tuple const &tuple) const { hash_combine(seed, std::get<0>(tuple)); } }; template <typename... TT> struct hash<std::tuple<TT...>> { size_t operator()(std::tuple<TT...> const &tt) const { size_t seed = 0; HashValueImpl<std::tuple<TT...>>{}(seed, tt); return seed; } }; // 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); } }; } // namespace hash_tuple #include <bits/stdc++.h> int main() { using ll = long long; // std::unordered_map<std::pair<ll, ll>, ll, hash_tuple::pair_hash> // hashmapPair; std::unordered_set<std::pair<ll, ll>, hash_tuple::pair_hash> // hashsetPair; std::unordered_map<std::pair<ll, ll>, ll, hash_tuple::pair_hash> hashmapPair; hashmapPair[{0, 0}] = 10; std::unordered_set<std::pair<ll, ll>, hash_tuple::pair_hash> hashsetPair; hashsetPair.insert({1, 1}); using TI = std::tuple<ll, ll, ll, ll>; std::unordered_map<TI, ll, hash_tuple::hash<TI>> hashmapTuple; hashmapTuple[{0, 1, 2, 3}] = 10; std::unordered_set<TI, hash_tuple::hash<TI>> hashsetTuple; hashsetTuple.emplace(0, 1, 2, 3); return 0; }
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