在c++11 的unordered_set和unordered_map中插入pair或tuple作为键值

想完成的任务 与 遇到的问题

想在c++11 的unordered_set和unordered_map中插入pair或tuple作为键值

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

会报错
/usr/include/c++/4.9/bits/hashtable_policy.h: In instantiation of 'struct std::__detail::__is_noexcept_hash<std::tuple<int, int>, std::hash<std::tuple<int, int> > >'
或者
/usr/include/c++/4.9/bits/hashtable_policy.h: In instantiation of 'struct std::__detail::__is_noexcept_hash<std::pair<std::basic_string<char>, std::basic_string<char> >, std::hash<std::pair<std::basic_string<char>, std::basic_string<char> > > >'
C++的std::pair是无法std::hash的，为了在unordered_set和unordered_map中使用std::pair，有如下方法。还有个前提，pair 和 tuple 中的元素本身得是可以 std::hash 哈希的。

方法一：专门写个可用于std::pair的std::hash

#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 << '\n';
	}

	return 0;
}

输出

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

注意：上面的代码使用的异或(XOR)，由于x^x == 0并且x^y == y^x，所以应该配合一些位运算的shift或rotate来做。

方法二：使用boost::hash

boost::hash可以用于哈希integers, floats, pointers, strings, arrays, pairs 以及其它 STL 里的东西

#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;
}

输出

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

注意：boost的hash的位置改过，有网友说boost 1.72的hash在

#include <boost/container_hash/extensions.hpp>

原话是

By the way the functional hash has moved. I am not sure when, but in boost 1.72 it is in #include <boost/container_hash/extensions.hpp> I am not sure why the boost hash function for a tuple is not documented somewhere.

方法三:hash_combine

把下列代码放在任何你想实现本文目的代码头文件里
原话是

This works on gcc 4.5 allowing all c++0x tuples containing standard hashable types to be members of unordered_map and unordered_set without further ado. (I put the code in a header file and just include it.)
The function has to live in the std namespace so that it is picked up by argument-dependent name lookup (ADL).

#include <tuple>
namespace std{
    namespace
    {

        // Code from boost
        // Reciprocal of the golden ratio helps spread entropy
        //     and handles duplicates.
        // See Mike Seymour in magic-numbers-in-boosthash-combine:
        //     http://stackoverflow.com/questions/4948780

        template <class T>
        inline void hash_combine(std::size_t& seed, T const& v)
        {
            seed ^= std::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
        {
          static void apply(size_t& seed, Tuple const& tuple)
          {
            HashValueImpl<Tuple, Index-1>::apply(seed, tuple);
            hash_combine(seed, std::get<Index>(tuple));
          }
        };

        template <class Tuple>
        struct HashValueImpl<Tuple,0>
        {
          static void apply(size_t& seed, Tuple const& tuple)
          {
            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...> >::apply(seed, tt);    
            return seed;                                 
        }                                              

    };
}

参考资料

方法一、方法二：
《How to use std::pair as key to std::unordered_map in C++》
链接：https://www.techiedelight.com/use-std-pair-key-std-unordered_map-cpp/
方法三：
《Generic hash for tuples in unordered_map / unordered_set》问题下Leo Goodstadt的回答
链接：https://stackoverflow.com/questions/7110301/generic-hash-for-tuples-in-unordered-map-unordered-set?lq=1
其它可能有效的方法：
方法三的参考链接里的其它回答，还有 https://stackoverflow.com/questions/20834838/using-tuple-in-unordered-map 里的一些回答