C++ 使用std:：pair作为键创建std:：无序_映射

我正在尝试创建一个std:：unordered_映射，并将std:：pair作为键。可以想象，这需要我显式地提供一个类来为给定的键生成哈希，并为键提供一个相等比较器。以下是我目前的代码：

#include <unordered_map>
#include <memory>
#include <utility>

template <class T, typename U> 
struct PairHash{ 
    size_t operator()(const std::pair<T, U> &key){ 
        return std::hash<T>()(key.first) ^ std::hash<U>()(key.second);
    }
};

template <class T, typename U>
struct PairEqual{
    bool operator()(const std::pair<T, U> &lhs, const std::pair<T, U> &rhs) const{
        return lhs.first == rhs.first && lhs.second == rhs.second;
    }
};

struct GraphEdge{

};


int main(){

    std::unordered_map<std::pair<int, int>, 
                       std::unique_ptr<GraphEdge>, 
                       PairHash<int, int>,
                       PairEqual<int, int>>  edges; 

}

#包括
#包括
#包括
模板
结构PairHash{
size_t运算符（）（const std:：pair&key）{
返回std:：hash（）（key.first）^std:：hash（）（key.second）；
}
};
模板
结构PairEqual{
布尔运算符（）（常数std:：pair&lhs，常数std:：pair&rhs）常数{
返回lhs.first==rhs.first&&lhs.second==rhs.second；
}
};
结构图形边缘{
};
int main（）{
std：：无序的_映射边；
}

然而，这给了我一个相当（至少在我看来）难以理解的编译器错误：

In file included from /usr/include/c++/5/bits/hashtable.h:35:0,
                 from /usr/include/c++/5/unordered_map:47,
                 from prog.cpp:1:
/usr/include/c++/5/bits/hashtable_policy.h: In instantiation of 'struct std::__detail::__is_noexcept_hash<std::pair<int, int>, PairHash<int, int> >':
/usr/include/c++/5/type_traits:137:12:   required from 'struct std::__and_<std::__is_fast_hash<PairHash<int, int> >, std::__detail::__is_noexcept_hash<std::pair<int, int>, PairHash<int, int> > >'
/usr/include/c++/5/type_traits:148:38:   required from 'struct std::__not_<std::__and_<std::__is_fast_hash<PairHash<int, int> >, std::__detail::__is_noexcept_hash<std::pair<int, int>, PairHash<int, int> > > >'
/usr/include/c++/5/bits/unordered_map.h:100:66:   required from 'class std::unordered_map<std::pair<int, int>, std::unique_ptr<GraphEdge>, PairHash<int, int>, PairEqual<int, int> >'
prog.cpp:29:43:   required from here
/usr/include/c++/5/bits/hashtable_policy.h:85:34: error: no match for call to '(const PairHash<int, int>) (const std::pair<int, int>&)'
  noexcept(declval<const _Hash&>()(declval<const _Key&>()))>
                                  ^
prog.cpp:7:12: note: candidate: size_t PairHash<T, U>::operator()(const std::pair<_T1, _T2>&) [with T = int; U = int; size_t = unsigned int] <near match>
     size_t operator()(const std::pair<T, U> &key){ 
            ^
prog.cpp:7:12: note:   passing 'const PairHash<int, int>*' as 'this' argument discards qualifiers
In file included from /usr/include/c++/5/bits/move.h:57:0,
                 from /usr/include/c++/5/bits/stl_pair.h:59,
                 from /usr/include/c++/5/utility:70,
                 from /usr/include/c++/5/unordered_map:38,
                 from prog.cpp:1:
/usr/include/c++/5/type_traits: In instantiation of 'struct std::__not_<std::__and_<std::__is_fast_hash<PairHash<int, int> >, std::__detail::__is_noexcept_hash<std::pair<int, int>, PairHash<int, int> > > >':
/usr/include/c++/5/bits/unordered_map.h:100:66:   required from 'class std::unordered_map<std::pair<int, int>, std::unique_ptr<GraphEdge>, PairHash<int, int>, PairEqual<int, int> >'
prog.cpp:29:43:   required from here
/usr/include/c++/5/type_traits:148:38: error: 'value' is not a member of 'std::__and_<std::__is_fast_hash<PairHash<int, int> >, std::__detail::__is_noexcept_hash<std::pair<int, int>, PairHash<int, int> > >'
     : public integral_constant<bool, !_Pp::value>
                                      ^
In file included from /usr/include/c++/5/unordered_map:48:0,
                 from prog.cpp:1:
/usr/include/c++/5/bits/unordered_map.h: In instantiation of 'class std::unordered_map<std::pair<int, int>, std::unique_ptr<GraphEdge>, PairHash<int, int>, PairEqual<int, int> >':
prog.cpp:29:43:   required from here
/usr/include/c++/5/bits/unordered_map.h:100:66: error: 'value' is not a member of 'std::__not_<std::__and_<std::__is_fast_hash<PairHash<int, int> >, std::__detail::__is_noexcept_hash<std::pair<int, int>, PairHash<int, int> > > >'
       typedef __umap_hashtable<_Key, _Tp, _Hash, _Pred, _Alloc>  _Hashtable;
                                                                  ^
/usr/include/c++/5/bits/unordered_map.h:107:45: error: 'value' is not a member of 'std::__not_<std::__and_<std::__is_fast_hash<PairHash<int, int> >, std::__detail::__is_noexcept_hash<std::pair<int, int>, PairHash<int, int> > > >'
       typedef typename _Hashtable::key_type key_type;
                                             ^
/usr/include/c++/5/bits/unordered_map.h:108:47: error: 'value' is not a member of 'std::__not_<std::__and_<std::__is_fast_hash<PairHash<int, int> >, std::__detail::__is_noexcept_hash<std::pair<int, int>, PairHash<int, int> > > >'
       typedef typename _Hashtable::value_type value_type;
                                               ^
/usr/include/c++/5/bits/unordered_map.h:109:48: error: 'value' is not a member of 'std::__not_<std::__and_<std::__is_fast_hash<PairHash<int, int> >, std::__detail::__is_noexcept_hash<std::pair<int, int>, PairHash<int, int> > > >'
       typedef typename _Hashtable::mapped_type mapped_type;
                                                ^
/usr/include/c++/5/bits/unordered_map.h:110:43: error: 'value' is not a member of 'std::__not_<std::__and_<std::__is_fast_hash<PairHash<int, int> >, std::__detail::__is_noexcept_hash<std::pair<int, int>, PairHash<int, int> > > >'
       typedef typename _Hashtable::hasher hasher;
                                           ^
/usr/include/c++/5/bits/unordered_map.h:111:46: error: 'value' is not a member of 'std::__not_<std::__and_<std::__is_fast_hash<PairHash<int, int> >, std::__detail::__is_noexcept_hash<std::pair<int, int>, PairHash<int, int> > > >'
       typedef typename _Hashtable::key_equal key_equal;
                                              ^
/usr/include/c++/5/bits/unordered_map.h:112:51: error: 'value' is not a member of 'std::__not_<std::__and_<std::__is_fast_hash<PairHash<int, int> >, std::__detail::__is_noexcept_hash<std::pair<int, int>, PairHash<int, int> > > >'
       typedef typename _Hashtable::allocator_type allocator_type;
                                                   ^
/usr/include/c++/5/bits/unordered_map.h:117:45: error: 'value' is not a member of 'std::__not_<std::__and_<std::__is_fast_hash<PairHash<int, int> >, std::__detail::__is_noexcept_hash<std::pair<int, int>, PairHash<int, int> > > >'
       typedef typename _Hashtable::pointer  pointer;
                                             ^
/usr/include/c++/5/bits/unordered_map.h:118:50: error: 'value' is not a member of 'std::__not_<std::__and_<std::__is_fast_hash<PairHash<int, int> >, std::__detail::__is_noexcept_hash<std::pair<int, int>, PairHash<int, int> > > >'
       typedef typename _Hashtable::const_pointer const_pointer;
                                                  ^
/usr/include/c++/5/bits/unordered_map.h:119:47: error: 'value' is not a member of 'std::__not_<std::__and_<std::__is_fast_hash<PairHash<int, int> >, std::__detail::__is_noexcept_hash<std::pair<int, int>, PairHash<int, int> > > >'
       typedef typename _Hashtable::reference  reference;
                                               ^
/usr/include/c++/5/bits/unordered_map.h:120:52: error: 'value' is not a member of 'std::__not_<std::__and_<std::__is_fast_hash<PairHash<int, int> >, std::__detail::__is_noexcept_hash<std::pair<int, int>, PairHash<int, int> > > >'
       typedef typename _Hashtable::const_reference const_reference;
                                                    ^
/usr/include/c++/5/bits/unordered_map.h:121:46: error: 'value' is not a member of 'std::__not_<std::__and_<std::__is_fast_hash<PairHash<int, int> >, std::__detail::__is_noexcept_hash<std::pair<int, int>, PairHash<int, int> > > >'
       typedef typename _Hashtable::iterator  iterator;
                                              ^
/usr/include/c++/5/bits/unordered_map.h:122:51: error: 'value' is not a member of 'std::__not_<std::__and_<std::__is_fast_hash<PairHash<int, int> >, std::__detail::__is_noexcept_hash<std::pair<int, int>, PairHash<int, int> > > >'
       typedef typename _Hashtable::const_iterator const_iterator;
                                                   ^
/usr/include/c++/5/bits/unordered_map.h:123:51: error: 'value' is not a member of 'std::__not_<std::__and_<std::__is_fast_hash<PairHash<int, int> >, std::__detail::__is_noexcept_hash<std::pair<int, int>, PairHash<int, int> > > >'
       typedef typename _Hashtable::local_iterator local_iterator;
                                                   ^
/usr/include/c++/5/bits/unordered_map.h:124:57: error: 'value' is not a member of 'std::__not_<std::__and_<std::__is_fast_hash<PairHash<int, int> >, std::__detail::__is_noexcept_hash<std::pair<int, int>, PairHash<int, int> > > >'
       typedef typename _Hashtable::const_local_iterator const_local_iterator;
                                                         ^
/usr/include/c++/5/bits/unordered_map.h:125:47: error: 'value' is not a member of 'std::__not_<std::__and_<std::__is_fast_hash<PairHash<int, int> >, std::__detail::__is_noexcept_hash<std::pair<int, int>, PairHash<int, int> > > >'
       typedef typename _Hashtable::size_type  size_type;
                                               ^
/usr/include/c++/5/bits/unordered_map.h:126:52: error: 'value' is not a member of 'std::__not_<std::__and_<std::__is_fast_hash<PairHash<int, int> >, std::__detail::__is_noexcept_hash<std::pair<int, int>, PairHash<int, int> > > >'
       typedef typename _Hashtable::difference_type difference_type;
                                                    ^
/usr/include/c++/5/bits/unordered_map.h:280:7: error: 'value' is not a member of 'std::__not_<std::__and_<std::__is_fast_hash<PairHash<int, int> >, std::__detail::__is_noexcept_hash<std::pair<int, int>, PairHash<int, int> > > >'
       operator=(initializer_list<value_type> __l)
       ^
/usr/include/c++/5/bits/unordered_map.h:379:2: error: 'value' is not a member of 'std::__not_<std::__and_<std::__is_fast_hash<PairHash<int, int> >, std::__detail::__is_noexcept_hash<std::pair<int, int>, PairHash<int, int> > > >'
  emplace(_Args&&... __args)
  ^
/usr/include/c++/5/bits/unordered_map.h:432:7: error: 'value' is not a member of 'std::__not_<std::__and_<std::__is_fast_hash<PairHash<int, int> >, std::__detail::__is_noexcept_hash<std::pair<int, int>, PairHash<int, int> > > >'
       insert(const value_type& __x)
       ^
/usr/include/c++/5/bits/unordered_map.h:439:2: error: 'value' is not a member of 'std::__not_<std::__and_<std::__is_fast_hash<PairHash<int, int> >, std::__detail::__is_noexcept_hash<std::pair<int, int>, PairHash<int, int> > > >'
  insert(_Pair&& __x)
  ^
/usr/include/c++/5/bits/unordered_map.h:499:7: error: 'value' is not a member of 'std::__not_<std::__and_<std::__is_fast_hash<PairHash<int, int> >, std::__detail::__is_noexcept_hash<std::pair<int, int>, PairHash<int, int> > > >'
       insert(initializer_list<value_type> __l)
       ^
/usr/include/c++/5/bits/unordered_map.h:645:7: error: 'value' is not a member of 'std::__not_<std::__and_<std::__is_fast_hash<PairHash<int, int> >, std::__detail::__is_noexcept_hash<std::pair<int, int>, PairHash<int, int> > > >'
       equal_range(const key_type& __x)
       ^
/usr/include/c++/5/bits/unordered_map.h:649:7: error: 'value' is not a member of 'std::__not_<std::__and_<std::__is_fast_hash<PairHash<int, int> >, std::__detail::__is_noexcept_hash<std::pair<int, int>, PairHash<int, int> > > >'
       equal_range(const key_type& __x) const
       ^

在/usr/include/c++/5/bits/hashtable.h:35:0中包含的文件中，
从/usr/include/c++/5/无序映射：47，
来自项目cpp:1：
/usr/include/c++/5/bits/hashtable_policy.h:在“struct std:：_detail:：_是_noexcept_hash”的实例化中：
/usr/include/c++/5/type_traits:137:12:必须来自“struct std:：u和u”
/usr/include/c++/5/type_traits:148:38:从“struct std:：_not_”中需要
/usr/include/c++/5/bits/unordered_-map.h:100:66:class std:：unordered_-map中的必填项
程序cpp:29:43：从这里开始需要
/usr/include/c++/5/bits/hashtable_policy.h:85:34:错误：调用“（const PairHash）（const std:：pair&）”不匹配
无例外（declval（）（declval（））>
^
prog.cpp:7:12：注：候选：size\t PairHash:：operator（）（const std:：pair&）[带t=int；U=int；size\U t=unsigned int]
size_t运算符（）（const std:：pair&key）{
^
prog.cpp:7:12:注意：将“const PairHash*”传递为“this”参数将丢弃限定符
在/usr/include/c++/5/bits/move.h:57:0中包含的文件中，
从/usr/include/c++/5/bits/stl_pair.h:59，
从/usr/include/c++/5/utility:70，
从/usr/include/c++/5/unordered_map:38，
来自项目cpp:1：
/usr/include/c++/5/type_traits:在“struct std:：__not_”的实例化中：
/usr/include/c++/5/bits/unordered_-map.h:100:66:class std:：unordered_-map中的必填项
程序cpp:29:43：从这里开始需要
/usr/include/c++/5/type_traits:148:38:错误：“value”不是“std:：u和_u”的成员
：公共积分常数
^
在/usr/include/c++/5/unordered_map:48:0中包含的文件中，
来自项目cpp:1：
/usr/include/c++/5/bits/unordered_-map.h:在“class std:：unordered_-map”的实例化中：
程序cpp:29:43：从这里开始需要
/usr/include/c++/5/bits/unordered\u map.h:100:66:错误：“value”不是“std:”的成员
typedef_uuumap_hashtable_hashtable；
^
/usr/include/c++/5/bits/unordered\u map.h:107:45:错误：“value”不是“std:”的成员
typedef typename\u哈希表：：key\u type key\u type；
^
/usr/include/c++/5/bits/unordered\u map.h:108:47:错误：“value”不是“std:”的成员
typedef typename_Hashtable:：value_type value_type；
^
/usr/include/c++/5/bits/unordered\u map.h:109:48:错误：“value”不是“std:”的成员
typedef typename_Hashtable:：mapped_type mapped_type；
^
/usr/include/c++/5/bits/unordered\u map.h:110:43:错误：“value”不是“std:”的成员
typedef typename_Hashtable:：hasher-hasher；
^
/usr/include/c++/5/bits/unordered\u map.h:111:46:错误：“value”不是“std:”的成员
typedef typename_Hashtable:：key_equal key_equal；
^
/usr/include/c++/5/bits/unordered\u map.h:112:51:错误：“value”不是“std:”的成员
typedef typename\u哈希表：：分配器\u类型分配器\u类型；
^
/usr/include/c++/5/bits/unordered\u map.h:117:45:错误：“value”不是“std:”的成员
typedef typename_哈希表：：指针；
^
/usr/include/c++/5/bits/unordered\u map.h:118:50:错误：“value”不是“std:”的成员
typedef typename_哈希表：：常量指针常量指针；
^
/usr/include/c++/5/bits/unordered\u map.h:119:47:错误：“value”不是“std:”的成员
typedef typename_Hashtable:：reference；
^
/usr/include/c++/5/bits/unordered\u map.h:120:52:错误：“value”不是“std:”的成员
typedef typename_哈希表：：const_reference const_reference；
^
/usr/include/c++/5/bits/unordered\u map.h:121:46:错误：“value”不是“std:”的成员
typedef typename_Hashtable:：迭代器迭代器；
^
/usr/include/c++/5/bits/unordered\u map.h:122:51:错误：“value”不是“std:”的成员
typedef typename_哈希表：：常量迭代器常量迭代器；
^
/usr/include/c++/5/bits/unordered\u map.h:123:51:错误：“value”不是“std:”的成员
typedef typename_Hashtable:：local_iterator local_iterator；
^
/usr/include/c++/5/bits/unordered\u map.h:124:57:错误：“value”不是“std:”的成员
typedef typename_Hashtable:：const_local_迭代器const_local_迭代器；
^
/usr/include/c++/5/bits/unordered\u map.h:125:47:错误：“value”不是“std:”的成员
typedef typename\u哈希表：：size\u type size\u type；