使用std :: pair作为键来创建std :: unordered_map [英] Creating an std::unordered_map with an std::pair as key
本文介绍了使用std :: pair作为键来创建std :: unordered_map的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！
问题描述
我试图创建一个std :: unordered_map与std ::对作为键。你可以想象，这将要求我明确地提供一个类来生成一个给定的键的哈希，以及一个键的相等比较器。这里是我的代码到目前为止：
#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& ^ 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>边缘;
}
但是，这给了我一个最小）inscrutable编译器错误：
在/usr/include/c++/5/bits/hashtable.h:35 ：0，
来自/ usr / include / c ++ / 5 / unordered_map：47，
来自prog.cpp：1：
/usr/include/c++/5/bits/hashtable_policy.h ：在实例化'struct std :: __ detail :: __ is_noexcept_hash< std :: pair< int，int>，PairHash< int，int> >'：
/ usr / include / c ++ / 5 / type_traits：137：12：需要从'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：需要从'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：需要从'std :: unordered_map< std :: pair< int，int>，std :: unique_ptr< GraphEdge>，PairHash< int，int>，PairEqual< int，int> >'
prog.cpp：29：43：从这里需要
/usr/include/c++/5/bits/hashtable_policy.h:85:34：错误：没有匹配到调用' const（）const Hash< int，int>）（const std :: pair< int，int>&）'
noexcept（declat< const _Hash&> >
^
prog.cpp：7：12：note注意：候选者：size_t PairHash< T，U> :: operator（）（const std :: pair< _T1，_T2& = 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> *'as'this'argument放弃限定符
在/usr/include/c++/5/bits/move.h:57:0包含的文件中，
来自/ usr / include / c ++ / 5 / bits / stl_pair.h：59，
来自/ usr / include / c ++ / 5 / utility：70，
来自/ usr / include / c ++ / 5 / unordered_map：38，
从prog.cpp：1：
/ usr / include / c ++ / 5 / type_traits：在实例化'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：需要从'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'不是'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>
^
在/ usr / include / c ++ / 5 / unordered_map：48：0，
从prog.cpp：1：
包含的文件/ usr / include / c ++ / 5 / bits / unordered_map.h：在实例化'class std :: unordered_map< std :: pair< int，int>，std :: unique_ptr< GraphEdge> ;, PairHash< int，int>，PairEqual< int，int& >'：
prog.cpp：29：43：从这里需要
/usr/include/c++/5/bits/unordered_map.h:100:66：error：'value'不是'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'不是'std :: __ not_< std :: __ and_< std的成员:: __ 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'不是'std :: __ not_< std :: __ and_< std的成员:: __ 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'不是'std :: __ not_< std :: __ and_< std的成员:: __ 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'不是'std :: __ not_< std :: __ and_< std的成员:: __ 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'不是'std :: __ not_< std :: __ and_< std的成员:: __ 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'不是'std :: __ not_< std :: __ and_< std的成员:: __ 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'不是'std :: __ not_< std :: __ and_< std的成员:: __ 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'不是'std :: __ not_< std :: __ and_< std的成员:: __ 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'不是'std :: __ not_< std :: __ and_< std的成员:: __ 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'不是'std :: __ not_< std :: __ and_< std的成员:: __ 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'不是'std :: __ not_< std :: __ and_< std的成员:: __ 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'不是'std :: __ not_< std :: __ and_< std的成员:: __ 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'不是'std :: __ not_< std :: __ and_< std的成员:: __ 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'不是'std :: __ not_< std :: __ and_< std的成员:: __ 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'不是'std :: __ not_< std :: __ and_< std的成员:: __ 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'不是'std :: __ not_< std :: __ and_< std的成员:: __ 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：错误：'value'不是'std :: __ not_< std :: __ and_< std的成员:: __ 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不是'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 '不是'std :: __ not_< std :: __和_< 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'不是'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：错误：'value'不是'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：错误：'value'不是'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：错误：'value'不是'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
^
显然，libstdc ++通过 const PairHash< int指向你的哈希对象， int> * 。因此，调用程序中未标记为 const 的 operator（）是编译器错误。
您可以使用libstdc ++编译 operator（） const 。
从Hash要求开始，Hash类型必须提供一个 size_t运算符（KeyType）const; ，因此您的代码确实不正确。
I am trying to create an std::unordered_map with an std::pair as key. As you can imagine, this would require me to explicitly provide a class to generate a hash for a given key, as well as an equality comparator for the keys. Here's my code so far:
#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;
}
However, this gives me a rather (to my eyes at least) inscrutable compiler error:
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
^
What am I doing wrong?
解决方案
Apparently, libstdc++ refers to your hash object via const PairHash<int, int>*. Thus calling the operator () which is not marked const in your program is a compiler error.
You can get your code to compile with libstdc++ by making operator() const.
As of 17.6.3.4 (Hash Requirements), a Hash type must provide a size_t operator(KeyType) const;, so your code is indeed incorrect.
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