std：：unordered_map<boost：：any， boost：：any> 抛出烦人的编译错误
std::unordered_map<boost::any, boost::any> throws annoying compile errors
当我声明std::unordered_map<boost::any, boost::any>类型的变量时，它会引发令人讨厌的编译错误。
例如，any.cc:
#include <map>
#include <boost/any.hpp>
int main() {
std::map<boost::any, boost::any> dict;
return 0;
}
将上面的代码编译为g++ any.cc -std=c++11 -I/usr/include/boost会出现很多错误，如下所示：
In file included from /usr/include/c++/5/bits/hashtable.h:35:0,
from /usr/include/c++/5/unordered_map:47,
from any.cc:1:
/usr/include/c++/5/bits/hashtable_policy.h: In instantiation of ‘struct std::__detail::__is_noexcept_hash<boost::any, std::hash<boost::any> >’:
/usr/include/c++/5/type_traits:137:12: required from ‘struct std::__and_<std::__is_fast_hash<std::hash<boost::any> >, std::__detail::__is_noexcept_hash<boost::any, std::hash<boost::any> > >’
/usr/include/c++/5/type_traits:148:38: required from ‘struct std::__not_<std::__and_<std::__is_fast_hash<std::hash<boost::any> >, std::__detail::__is_noexcept_hash<boost::any, std::hash<boost::any> > > >’
/usr/include/c++/5/bits/unordered_map.h:100:66: required from ‘class std::unordered_map<boost::any, boost::any>’
any.cc:5:48: required from here
/usr/include/c++/5/bits/hashtable_policy.h:85:34: error: no match for call to ‘(const std::hash<boost::any>) (const boost::any&)’
noexcept(declval<const _Hash&>()(declval<const _Key&>()))>
^
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 any.cc:1:
/usr/include/c++/5/type_traits: In instantiation of ‘struct std::__not_<std::__and_<std::__is_fast_hash<std::hash<boost::any> >, std::__detail::__is_noexcept_hash<boost::any, std::hash<boost::any> > > >’:
/usr/include/c++/5/bits/unordered_map.h:100:66: required from ‘class std::unordered_map<boost::any, boost::any>’
any.cc:5:48: required from here
/usr/include/c++/5/type_traits:148:38: error: ‘value’ is not a member of ‘std::__and_<std::__is_fast_hash<std::hash<boost::any> >, std::__detail::__is_noexcept_hash<boost::any, std::hash<boost::any> > >’
: public integral_constant<bool, !_Pp::value>
^
In file included from /usr/include/c++/5/unordered_map:48:0,
from any.cc:1:
/usr/include/c++/5/bits/unordered_map.h: In instantiation of ‘class std::unordered_map<boost::any, boost::any>’:
any.cc:5:48: 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<std::hash<boost::any> >, std::__detail::__is_noexcept_hash<boost::any, std::hash<boost::any> > > >’
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<std::hash<boost::any> >, std::__detail::__is_noexcept_hash<boost::any, std::hash<boost::any> > > >’
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<std::hash<boost::any> >, std::__detail::__is_noexcept_hash<boost::any, std::hash<boost::any> > > >’
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<std::hash<boost::any> >, std::__detail::__is_noexcept_hash<boost::any, std::hash<boost::any> > > >’
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<std::hash<boost::any> >, std::__detail::__is_noexcept_hash<boost::any, std::hash<boost::any> > > >’
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<std::hash<boost::any> >, std::__detail::__is_noexcept_hash<boost::any, std::hash<boost::any> > > >’
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<std::hash<boost::any> >, std::__detail::__is_noexcept_hash<boost::any, std::hash<boost::any> > > >’
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<std::hash<boost::any> >, std::__detail::__is_noexcept_hash<boost::any, std::hash<boost::any> > > >’
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<std::hash<boost::any> >, std::__detail::__is_noexcept_hash<boost::any, std::hash<boost::any> > > >’
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<std::hash<boost::any> >, std::__detail::__is_noexcept_hash<boost::any, std::hash<boost::any> > > >’
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<std::hash<boost::any> >, std::__detail::__is_noexcept_hash<boost::any, std::hash<boost::any> > > >’
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<std::hash<boost::any> >, std::__detail::__is_noexcept_hash<boost::any, std::hash<boost::any> > > >’
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<std::hash<boost::any> >, std::__detail::__is_noexcept_hash<boost::any, std::hash<boost::any> > > >’
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<std::hash<boost::any> >, std::__detail::__is_noexcept_hash<boost::any, std::hash<boost::any> > > >’
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<std::hash<boost::any> >, std::__detail::__is_noexcept_hash<boost::any, std::hash<boost::any> > > >’
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<std::hash<boost::any> >, std::__detail::__is_noexcept_hash<boost::any, std::hash<boost::any> > > >’
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<std::hash<boost::any> >, std::__detail::__is_noexcept_hash<boost::any, std::hash<boost::any> > > >’
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<std::hash<boost::any> >, std::__detail::__is_noexcept_hash<boost::any, std::hash<boost::any> > > >’
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<std::hash<boost::any> >, std::__detail::__is_noexcept_hash<boost::any, std::hash<boost::any> > > >’
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<std::hash<boost::any> >, std::__detail::__is_noexcept_hash<boost::any, std::hash<boost::any> > > >’
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<std::hash<boost::any> >, std::__detail::__is_noexcept_hash<boost::any, std::hash<boost::any> > > >’
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<std::hash<boost::any> >, std::__detail::__is_noexcept_hash<boost::any, std::hash<boost::any> > > >’
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<std::hash<boost::any> >, std::__detail::__is_noexcept_hash<boost::any, std::hash<boost::any> > > >’
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<std::hash<boost::any> >, std::__detail::__is_noexcept_hash<boost::any, std::hash<boost::any> > > >’
equal_range(const key_type& __x) const
^
但当我使用boost::unordered_map而不是std时，它可以正常工作，没有任何错误。
为什么？
std::unordered_map使用std::hash作为默认哈希器。std::hash对boost::any没有专门化。因为STanDard库对第三方库中的实体一无所知。
boost::unordered_map使用boost::hash作为默认散列器，它对boost库中的类型进行了专门化。
解决方案：明确告知std::unordered_map使用boost::hash<boost::any>作为hasher。
当您声明unordered_map<Key, Value>时，您需要一个哈希函数。默认的哈希函数是const hash<Key>(const Key&)。
boost::any似乎有这样的哈希模板专门化，但对于boost::hash来说。
因此，实际上编译器试图为boost::any找到std::hash的专用化，但只找到boost::hash的专用化。
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