当我声明一个std::unordered_map<boost::any, boost::any>类型的变量时，它会引发恼人的编译错误。
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
^
但是，当我使用boost::unordered_map而不是std时，它可以正常运行。
为什么？
答案 0 :(得分：4)
std::unordered_map使用std::hash作为默认的哈希。 std::hash没有boost::any的专业化。因为STanDard库对第三方库中的实体一无所知。
boost::unordered_map使用boost::hash作为默认的hasher，它具有来自boost库的类型的特化。
解决方案：明确告诉std::unordered_map使用boost::hash<boost::any>作为哈希。
答案 1 :(得分：1)
声明unordered_map<Key, Value>时，需要散列函数。默认的散列函数是const hash<Key>(const Key&)。
boost::any具有此类散列模板专精，但适用于boost::hash。
因此，编译器实际上会尝试为std::hash找到boost::any的专门化，但只查找boost::hash专门化。