In file included from /usr/include/c++/5/utility:70:0,
from /usr/include/c++/5/unordered_map:38,
from prog.cpp:1:
/usr/include/c++/5/bits/stl_pair.h: In instantiation of 'struct std::pair<const int, test>':
/usr/include/c++/5/ext/aligned_buffer.h:85:34: required from 'struct __gnu_cxx::__aligned_buffer<std::pair<const int, test> >'
/usr/include/c++/5/bits/hashtable_policy.h:246:43: required from 'struct std::__detail::_Hash_node_value_base<std::pair<const int, test> >'
/usr/include/c++/5/bits/hashtable_policy.h:292:12: required from 'struct std::__detail::_Hash_node<std::pair<const int, test>, false>'
/usr/include/c++/5/bits/hashtable_policy.h:1896:60: required from 'struct std::__detail::_Hashtable_alloc<std::allocator<std::__detail::_Hash_node<std::pair<const int, test>, false> > >'
/usr/include/c++/5/bits/hashtable.h:170:11: required from 'class std::_Hashtable<int, std::pair<const int, test>, std::allocator<std::pair<const int, test> >, std::__detail::_Select1st, std::equal_to<int>, std::hash<int>, std::__detail::_Mod_range_hashing, std::__detail::_Default_ranged_hash, std::__detail::_Prime_rehash_policy, std::__detail::_Hashtable_traits<false, false, true> >'
/usr/include/c++/5/bits/unordered_map.h:101:18: required from 'class std::unordered_map<int, test>'
prog.cpp:5:17: required from here
/usr/include/c++/5/bits/stl_pair.h:102:11: error: 'std::pair<_T1, _T2>::second' has incomplete type
_T2 second; /// @c second is a copy of the second object
^
prog.cpp:3:7: note: forward declaration of 'class test'
class test;
^