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I'd like to know if it is possible to use an object reference as a key in an unordered_map container in C++. #include <unordered_map> class Object { int value; }; struct object_hash { inline size_t operator()(const Object& o) const { return 0; } }; std::unordered_map<Object&, int, object_hash> map; When trying to compile this simple snippet, I got some errors about methods redefinition: Using clang with libc++ /usr/include/c++/v1/unordered_map:352:12: error: class member cannot be redeclared size_t operator()(const _Cp& __x) const Using gcc 4.6 with libstdc++ /usr/include/c++/4.6/bits/hashtable_policy.h:556:5: error: ‘std::__detail::_Map_base<_Key, _Pair, std::_Select1st<_Pair>, true, _Hashtable>::mapped_type& std::__detail::_Map_base<_Key, _Pair, std::_Select1st<_Pair>, true, _Hashtable>::operator [with _Key = Object&, _Pair = std::pair, _Hashtable = std::_Hashtable, std::allocator >, std::_Select1st >, std::equal_to, object_hash, std::__detail::_Mod_range_hashing, std::__detail::_Default_ranged_hash, std::__detail::_Prime_rehash_policy, false, false, true>, std::__detail::_Map_base<_Key, _Pair, std::_Select1st<_Pair>, true, _Hashtable>::mapped_type = int]’ cannot be overloaded /usr/include/c++/4.6/bits/hashtable_policy.h:537:5 : error: with ‘std::__detail::_Map_base<_Key, _Pair, std::_Select1st<_Pair>, true, _Hashtable>::mapped_type& std::__detail::_Map_base<_Key, _Pair, std::_Select1st<_Pair>, true, _Hashtable>::operator[](const _Key&) [with _Key = Object&, _Pair = std::pair, _Hashtable = std::_Hashtable, std::allocator >, std::_Select1st >, std::equal_to, object_hash, std::__detail::_Mod_range_hashing, std::__detail::_Default_ra

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