C++ unordered_map user defined type

I am having a class which is used as the key in the unordered_map. When I tried to compiled the code, it shows undefined reference to std::hash<typeName>::operator()(typename) const. How could I go to fix it? What additional function do I need to overload to make the user defined type to be used in an unordered_map?

I have a dateTime struct which stores the info of date and time.

The error message is as follows:

In function 'std::__detail::_Hash_code_base<DateTime, std::pair<DateTime const, int>, std::_Select1st<std::pair<DateTime const, int> >, std::equal_to<DateTime>, std::hash<DateTime>, std::__detail::_Mod_range_hashing, std::__detail::_Default_ranged_hash, false>::_M_hash_code(DateTime const&) const': testing.cpp:(.text._ZNKSt8__detail15_Hash_code_baseI10DateTimeSt4pairIKS1_DeESt10_Select1stIS4_ESt8equal_toIS1_ESt4hashIS1_ENS_18_Mod_range_hashingENS_20_Default_ranged_hashELb0EE12_M_hash_codeERS3_[开发者_如何学Cstd::__detail::_Hash_code_base<DateTime, std::pair<DateTime const, int>, std::_Select1st<std::pair<DateTime const, int> >, std::equal_to<DateTime>, std::hash<DateTime>, std::__detail::_Mod_range_hashing, std::__detail::_Default_ranged_hash, false>::_M_hash_code(DateTime const&) const]+0x23): undefined reference to 'std::hash<DateTime>::operator()(DateTime) const'

Thanks.

you have to implement hash algorithm, otherwise standard container will not pick your type, because it has no idea how to calculate hash code for it.

namespace std
{    
   template <>
   struct hash<DateTime> : public unary_function<DateTime, size_t>
   {
       size_t operator()(const DateTime& v) const
       {
           return /* my hash algorithm */;
       }
   };
}