fell, 
C++ compiler be like:
error LNK2001: unresolved external symbol "public: static class std::unordered_map<class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >,unsigned int,struct std::hash<class std::basic_string<char,struct std::char_traits<cha r>,class std::allocator<char> > >,struct std::equal_to<class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > >,class std::allocator<struct std::pair<class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > const ,unsigned int> > > TextureStore::texture_cache" (?texture_cache@TextureStore@@2V?$unordered_map@V?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@IU?$hash@V?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@@2@U?$equal_to@V?$basic_string@DU?$char_traits @D@std@@V?$allocator@D@2@@std@@@2@V?$allocator@U?$pair@$$CBV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@I@std@@@2@@std@@A)And expect me to casually read that and go "Ah, I see".