Holy linker error, batman!

main.obj : error LNK2001: unresolved external symbol "public: class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > __thiscall FieldExpression<class FieldExpression<class Field<class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > >,class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > >,class FieldExpression<class Field<class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > >,class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > > >::toString(void)const " (?toString@?$FieldExpression@V?$FieldExpression@V?$Field@V?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@@@V?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@@@V1@@@QBE?AV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@XZ)