void assign(size_t, const value_type&);

inline void List::assign(size_t count, const value_type& val) {
   this->List::clear();
   this->List::List(count, val);
}

Error   LNK2019 unresolved external symbol "public: void __cdecl List::assign(unsigned __int64,int const &)" (?assign@List@@QEAAX_KAEBH@Z) referenced in function main  list    C:UsersAdminDocumentsVisual Studio 2017ProjectslistlistSource.obj   1

List ls(10,9);
ls.assign(8, 8);