c++ - Structure initialization -

i have used following structure

template <typename item> struct tset {     typedef std::set <int, comparator <item>  >  type; }; 

as data member of the structure

template <typename item> struct tobject {     int code;     ...    typename tset <item> ::type indices;     tobject ( const list <item> *list ) : code( 0 ), indices ( list ) {}  }; 

where

 template <typename item>  struct tlist  {     typedef std::vector <item> type;  };  template <typename item> class list {     private:             typename tlist <item>::type items; }; 

but have changed data model to

template <typename item>  class tset : public std::set <int, comparator <item>  > { };  template <typename item> struct tobject {    int code;    ...   typename tset <item> indices;    tobject ( const list <item> *list ) : code ( 0 ), indices ( list ) {} //error: can not convert parameter 1 const list <item> const tset <item>  }; 

and there problems structure initialization.

error: can not convert parameter 1 const list <item> const tset <item> 

where problem?

why expect conversion list tset if did not write one? also, type of indices needs typename on front since dependent type.