Structure initialization

I have used the following structure

template <typename Item>
struct TSet
{
    typedef std::set <int, comparator <Item>  >  Type;
};

as a data member of the 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 I have changed the 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 c开发者_如何学编程onvert parameter 1 const List <Item> to const TSet <Item>
 };

and there are problems with the structure initialization.

Error: Can not convert parameter 1 const List <Item> to const TSet <Item>

Where is the problem?

Why would you expect a conversion from List to TSet if you did not write one? Also, the type of indices needs a typename on the front since it is a dependent type.