template parameter to boost multi index container

Question

I need to create a generic class containing multiindex container as a storage. when I compile, it gives error as below where I have defined nth index view.

error: non-template ‘nth_index’ used as template

/**
 * connection manager
 */

template < typename T, typename C >
class conn_mgr: boost::noncopyable {
public:
    /**
     * connection ptr
     */
    typedef boost::shared_ptr conn_ptr_t;

    /**
     * connection table type
     * It's a multi index container
     */
    typedef  boost::multi_index::multi_index_container <
            conn_ptr_t,
            boost::multi_index::indexed_by <
                    //sequenced < >,
                    boost::multi_index::hashed_unique <
                            BOOST_MULTI_INDEX_CONST_MEM_FUN(T, std::string, T::id)  >,
                    boost::multi_index::hashed_non_unique <
                            BOOST_MULTI_INDEX_CONST_MEM_FUN(T, std::string,
                                    T::type)>,
                    boost::multi_index::hashed_non_unique <
                            boost::multi_index::composite_key < conn_ptr_t,
                                    BOOST_MULTI_INDEX_CONST_MEM_FUN(T,
                                            std::string, T::id),
                                    BOOST_MULTI_INDEX_CONST_MEM_FUN(T,
                                            std::string, T::type ) > > > >
            conn_table_t;

//typedef for ConnectionIdView
typedef conn_table_t::nth_index<0>::type conn_table_by_id_type;

typedef conn_table_t::nth_index<1>::type conn_table_by_type;

typedef conn_table_t::nth_index<2>::type conn_table_by_id_type;

private: conn_table_t conn_table_; };

and here how I am using in main.

int main( int argc, char** argv )
{
   typedef conn_mgr < smpp_conn, smpp_config > smpp_conn_mgr_t;
   smpp_conn_mgr_t conn_mgr;
}

Answer 1

Use this syntax instead for your nested typedefs:

typedef typename conn_table_t::template nth_index<0>::type conn_table_by_id_type;

The typename keyword is used here as a qualifier to let the compiler know that conn_table_t::template nth_index<0>::type is a type. This special use of typename is only necessary within templates.

The template keyword is used here as a qualifier to distingush member templates from other names.

---

Furthermore, this line is invalid:

typedef boost::shared_ptr conn_ptr_t;

You can't typedef templates. You can only typedef types. Perhaps you meant to write:

typedef typename boost::shared_ptr<T> conn_ptr_t;

---

One last error: You're attempting to give two typedefs the same name: conn_table_by_id_type

---

You should use BOOST_MULTI_INDEX_CONST_MEM_FUN(T, std::string, id) instead of BOOST_MULTI_INDEX_CONST_MEM_FUN(T, std::string, T::id), as documented here.

---

In response to your last comment: This snippet compiles for me:

void foo(std::string id)
{
    conn_table_by_id_type& id_type_view = conn_table_.template get<0>();
    typename conn_table_by_id_type::const_iterator it = id_type_view.find(id);
}

Where foo is a member function inside the conn_mgr template. I'm guessing that the above is what you were trying to do.

You should write helper methods that get references to your three different conn_table_ indices. This will make things much more concise. For example:

conn_table_by_id_type & by_id_type() {return conn_table_.template get<0>();}

void foo2(std::string id)
{
    typename conn_table_by_id_type::const_iterator it = by_id_type().find(id);
}