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;
}

You can't typedef a template. This line is invalid: `typedef boost::shared_ptr conn_ptr_t;` — Emile Cormier, Mar 10 '11 at 05:34

Emile Cormier · Accepted Answer · 2011-03-10T17:53:50.330

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);
}

Emile, thx a lot. Also I learnt a lesson of using typename. Eg. When I tried std::vector::iterator, it gave error but once I converted to typename std::vector::iterator, it worked. — rjoshi, Mar 10 '11 at 13:10
Hi Emile, now when I use the conm_mgr class as shown above, I get error "No type named "T" in class smpp_conn. I am passing "smpp_conn" class object as first template parameter "T" in conn_mgr class. — rjoshi, Mar 10 '11 at 13:57
@Emile: that worked. Need one more help. How do I find and iterator? e.g typedef typename conn_table_t::template nth_index<0>::type conn_table_by_id_type; conn_table_by_id_type *id_type_view; typename conn_table_by_id_type::const_iterator it = id_type_view->find(id); gives error on scalar conversion for it. — rjoshi, Mar 10 '11 at 15:58
@rjoshi: You can't invoke methods on the uniniatized pointer `*id_type_view`. See revised answer. — Emile Cormier, Mar 10 '11 at 17:46
@Emile: thx. I was getting iterator from index 2 and assigning to id_view which is index 1. But I like you idea of creating a wrapper function. Thx a lot again. — rjoshi, Mar 11 '11 at 14:33

template parameter to boost multi index container

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