Re: [Boost-users] Is this possible with boost::multi_index?

From: Joaquín Mª López Muñoz (joaquin_at_[hidden])
Date: 2006-05-03 12:23:29

Hello Jason,

Jason House ha escrito:

> Below is a small piece of code that I can't get to compile.
> gcc says: error: forward declaration of 'class search_node'
>
> Is there a way to get around this, or can I not embed a
> multi_index_container like that?  I'm using boost 1_32_0 and gcc version
> 4.0.1
>
> The code below is for a node within a search tree.  Each node stores a
>
> > class search_node{
> >   int      objective_value;                // Objective function
> >   position my_position;                    // Move location for this step in the search
> >   std::list<position> positions_to_visit;  // Future moves to consider in search
> >   boost::multi_index_container <
> >     search_node,
> >     indexed_by<
> >       ordered_unique    <search_node, position, &search_node::my_position>,
> >       ordered_non_unique<search_node, int,      &search_node::objective_value>
> >     > > search_results;
> > };
>

(First of all, you're missing the member<> part inside ordered_unique<...>
and  ordered_non_unique. I assume this is just a copy&paste error and that
you're actually using member<>.)

Unfortunately, multi_index_container cannot be instantiated for incomplete types,
as you're doing by defining the type of search_results inside search_node
itself. To escape this self-referentiality, you can add a level of indirection
like this:

   boost::multi_index_container <
     const search_node*,
     indexed_by<
       ordered_unique    <member<search_node, position, &search_node::my_position> >,
       ordered_non_unique<member<search_node, int,      &search_node::objective_value> >
   > > search_results;

or

   boost::multi_index_container <
     boost::reference_wrapper<const search_node>,
     indexed_by<
       ordered_unique    <member<search_node, position, &search_node::my_position> >,
       ordered_non_unique<member<search_node, int,      &search_node::objective_value> >
   > > search_results;

both of which are appropriate if the life cycle of the elements referenced by search_results
are
properly managed outside search_results. If this is not the case, i.e. if search_results must

take care of properly destroying the referenced search_node's, you can use:

   boost::multi_index_container <
     boost::shared_ptr<const search_node>,
     indexed_by<
       ordered_unique    <member<search_node, position, &search_node::my_position> >,
       ordered_non_unique<member<search_node, int,      &search_node::objective_value> >
   > > search_results;

Hope this helps,

Joaquín M López Muñoz
Telefónica, Investigación y Desarrollo