Subject: Re: [Boost-users] Multi Index: Nested std::pair
From: Etienne Philip Pretorius (icewolfhunter_at_[hidden])
Date: 2009-04-26 12:57:52
JOAQUIN M. LOPEZ MUÑOZ wrote:
> ________________________________________
> De: boost-users-bounces_at_[hidden] [boost-users-bounces_at_[hidden]] En nombre de Etienne Philip Pretorius [icewolfhunter_at_[hidden]]
> Enviado el: domingo, 26 de abril de 2009 16:17
> Para: boost-users_at_[hidden]
> Asunto: Re: [Boost-users] Multi Index: Nested std::pair
>
> JOAQUIN M. LOPEZ MUÑOZ wrote:
>
>>> First of all, you're defining a multi_index_container of *lists*,
>>> i.e. each element of the container is a list of triplets (value,x,y).
>>> Is this what you really meant, or maybe the std::list part has to be
>>> dropped?
>> Thank you. Yes, I'll drop the list. Each element should only be (value,x,y);
>
> OK, let's analyze the problem then. For the sake of brevity we
> first introduce the following typedef:
>
> typedef std::pair<
> /*value*/
> unsigned char,
> std::pair<
> /*x co-ordinate*/
> unsigned char,
> /*y co-ordinate*/
> unsigned char
> >
> > value_type;
>
> The value extractor can be specified using boost::multi_index::member:
>
> boost::multi_index::member<
> value_type,
> unsigned char,
> &value_type::first
> >
>
> As for x and y, there is no way to do it with predefined extractors and we
> have to write our own custom key extractor (http://tinyurl.com/dafk2f ):
>
> struct x_extractor
> {
> typedef unsigned char result_type;
> result_type operator()(const value_type& x)const
> {
> return x.second.first;
> }
> };
>
> struct y_extractor
> {
> typedef unsigned char result_type;
> result_type operator()(const value_type& x)const
> {
> return x.second.second;
> }
> };
>
> So the final declaration looks like this:
>
> boost::multi_index_container<
> value_type,
> boost::multi_index::indexed_by<
> /*value*/
> boost::multi_index::ordered_non_unique<
> boost::multi_index::member<
> value_type,
> unsigned char,
> &value_type::first
> >
> >,
> /*x co-ordinate*/
> boost::multi_index::ordered_non_unique<x_extractor>,
> /*y co-ordinate*/
> boost::multi_index::ordered_non_unique<y_extractor>
> >
> > m;
>
> Nevertheless, I don't see any good reason why you have to define
> your elements using such a convoluted combination of std::pairs.
> You can have it in a more user friendly way, say:
>
> struct value_type
> {
> value_type(unsigned char value,unsigned char x,unsigned char y):
> value(value),x(x),y(y){}
> unsigned char value;
> unsigned char x;
> unsigned char y;
> };
>
> which allows us you to define your multi_index_container without
> resorting to custom key extractors:
>
> boost::multi_index_container<
> value_type,
> boost::multi_index::indexed_by<
> /*value*/
> boost::multi_index::ordered_non_unique<
> boost::multi_index::member<
> value_type,
> unsigned char,
> &value_type::value
> >
> >,
> /*x co-ordinate*/
> boost::multi_index::ordered_non_unique<
> boost::multi_index::member<
> value_type,
> unsigned char,
> &value_type::x
> >
> >,
> /*y co-ordinate*/
> boost::multi_index::ordered_non_unique<
> boost::multi_index::member<
> value_type,
> unsigned char,
> &value_type::y
> >
> >
> >
> > m;
>
> HTH.
>
> Joaquín M López Muñoz
> Telefónica, Investigación y Desarrollo
> _______________________________________________
> Boost-users mailing list
> Boost-users_at_[hidden]
> http://listarchives.boost.org/mailman/listinfo.cgi/boost-users
Thank you!
It is starting to make sense. Just what I needed :).
Etienne