________________________________________ De: boost-users-bounces@lists.boost.org [boost-users-bounces@lists.boost.org] En nombre de Etienne Philip Pretorius [icewolfhunter@gmail.com] Enviado el: domingo, 26 de abril de 2009 16:17 Para: boost-users@lists.boost.org 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
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