Re: [Boost-users] multi_index_container for composite key

Uthpal Urubail escribió:

Hi

I am looking for a composite key container.

Though I found multi_index_container for composite key is the best I have few doubts in its use case:

*Question:01*

class Book

{

public:

size_t mID;

*size_t node[3];*

};

How to add the below into multi_index_container?

typedef multi_index_container<

Book,

indexed_by<

ordered_unique<

composite_key<

Book,

member,

member,

member

>

>,

ordered_unique<

member

>

>

...

beambook;

member does not work for syntactic reasons (Book::node[0] is not a member of Book), but you can overcome this difficulty by using a custom key extractor (http://tinyurl.com/dafk2f ) just like this: template<int N> struct BookNodeExtractor { typedef size_t result_type; result_type operator()(const Book& b)const { return b.node[N]; } }; typedef multi_index_container< Book, indexed_by< ordered_unique< composite_key< Book, BookNodeExtractor<0>, BookNodeExtractor<1>, BookNodeExtractor<2> > >, ordered_unique< member > >

beambook;

*Question:02*

My requirement is straight forward. I need to get the id by providing 3 nodes.

Should I always need to create a data structure? [class Book]

No, in fact you don't need to create a Book object. Suppose bb is a beambook and you want to retrieve the mID of an element in bb with nodes n0, n1 and n2. You only have to do the following: size_t mID=bb.find(boost::make_tuple(n0,n1,n2))->mID; As you can see, you simply pass a tuple with the values for the composite key, not a whole Book object. Hope this helps, Joaquín M López Muñoz Telefónica, Investigación y Desarrollo