id	summary	reporter	owner	description	type	status	milestone	component	version	severity	resolution	keywords	cc
5641	Multiple keys in same index for same value in multi_index	Aaron Moss <moss.aaron@…>	Joaquín M López Muñoz	"I have a collection of data objects analogous to this one:

{{{#!c++
struct foo {
  bar b;
  std::set<baz> s;
  quux q;
};
}}}

which I want to be able to efficiently search either by b, or any element of s (q is extra information). The natural way to do that would be some sort of multi_index-like container, where I have a hash-map of bar to foo (with one key per value), and another hash-map of baz to foo (with multiple keys mapping to the same foo).

I may be mistaken, but I don't believe the second map can be set up (efficiently, in its obvious implementation) in the current multi_index. However, if it is allowable to have different sizes for different views of the same collection / different numbers of keys of the same index mapping to the same value, it would perhaps be a useful capability.

Without looking at the Boost sources, this would probably involve generalizing key extractors to support multiple keys from the same value, with syntax looking something like the following:

{{{#!c++
typedef multi_index_container<
  foo,
  indexed_by<
    hashed_unique<
      member<foo, bar, &foo::b>
    >,
    hashed_unique<
      member_collection<foo, std::set<baz>::iterator, &foo::s.begin, &foo::s.end>
    >
  >
> my_map;
}}}"	Feature Requests	closed	To Be Determined	multi_index	Boost 1.46.1	Not Applicable	wontfix