Hi,
   First of all thanks for the great library, which had reduced our development time by a lot. 
   In boost multi-index, we currently have almost all the indexing concepts integrated. Hence i am requesting for a new feature called conditional indexes, where in an index/view inside multi-index will only index a portion of the entries inside the entire multi-index(Say something that satisfies a functor). This is in resemblance to the partial indexes in Postgresql.  
    I feel that would provide elegant solutions to lot of issues without forming round about solutions. For example i have a requirement where in there are two sets of data with the same search criteria, but different eviction criteria

Say i have two sets of data with same search condition, but one set needs a MRU(Most Recently Used) list of 100 and the other set requires a MRU of 200. Say the entry is like this

class Student

{

     int student_no;

     char sex;

     std::string address;

};

The search criteria is student_no, but for sex='m', we need MRU of 200 and for sex='f', we need a MRU of 100. Now i have a solution where in i introduce a new ordered index to maintain ordering.

For example the IndexSpecifierList will be something like this

typedef multi_index_container<

  Student,

  indexed_by<

    ordered_unique< member<Student, int, &Student::student_no> >,

    ordered_unique< composite_key<

                    member<Student, char, &Student::sex>,

                    member<Student, int,  &Student::sex_specific_student_counter> > >

  > 

> student_set

Here i have to introduce a new counter called sex_specific_student_counter, and during the eviction time, i need a logn search. Insertion also involves a extra logn complexity. Now instead if i have two sequenced indexes (like mentioned in the MRU example) in the place of second ordered index, but both with a condition on indexing only a few items, this would have looked more elegant and with better performance.

Thanks in advance,

Gokul.