Have you taken a look at the table in:

http://www.boost.org/libs/multi_index/doc/tutorial/
key_extraction.html#advanced_key_extractors

Regardless of whether the indexed-by member is const or not, when using
pointers to const elements the proper way of declaring the extractors is:


  member<
   ItemBase,
   const unsigned long,
   &ItemBase::some_member
  >

  member<
    ItemBase,
    const S2,
    &ItemBase::another_member
  >

This should clear problems away both in VS and Clang (please report
otherwise.)

Joaqu?n M L?pez Mu?oz
Telef?nica Digital

I read that part in detail (including the typo, T:gg instead of T::gg), going over all the MI docs again after my postings.  I agree this seems to address that use-case, but I really don't understand the table and section, and certainly had no clue what it was getting at when I read it without this issue specifically in mind. 

"The column "applicable to const elements?" states whether the corresponding key extractor can be used when passed constant elements (this relates to the elements specified in the first column, not the referenced T objects)."  What does that mean?
I was actually looking at this in regards to figuring out what a non-const member function extractor was good for.
"this also implies that multi_index_containers of elements of T cannot be sorted by T::g or T::gg,"  if you are not using it to sort by, what is the point of having the key extractor?

Anyway, I found that it works correctly on both compilers if the middle argument (the key type) is declared const for member and not for const_mem_fun.  I suppose that is due to differences in the metaprogramming, but really they should be treated the same.  Const on the first parameter, T, doesn't seem to matter if it's declared const or not.

—John