Re: [Boost-users] [MultiIndex] more on key extractor issues

Subject: Re: [Boost-users] [MultiIndex] more on key extractor issues
From: John M. Dlugosz (mpbecey7gu_at_[hidden])
Date: 2013-01-20 05:08:39

> 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_container|s 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