Re: [Boost-users] multi-index-container with nested templates => problem getting iterator on 2nd index.

Subject: Re: [Boost-users] multi-index-container with nested templates => problem getting iterator on 2nd index.
From: joaquin_at_[hidden]
Date: 2009-05-14 02:16:24

JohnQ escribió:
>
>
>     >   typedef boost::multi_index_container<
>     >     value_type,
>     >     indexed_by<
>     >       ordered_non_unique<
>     >         tag<index1>,member<value_type,Key1,&value_type::first> >,
>
>     This cannot possibly work, as value_type does not have a
>     member called first (you're using a tuple, not an std::pair).
>     Anyway, the problem does not lie here.
>
>
> Yes, I've been ignoring this while I figured out the iterator... it's 
> leftover from the example and I just named things first,second,third 
> while I get the rest figured out. What I did instead was use the 
> structure below... I didn't want to mess with stuff like 
> tuple_element<N>::template get<RET>(x) (or whatever the correct 
> type-independent tuple-element accessor would be) becuase I'm really 
> not that familiar with this sort of meta programming... this seems 
> like it will work but if you're familiar with a better way, I'd love 
> to hear about it.

You have to define a custom key extractor to work with tuples:
for an example, look for "tuple_member_extractor" at
http://lists.boost.org/boost-users/2005/12/16088.php

[...]
>
>     >       typedef typename bimultimap<Key1,Key2,Data>::
>     >          bimimap::index<index2>::type::iterator bimimapIterator2;
>
>     You need a disambiguating template keyword:
>
>            typedef typename bimultimap<Key1,Key2,Data>::
>              bimimap::template index<index2>::type::iterator
>     bimimapIterator2;
>
>
> That compiles (BIG THANK YOU). Next step is for me to figure out what 
> this actually does. :)  Can you give me some background (or point me 
> to the correct reading material) on why the "template" qualifier is 
> needed and what effect that has? I need to learn more about this...

The need for diambiguating "template"s is related to the usage of "typename"
in expressions like "typedef typename bimultimap..." that appear in 
so-called
dependent contexts. Some further explanations at

http://www.comeaucomputing.com/techtalk/templates/#templateprefix

Joaquín M López Muñoz
Telefónica, Investigación y Desarrollo