Re: [Boost-users] [MultiIndex] Indexing integers

Subject: Re: [Boost-users] [MultiIndex] Indexing integers
From: Sensei (senseiwa_at_[hidden])
Date: 2014-07-29 11:26:39

As Joaquín helped me out, I'm currently using his solution to test multi 
index containers.

Playing with that, I'm now moving to hashed indices and 128 bits 
integers, just for fun.

So now, in my class Data, I have a 128bit integer, indexed by 
most/half/least significant bits, with access functions. It works 
nicely. I added a fourth index, a hashed one, hashed by the MSB, and now 
I'd like to access all items that have MSB equal to 3, for instance.

You will find my badly-written code below.

Following the example [1], I proceeded in defining the index type, but 
here my compiler throws me two errors:

    error: missing 'typename' prior to dependent type name
    'DataWithIndices::nth_index'
     ^

and

    error: expected unqualified-id
    typedef DataWithIndices::nth_index<3>::type byHashedIndex;
                                      ^

So I followed the suggestion and used typename, but an error is there:

    error: use 'template' keyword to treat 'nth_index' as a dependent 
template name
    typedef typename DataWithIndices::nth_index<3>::type byHashedIndex;
                                      ^

Again, I follow the suggestion, use template and it works. Adding the 
following line from the example gives me an error:

error: expected expression
         byHashedIndex &p = multiIndex_.get<3>();
                                            ^

error: declaration of reference variable 'p' requires an initializer
         byHashedIndex &p = multiIndex_.get<3>();
                        ^

Sincerely, I think I'm translating the example in a right manner, but 
obviously I am not.

Can you help me? I am learning still, so please excuse me if this is 
trivial!

Thanks & Cheers!

=== CODE ===

     typedef boost::multi_index::multi_index_container<
         __uint128_t,
         boost::multi_index::indexed_by<
 
boost::multi_index::ordered_non_unique<boost::multi_index::global_fun<__uint128_t, 
uint64, &Data::msb>>,
 
boost::multi_index::ordered_non_unique<boost::multi_index::global_fun<__uint128_t, 
uint64, &Data::hsb>>,
 
boost::multi_index::ordered_non_unique<boost::multi_index::global_fun<__uint128_t, 
uint64, &Data::lsb>>,
 
boost::multi_index::hashed_non_unique<boost::multi_index::global_fun<__uint128_t, 
uint64, &Data::msb>>
         >
     > DataWithIndices;

     DataWithIndices multiIndex_;

     void objs()
     {
         typedef typename DataWithIndices::template nth_index<3>::type 
byHashedIndex;

         byHashedIndex &p = multiIndex_.get<3>();
     }

[1] 
http://www.boost.org/doc/libs/1_55_0/libs/multi_index/doc/tutorial/indices.html#hash_lookup