Multi-Index: custom iterator

25 Aug 2014

      Dear all,

I'm not yet satisfied with my MultiIndex games, so now I am trying to do 
something that is purely cosmetic. However, I have *no idea* how I can 
do this.

How can I build a custom iterator on a MultiIndex?

The objective is simple. As you may recall, I am building a MultiIndex 
with 128 bits, indexing them by MSB, LSB, and HSB (in the middle). It 
works like a charm.

Now I'd like to write something like this (or similar):

	auto it = storage.myfind<byMSB>(MSB(value128bits));

	for (; i != it.end(); i++)

In essence, I'd like to hide the get<> and everything I am using right 
now (as shown below). So I need an iterator for this data structure:

     typedef __uint128_t Storage;

     // Indexing placeholders
     struct byMSB { };
     struct byHSB { };
     struct byLSB { };

     // Storage type with indexing
     typedef boost::multi_index::multi_index_container<
         Storage,
         boost::multi_index::indexed_by<
             // Hashed by subject/predicate/object

boost::multi_index::hashed_non_unique>,     // 0

boost::multi_index::hashed_non_unique>,   // 1

boost::multi_index::hashed_non_unique>       // 2
         >
     > DataStorage;

     DataStorage storage_;

With your suggestions I've achieved to find all items by MSB, but 
sincerely, now I don't know how to write a custom iterator for this type.

The code for finding elements is this:

         //typedef typename DataStorage::template nth_index<0>::type 
byHashedIndex;
         //typedef typename DataStorage::template index<byMSB>::type 
byHashedIndex;

         //byHashedIndex &p = storage_.template get<byMSB>();

         auto &p = storage_.template get<byMSB>();

         auto q = p.equal_range(subject(v));

         while (q.first != q.second)
         {
             std::cout << " EQID " << *(q.first) << " | " << 
msb(*(q.first)) << " | " << hsb(*(q.first)) << " | " << lsb(*(q.first)) 
<< std::endl;

             l.push_back(*(q.first));

             q.first++;
         }

Note: I *have* written iterators, but they were much easier (for 
instance char*). This is far too complex for making my way on my own!

 From the code above, I guess I have begin() and end() point to an 
equal_range output .first and .second. But what will the "pointer" I 
need in the iterator will be?

Of course, my first attempt is a complete failure:

     // Iterator
     template <class T>
     class iterator : public std::iterator
     {
         typedef typename DataStorage::template index<T>::type 
byHashedIndex;

         byHashedIndex &p;

         // Default constructor
         iterator() : p(storage_.get<T>()) { };

         // Copy constructor
         iterator(const iterator& i) : p(i.p) { };

         // Pre-increment ++it
         iterator& operator++() { ++p; return *this; };

         // Post-increment it++
         iterator operator++(int) {iterator tmp(*this); operator++(); 
return tmp; };

         // Equality
         bool operator==(const iterator& rhs) { return p == rhs.p; };

         // Non-equality
         bool operator!=(const iterator& rhs) { return p != rhs.p; };

         // Access
         Storage& operator*() { return *p; };

         // Difference
         iterator operator-(int i) { return iterator(p - i); };

         // Addition
         iterator operator+(int i) { return iterator(p + i); };
     };

Can you point me in the right direction?

Thanks!

Sensei

Joaquin M Lopez Munoz

Sensei

Joaquin M Lopez Munoz

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