Sensei <senseiwa <at> gmail.com> writes:
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++)
Hi Sensei, One easy way to get something roughly equivalent in many situations to what you want is (warning, uncompiled): template<typename Tag> typename boost::multi_index::index<DataStorage,Tag>::index::const_iterator myfind(const DataStorage& s,unsigned int v) { return s.template get<Tag>().find(v); } ... auto it=myfind<byMSB>(storage,MSB(value128bits)); myfind returns a different iterator type depending on the tag you specify when calling it. Note that this is *not* the same as returning a custom iterator type that is able to traverse the different indices DataStorage is comprised of: in order to do that you need to implement an iterator class with *type erasure*, which has a run-time penalty and is not entirely trivial to write. If you want to know more about type-erasing iterators, take a look at, for instance, adobe::any_iterator: http://stlab.adobe.com/classadobe_1_1any__iterator.html and Boost.TypeErasure: http://www.boost.org/doc/html/boost_typeerasure.html Joaquín M López Muñoz Telefónica