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::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
