Re: [Boost-users] [Multi-Index] Get container view of a non-unique-index equal_range?

11 Jun 2009


      On Thu, Jun 11, 2009 at 3:12 PM, Steven Watanabe wrote:
...
AMDG
Dominique Devienne wrote:
...
 <snip>
I'd like to be able to have a "view" of VehicleIndex, for example
Because of the way multi_index implements hashed_non_unique,
there is no sequence of equal elements.
What if I switch to using an ordered rather than hashed non-unique index?
Would that change your answer?
...
If you try to store a pair of iterators,
they may be invalidated or cease to delimit the equal_range.
Aren't B.MI iterators pretty resilient in the face of changes to the
container? I remember reading that in the doc, and was actually
thinking of replacing some yummy code (below) that takes copies of a
container with a B.MI, since my reading of the doc was that i, as an
iterator from the B.MI, would not be invalidated by side-effects of
calling detach() which end up erasing other elements (or itself) from
observers_. Did I read that wrong?

        ObserverList copy = *observers_ ;
        for (ObserverList::iterator i(copy.begin()); i != copy.end(); ++i) {
           Observer* obs = *i;
           // still a valid observer?
           if (observers_->contains(obs)) {
               if (obs) {
                   obs->detach(this) ;
               } else {
                   cerr << "null observer ignored" << endl;
               }
           }
        }
...
You could try looking up the iterators on demand,
if you don't mind the extra overhead.
template
struct equal_range_view {
public:
  equal_range_view(MultiIndexContainer& c, const Key& k) : container_(c),
key_(k) {}
  typedef ... iterator;
  iterator begin() const { return(container_.equal_range(key_).first); }
  iterator end() const { return(container_.equal_range(key_).second); }
private:
  MultiIndexContainer& container_;
  Key key_;
};
That's two equal_range lookups everytime to iterate the range. I was
hoping that B.MI had some kind of "logical" begin-range, end-range
iterators that adjusted themselves automatically upon inserts and
updates in fact. Am I in luck and using an ordered_non_unique index
give me that? Maybe in 1.40 ;-) --DD
...
In Christ,
Steven Watanabe