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Subject: Re: [boost] [Intrusive] [MultiIndex] Random Access mental model?
From: Ion Gaztañaga (igaztanaga_at_[hidden])
Date: 2008-11-01 19:22:39

David Abrahams wrote:
> I know. I think you must misunderstand me. When I wrote of using "a
> value_type that holds an additional pointer" I meant that I would build
> the FIFO queue using the hash nodes, roughly:

Ok, thanks for the example. The only advantages I see to implement this
class with Boost.Intrusive is that pop() could be faster (and no-throw):

           void pop()
               value_type & v = fifo->front();

and that you avoid copies (what happens if the key is already in the map
in your example?):

> // push a value for the given key onto the FIFO
> void push(Key const& k, Value const& v)
> {
> typename map_type::iterator pos
> = map.insert(k, fifo_value(v, &last));
> *tail = &pos->first;
> tail = &pos->;
> }

Supposing you want to avoid duplicate values then

           // push a value for the given key onto the FIFO
           void push(Key const& k, Value const& v)
               typename map_type::insert_commit_data data;
               //Test if the key is there without constructing
               //the value
               if(!map.insert_check(key, key_hasher()
                  , key_mapped_compare(), data).second){
                 //Already present, return
               //Success guaranteed, now commit
               mapped_type *m= new mapped_type(k, v);
               map.insert_commit(*m, data);
               //If slist is implemented with cachelast<true> option
               //you have O(1) push_back()

If you allow duplicates then I suppose you meant unordered_multimap
instead of unordered_map, so just forget insert_check/insert_commit and
do direct insertion:

               mapped_type *m= new mapped_type(k, v);
               map.insert(*m, data);

The drawback is that you need to define a "mapped_type" that stores
Value and Key and do the memory management. Please let me know if the
example was helpful or I'm wrong (again).



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