Re: [Boost-users] any boost iterator adapter to make it java styleiterator ?

From: abir basak (abirbasak_at_[hidden])
Date: 2007-03-03 00:38:32

Jeff F wrote:
> abir basak wrote:
>> Hi,
>>   In some case I need a iterator which knows its position (index ) in
>> the container, so that I can get the index back from iterator.
>> The need is two fold.
>> 1) This is needed for simultaneous traversal to different container.
>>  i.e i have vector<point> and vector<velocity> where each point
>> corresponds to velocity at same position. Thus converting the iterator
>> to the index helps.
> 
> Since your dealing with random access iterators, std::distance( c.begin(), 
> itr ) gives you that in constant time doesn't it, at least when you have 
> access to the container.
> 
Yes, "when I specify the container", for random access iterator 
std::distance is const time.
For container I am using boost::circular_buffer. From the implementation 
  of its iterator it looks it knows the container already as its member 
variables are const Buff* m_buff, and pointer m_it;
Thus if it can return the container reference also , I don't need to 
specify the container (Though it is not a general solution esp for 
std::vector or native pointers, where it doesn't know the container ).

>>   But even better if I have a multi iterator, which can club a set of
>> iterators, and allow to return all of them (like (*it).point() returns
>> point, (*it).velocity() returns velocity etc. Also it defines ++, --
>> etc the same way (i.e increments all these, or one in a logic , say
>> next point is a jump in velocity by 2 etc), so that I can use
>> boost::sub_range for these kind of iterator.
> 
> Doesn't http://www.boost.org/libs/iterator/doc/zip_iterator.html do what you 
> want?
> 
This will do exactly what I need (I skipped this one thinking it might 
be used to iterate a zip stream :( ) Thanks for pointing it.
>> 2) I need to convert iterator to index, as I need to store certain
>> portion of a container (which changes its size) in a class. There I
>> can only store index as iterator may get invalidated.
>> It is something like,
>>   void mark_if_useful(point_sub_range r){///an portion of the
>>      container point_iterator it = r.begin();
>>      for(; it != r.end() ; ++it){
>>       if ((*it).x == 123 ) {
>>           ///ok certain criterion is fulfilled
>>           ///I need to mark (ans store tis position)
> 
>     std::size_t idx = std::distance( r.begin(), it );
> 
>>           ///I can't store *it as it is a pointer to a movable object
>>           ///However I can store (*it).index() may be it is 5th
>> location                in the vector, and (*it)>container() also the
>> reference to the container
>> ///may not be needed if the container is globally available.
>> ///if some object also gets removed the the remove count from the
>> container can be added to have a unique linear index (that can me done
>> adding additional method to the container :) )
>>       }
>>      }
>>   }
> 
> Jeff 

-- 
Abir Basak, Member IEEE
Software Engineer, Read Ink Technologies
B. Tech, IIT Kharagpur
email: abir_at_[hidden]
homepage: www.abirbasak.com