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@abirbasak.com homepage: www.abirbasak.com