Re: [Boost-bugs] [Boost C++ Libraries] #8011: iterator_range::size() is not SFINAE-friendly

Subject: Re: [Boost-bugs] [Boost C++ Libraries] #8011: iterator_range::size() is not SFINAE-friendly
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2013-02-13 01:06:04

#8011: iterator_range::size() is not SFINAE-friendly
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  Reporter:  Jonathan Wakely <jwakely.boost@…>           |       Owner:  neilgroves
      Type:  Feature Requests                            |      Status:  new       
 Milestone:  To Be Determined                            |   Component:  range     
   Version:  Boost 1.52.0                                |    Severity:  Problem   
Resolution:                                              |    Keywords:            
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Comment (by nathanridge):

 Replying to [comment:4 jwakely.boost@…]:
 > I use `boost::zip_iterator`so to construct an end iterator that is
 reachable from  {r1.begin(), r2.begin(), ...}  it needs to be
 {r1.begin()+N, r2.begin()+N, ...} (where N is the size of the smallest
 range) not {r1.end(), r2.end(), ...} because `operator==` for
 `zip_iterator` is only true if all members of the tuple are equal.
 >
 > https://gitorious.org/redistd/redistd/blobs/master/include/redi/zip.h

 > Since I switched from using the size() member it needs fixing to avoid
 calling both `std::distance()` and `std::advance()`, but the point of this
 ticket is that I'd prefer to use size() when possible

 Let me suggest an alternative way of implementing such a zipped range:

 - Use a custom iterator class rather than boost::zip_iterator
    - Use a special marker value to represent 'end'
    - In the begin iterator, store the end of each component range as well
 as the current position in each
 - When incrementing the begin iterator, increment each component iterator.
 When any one of them compare equal to the corresponding end iterator, set
 the iterator's value to the special marker value that represents 'end'

 This way:
 - You don't do an extra traversal of forward and bidirectional component
 ranges to figure out their size
 - You range is usable with single-pass component ranges

 The only disadvantage I can see is the extra memory to store the component
 end iterators in the zipped range's iterator.

-- 
Ticket URL: <https://svn.boost.org/trac/boost/ticket/8011#comment:5>
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