[Boost-bugs] [Boost C++ Libraries] #13094: boost::adaptors::transform fails to preserve bidirectional behavior of range

Subject: [Boost-bugs] [Boost C++ Libraries] #13094: boost::adaptors::transform fails to preserve bidirectional behavior of range
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2017-06-23 23:58:32

#13094: boost::adaptors::transform fails to preserve bidirectional behavior of
range
--------------------------------------+------------------------
 Reporter:  Jake Cobb <jake.cobb@…>   |      Owner:  neilgroves
     Type:  Bugs                      |     Status:  new
Milestone:  To Be Determined          |  Component:  range
  Version:  Boost 1.64.0              |   Severity:  Problem
 Keywords:  range adaptors transform  |
--------------------------------------+------------------------
 The documentation of `boost::adaptors::transform` states the returned
 range type is the same as the argument range type.  However, when wrapping
 `std::vector`, the iterators returned are not bidirectional as the
 `std::vector` iterators are.  `std::advance` with a negative integer
 leaves them unchanged and `std::prev` fails to compile with them.  They do
 work as expected with `operator-`.

 {{{#!c++
 #include <iostream>
 #include <vector>
 #include <iterator>
 #include <boost/range/adaptor/transformed.hpp>

 int main() {
     std::vector<int> data{1,2,3};
     auto transformed = boost::adaptors::transform(data, [](int x) { return
 x; });
     auto iter = transformed.begin();
     // auto iter = data.begin(); // swap to compare
     std::cout << "begin: " << *iter << std::endl; // prints 1 both cases
     std::advance(iter, 1);
     std::cout << "begin+1: " << *iter << std::endl; // prints 2 both cases
     std::advance(iter, -1);
     // prints 2 for boost adaptor, 1 for std::vector
     std::cout << "begin+1-1: " << *iter << std::endl;
     std::cout << "begin+1-1 (v2): " << *(iter - 1) << std::endl; // prints
 1 both cases
     // std::prev(iter); // boost fails bidirectional test, won't compile
     return 0;
 }
 }}}

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Ticket URL: <https://svn.boost.org/trac10/boost/ticket/13094>
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