[Boost-bugs] [Boost C++ Libraries] #7630: Range adaptors do not play nicely with range-based for loops

Subject: [Boost-bugs] [Boost C++ Libraries] #7630: Range adaptors do not play nicely with range-based for loops
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2012-11-02 18:37:22

#7630: Range adaptors do not play nicely with range-based for loops
-------------------------------+--------------------------------------------
 Reporter:  gromer@…           |       Owner:  neilgroves
     Type:  Bugs               |      Status:  new       
Milestone:  To Be Determined   |   Component:  range     
  Version:  Boost 1.51.0       |    Severity:  Problem   
 Keywords:                     |  
-------------------------------+--------------------------------------------
 Consider the following C++11 code, adapted from the Range documentation:


 {{{
 std::vector<int> vec;
 for (int val : vec | boost::adaptors::reversed
                    | boost::adaptors::uniqued) {
   // Do stuff with val
 }
 }}}

 The behavior of this natural-seeming code is actually undefined, due to a
 dangling reference: per the C++ standard, it is equivalent to


 {{{
 {
   auto && __range = (vec | boost::adaptors::reversed
                          | boost::adaptors::uniqued);
   for ( auto __begin = __range.begin(),
              __end = __range.end();
         __begin != __end;
         ++__begin ) {
     int val = *__begin;
     // Do stuff with val
   }
 }
 }}}


 The problem is that the value returned by the subexpression `vec |
 boost::adaptors::reversed` is a temporary, so its lifetime ends at the end
 of the statement containing it, namely the declaration of `__range`. Thus,
 `__range` is left holding a dangling reference to the range it's adapting.

 The fix is for each range adaptor to use an rvalue-reference overload to
 detect whether the input range is a temporary, and if so, move it into the
 adaptor (i.e. with `std::move`) in order to extend its lifetime.

-- 
Ticket URL: <https://svn.boost.org/trac/boost/ticket/7630>
Boost C++ Libraries <http://www.boost.org/>
Boost provides free peer-reviewed portable C++ source libraries.