id,summary,reporter,owner,description,type,status,milestone,component,version,severity,resolution,keywords,cc
7630,Range adaptors do not play nicely with range-based for loops,gromer@…,Neil Groves,"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.
",Bugs,assigned,To Be Determined,range,Boost 1.51.0,Problem,,range,jyasskin@… roman.perepelitsa@… mikhailberis@…