Re: [Boost-users] Reverse for loop with boost::adaptors::reverse crashes

Subject: Re: [Boost-users] Reverse for loop with boost::adaptors::reverse crashes
From: Neil Groves (neil_at_[hidden])
Date: 2014-11-25 11:41:02

On 25 November 2014 at 16:11, Filip Konvička <filip.konvicka_at_[hidden]>
wrote:

> Hi,
>
> I was trying to replace BOOST_REVERSE_FOREACH with the standard
> range-based for loop.
>
> #include <boost/range/adaptor/reversed.hpp>
> #include <boost/foreach.hpp>
> #include <vector>
> #include <iostream>
>
> std::vector<int> getv() {
>   return std::vector<int>{1,2,3};
> }
>
> int main(int argc, char* argv[]) {
>   // works fine:
>   BOOST_REVERSE_FOREACH(int i, getv())
>     std::cout << i << std::endl;
>   // crashes:
>   for(int i : boost::adaptors::reverse(getv()))
>     std::cout << i << std::endl;
>   return 0;
> }
>
> Maybe this is basic C++ I should understand; it looks like the temporary
> vector returned by getv() gets destroyed in the 2nd loop too soon.  I was
> wondering whether the adaptor could warn me that I'm using it in a wrong
> way (the code looks so nice and compiles cleanly...).
>
>
Hi, I maintain the Boost.Range library and this issue is the most
frustrating one. I've been unable to come up with a solution that does not
unacceptably deteriorate performance in valid cases. I've thrown the
problem wide open to the list and a number of other people and there
haven't been any solutions that wouldn't ruin the interface or performance.
I can only apologise for failing to address this. I've tried for many hours
to find a solution, but failed.

Our hope is with Range V3 and the standardisation proposal from Eric
Niebler. He has defined the lifetime of new Iterables and Range Concepts to
avoid this problem. With a new interface we can fix the problem.

I'm still open to solutions for the V2 Boost.Range. I'd certainly not like
to be irrationally blocking progress. The current recommendation is to take
a temporary outside of the loop. This can often be a const reference
temporary to extend the lifetime sufficiently.

> Thanks,
> Filip
>

Regards,
Neil Groves