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

On Tue, 25 Nov 2014 13:55:15 -0500 Matei David wrote:

On Tue, 25 Nov 2014 16:41:02 +0000 Neil Groves wrote:

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On 25 November 2014 at 16:11, Filip Konvička wrote:

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Hi,

I was trying to replace BOOST_REVERSE_FOREACH with the standard range-based for loop.

#include #include #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.

Hi,

Is this related to the issue I reported here? https://svn.boost.org/trac/boost/ticket/10789

Looking more into this issue, I found this bug report from 2 years ago that describes what I think is the underlying problem: https://svn.boost.org/trac/boost/ticket/7630 Given that the issue (bug?) involves a use case as simple as the one described by OP, many people might be bothered by it. Not everyone will know to ask on this list. Worse still, some might invest time in uselessly reporting the same issue all over again. For these reasons, I strongly agree with OP in suggesting that a *big warning sign* should be included in the Boost Range 2.0 documentation about this issue: "Thou shalt not use boost range adaptors on rvalue containers in C++11 range-based for loops." If the note is already there, then it is not prominent enough. Regarding solutions or workarounds, the last post by neilgroves in the bug report is from 9 months ago, suggesting he has a solution. Are there any updates on that?