Boost C++ Libraries: Ticket #6067: is broken

attachment set

anonymous — Thu, 27 Oct 2011 14:41:46 GMT

attachment → raw.cpp

owner changed

Joel de Guzman — Fri, 28 Oct 2011 23:41:39 GMT

owner changed from Joel de Guzman to Eric Niebler

Eric, could you please have a quick check?

The error points me to:

C:\dev\boost\boost/fusion/view/iterator_range/detail/segmented_iterator_range.hpp(261) : error C2146: syntax error : missing ',' before identifier 'push_back'

I'm not familiar enough with segmented_iterator.

Thanks!

owner changed

Eric Niebler — Sat, 29 Oct 2011 05:48:17 GMT

owner changed from Eric Niebler to Joel de Guzman

The problem is simple, and the fix is also simple but tedious and requires discipline. The problem is circular includes. It's nearly impossible to avoid. Consider that fusion::begin should automatically return a segmented_iterator for segmented sequences. But the implementation of segmented_iterator itself need to use fusion::begin() to find the start iterator of the sequence's internal segments. Hence, you end up with a circular dependency: begin.hpp needs segmented_iterator, but segmented_iterator.hpp needs begin. In this case, it's the fact that segmented_iterator's implementation uses push_back, which uses joint_view, which uses begin, which uses segmented_iterator, closing the circle.

The solution is consistent use of _fwd.hpp headers. Every .hpp gets an associated _fwd.hpp that forward-declares the things it publishes. The _fwd.hpp must do nothing but forward-declare stuff and include other _fwd.hpp headers. Then, for instance, foo.hpp includes foo_fwd.hpp before it includes anything else. When you do that consistently, circular includes happen, but they never lead to this strange situation where a file appears to be included but its symbols are not visible -- as is happening here.

In Proto, I solve this problem by having one big proto_fwd.hpp file that forward-declares everything in Proto. That gets included from every other file in Proto. Once that's done, headers are free to include each other willy-nilly and the circular include problem is vanquished. You may choose to do something similar, or individual _fwd.hpp headers, or maybe granular ones like algorithm_fwd.hpp and intrinsic_fwd.hpp. You get the idea.

HTH, Eric

attachment set

Eric Niebler — Sat, 29 Oct 2011 05:55:25 GMT

attachment → segmented_iterator_range.hpp.patch

Hack to solve the immediate problem

Eric Niebler — Sat, 29 Oct 2011 05:59:22 GMT

Sorry, forgot to mention: the above is a description of how to address the larger issue. For the immediate problem, we can hack some forward declarations directly into segmented_iterator_range.hpp, as in the patch I just attached.

Also, my apologies for not catching this issue sooner. I already added a bunch of _fwd.hpp headers to solve many of these problems and thought I had caught them all. Clearly I was wrong. A consistent policy is needed, but I'll leave that for you to decide.

Joel de Guzman — Fri, 04 Nov 2011 20:39:22 GMT

Thanks Eric. This will do for now. Hmm, I guess it's too late to merge to release huh?

Eric Niebler — Fri, 04 Nov 2011 21:01:50 GMT

I don't think it's too late, and this is a very low risk fix with a potentially large positive impact. As a release manager, you have my permission.

Joel de Guzman — Sat, 05 Nov 2011 02:03:31 GMT

Thanks, Eric! Done.

Wed, 05 Nov 2014 13:35:07 GMT

This is already fixed. djowel, please close.

status changed; resolution set

Joel de Guzman — Wed, 05 Nov 2014 15:06:29 GMT

status new → closed
resolution → fixed