The November 2010 mailing for the C++ Standards Committee was published last week. This is the post-meeting mailing for the November 2010 committee meeting.

As well as the usual core and library issues lists, this mailing also includes an updated summary of the status of the FCD comments, along with a whole host of papers attempting to address some the remaining FCD comments and a new C++0x working draft.

To move or not to move

In my blog post on the October mailing, I mentioned that the implicit generation of move constructors was a big issue. I even contributed a paper with proposed wording for removing implicit move generation from the draft — with expert core wording guidance from Jason Merrill, this became N3216. My paper was just to give the committee something concrete to vote on — it doesn't matter how good your arguments are; if there isn't a concrete proposal for wording changes then the committee can't vote on it. In the end, the decision was that implicit move generation was a good thing, even though there was the potential for breaking existing code. However, the conditions under which move operations are implicitly generated have been tightened: the accepted proposal was N3203: Tightening the conditions for generating implicit moves by Jens Maurer, which provides wording for Bjarne's paper N3201: Moving Right Along. The proposal effectively treats copy, move and destruction as a group: if you specify any of them manually then the compiler won't generate any move operations, and if you specify a move operation then the compiler won't generate a copy. For consistency and safety, it would have been nice to prevent implicit generation of copy operations under the same circumstances, but for backwards compatibility this is still done when it would be done under C++03, though this is deprecated if the user specifies a destructor or only one of the copy operations.

Exceptions and Destructors

The second big issue from the October mailing was the issue of implicitly adding noexcept to destructors. In the end, the committee went for Jens Maurer's paper N3204: Deducing "noexcept" for destructors — destructors have the same exception specification they would have if they were implicitly generated, unless the user explicitly says otherwise. This will break code, but not much — the fraction of code that intentionally throws an exception from a destructor is small, and easily annotated with noexcept(false) to fix it.

Concurrency-related papers

There are 9 concurrency-related papers in this mailing, which I've summarised below. 8 of them were adopted at this meeting, and are now in the new C++0x working draft.

Other adopted papers

Of course, the committee did more than just address implicit move, exceptions in destructors and concurrency. The full minutes are available as N3212 in the mailing. Here is a quick summary of some of the other changes made:

Library functions that don't throw exceptions have been changed to use noexcept

The ratio arithmetic facilities have been changed to allow libraries to try and give the correct result if the result is representable, but the intermediate calculations may overflow (e.g. ratio_add<ratio<1,INTMAX_MAX>,ratio<1,INTMAX_MAX>> ) (N3210)

) (N3210) New functions have been added to retrieve the current new handler, terminate handler or unexpected handler (N3189)

Alignment control is now done with the alignas keyword, rather than an attribute (N3190)

keyword, rather than an attribute (N3190) Virtual function override control is now done with keywords (including the first context sensitive keywords: override amd final ) rather than attributes (N3206)

For the remaining changes, see the full minutes.

FCD comment status

The count of unresolved FCD comments is dropping rapidly, and now stands at 75 (out of 564 total), of which only 56 have any technical content. See N3224: C++ FCD Comment Status from the mailing for the full list.

Your comments

If you have any opinions on any of the papers listed here, or the resolution of any NB comments, please add them to the comments for this post.

Posted by Anthony Williams

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Tags: C++0x, C++, standards, concurrency

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