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Archive for March 13th, 2010

[Note: I usually post trip reports after the public post-meeting mailing goes live a few weeks after the meeting, so that I can provide links to minutes and papers. This time, I wanted to post the report right away to share the news. If you’re interested in the post-meeting papers, including the official minutes, watch the 2010 papers page which is where they will appear in a few weeks.]

 

The ISO C++ committee met in Pittsburgh, PA, USA on March 8-13, 2010, hosted by the CERT Software Engineering Institute at Carnegie Mellon University. As usual, about 50 experts attended, and we had eight official national body delegations from Canada, Finland, France, Netherlands, Spain, Switzerland, United Kingdom, and United States.

IMG_0396The meeting just concluded a few minutes ago (yes, the meetings run through Saturday afternoon). Here are some highlights of what happened today, which was another landmark in the progression of the C++0x standard.

Approved Final Committee Draft (FCD) for C++0x

The biggest news is that this afternoon we voted in the final remaining feature changes to C++0x, and to much applause then unanimously approved the text for international ballot as a Final Committee Draft (FCD). FCD means that, assuming no surprises, we intend to do only bug fixes and editorial corrections for the next year or so, and then ballot a final standard. If we can do that, assuming all goes well, C++0x could officially be published as soon as next year as ISO C++ 2011, and we can stop with the “x-is-hex” jokes and just start calling it C++11.

This is a big milestone, and it was achieved thanks to removing a couple of controversial features last summer and a whole lot of work by the ISO C++ committee members over the past six months in particular. That work includes countless hours spent between our full face-to-face meetings at face-to-face ad-hoc meetings to swat library bugs, teleconferences on resolving core language questions, and triple-digit person-hours invested in four teleconferences during December-February purely about C and C++ compatibility that have greatly helped to identify and attempt to resolve minor areas of divergence between the C++0x draft standard and the C1x draft standard (as both are now in progress; C1x is targeting completion and publication in 2012).

All in all, your committee members have put in an enormous amount of effort to bring this in, and the draft is in far better shape for this meeting than anyone could have expected last summer. For comparison, in my and several others’ opinions, it’s in better shape than the FCD of the C++98 standard.

Since we’re closing down this round of standardization, we didn’t make many exciting technical changes. Here are two technical highlights of the meeting that are likely of general interest, finally adopting changes we’ve contemplated before.

Removed Export Template

As I reported after the last meeting (here and here), the committee considered the question of whether to deprecate, remove, or leave in the “export template” feature.

For context, the only reason we’re even considering this is because Edison Design Group (EDG), the only company to ever implement export, is recommending export be removed or deprecated. Recall that back in the 1990s the committee originally voted the feature in over EDG’s objections in the first place, then in the late 1990s and early 2000s EDG graciously and gallantly went on to invest enormous effort to implement the feature in order to conform to the standard, and so the committee was loath to punish them again by now removing the feature on them. However, given the passage of time, EDG reports that their experience in the field has been that nearly no one actually uses the feature, and that it would be right (and okay with EDG) to deprecate or remove it.

At our previous meeting, the general sentiment was in favor of deprecation only. However, at this meeting EDG reported that they would prefer the feature to be removed rather than just deprecated, because a deprecated feature is still part of the standard and required for conformance. By removing it from C++0x, it removes the maintenance burden of being forced to support export indefinitely to maintain full standards conformance (though of course EDG will continue to support it as an extension in their compiler front end for however long seems right based on their own customers’ demand).

The committee agreed, and today voted to remove export entirely from C++0x. The “export” keyword is still reserved for future use, but has no defined meaning in the C++0x standard.

Deprecated Exception Specifications, Added noexcept

As also considered at our previous meeting, this week we went ahead and deprecated throw-specifications (e.g., throw(), throw( Base, Derived1 )). For rationale, see the two background references referred to in the proposal, one of which is an article by me:

As noted in the second reference, “A non-inline function is the one place a ‘throws nothing’ [i.e., throw()] exception-specification may have some benefit with some compilers.” Because this would have some utility, there is a replacement for empty exception-specifications… the noexcept keyword, which has the basic use syntax “void f() noexcept {…}”. It optionally takes a compile-time constant true/false expression that if true means the noexcept is in force, which helps libraries to write efficient overloads. This feature will be written about for years to come, but that’s the basic idea.

Here’s the concise summary from the core working group report by Steve Adamczyk (of EDG fame and longtime core working group chair):

We discussed two things related to exception specifications: noexcept (N3050), and deprecating exception specifications (N3051). There was some spirited discussion of what happens when you violate a promise not to throw. In the end, we decided that in such a circumstance terminate() gets called, and it’s unspecified whether any cleanup is done for local variables between the throw point and the point of the violated noexcept. It’s believed that for efficient implementations of exception handling a noexcept will not add any overhead and will not restrict optimization opportunities.

The old-style exception specifications, including the throw() form, will now be deprecated. The library specification needs to be updated to use noexcept instead, and that won’t happen at this meeting.

Note that the decision to call terminate() is not final and alternatives will be considered at future meetings. There was general (but not universal) agreement that the program should not be allowed to continue executing if a noexcept is violated because in that case the program will almost certainly be in a corrupt state, and so the consensus was in favor of guaranteeing to call terminate() instead of allowing arbitrary undefined behavior, but it’s possible that a comfortable middle ground between those two options may yet be found.

Looking Forward

We wrapped up with a presentation of the scheduled dates and locations for the next few ISO C++ standards committee meetings (meeting information link included where available):

Finally, here’s a copy of summary slide I presented, which will also be in the post-meeting mailing. Here’s to C++11 (we hope)!

 

Pittsburgh slide.jpg

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Desktop part with 12 hardware threads (6 cores x 2 threads/core), 32nm process, >1.1B transistors.

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