+1 On Thu, Nov 5, 2015 at 7:58 PM, Bernd Mathiske <[email protected]> wrote:
> +1 > > This site has a list of compilers that support #pragma once. > > https://en.wikipedia.org/wiki/Pragma_once#Portability > > Clang, MS V C++, GCC as of 3.4. from 2006. OK! > > (Too bad for Sun/Oracle Studio C++. But you can use GCC on Solaris, right?) > > We could tackle this change at the same time as correcting the copyright > notes, then we have the history clutter only once: > > https://issues.apache.org/jira/browse/MESOS-3581 > > Any other projects of that nature that can be bundled? > > Bernd > > On Nov 5, 2015, at 6:36 AM, Alex Clemmer <[email protected]> > wrote: > > Hey folks. > > In r/39803[1], Mike Hopcroft (in quintessential MSFT style, heh) > brought up the issue of moving away from #include guards and towards > `#pragma once`. > > As this has been brought up before, I will be brief: we think it's > revisiting because the primary objection in previous threads appears > to be that, though `#pragma once` is a cleaner solution to the > multiple-include problem, it's not so much better that it's worth the > code churn. However, the ongoing Windows integration work means we > have to touch these files anyway, so if we agree this is cleaner and > desirable, then this is an opportunity to obtain that additional code > clarity, without the cost of the churn. > > For the remainder of the email, I will summarize the history of our > discussion of this issue, who will do the work, and what the next > steps are. > > PROPOSAL: We propose that all new code use `#pragma once` instead of > #include guards; for existing files, we propose that you change > #include guards when you touch them. > > HISTORY: This has been discussed before, most recently a year ago on > the mailing list[2]. There is a relevant JIRA[3] and discarded > review[4] that changes style guide's recommendation on the matter. > > SUMMARIZED OBJECTIONS: > 1. The Google style guide explicitly forbids `#pragma once`. > 2. This results in a lot of code churn, but is only marginally better. > 3. It's not C++ standardized/it's platform dependent/IBM's compiler > doesn't support it. > 4. Popular projects like Chrome don't do `#pragma once` because of > history clutter. > 5. Intermediate state of inconsistency as we transition to `#pragma > once` from #include guards. > > OUR RESPONSE: > Objections (1), (2), and (4) are essentially the same -- Dominic Hamon > points out in a previous thread that the Google style guide was > canonized when `#pragma once` was Windows-only, and the guidance has > not changed since because of the history churn problem. As noted > above, we think the history churn problem is minimized by the fact > that it can be wrapped up into the Windows integration work. > > For objection (3), the consensus seems to be that the vast majority of > compilers we care about (in particular, the ones supporting C++ 11) do > support it. > > For objection (5) we believe the inconsistent state is likely to not > be long lived, as long as we commit to wrapping this work up into the > Windows integration work. > > SUMMARIZED ADVANTAGES: > * Basically fool-proof. Communicates simply what its function is (you > include this file once). Semantically it is "the right tool for the > job". > * No need for namespacing conventions for #include guards. > * No conflicts with reserved identifiers[5]. > * No internal conflicts between include guards in Stout, Process > library, and Mesos (this is one reason we need the namespacing > conventions) > * Reduces preprocessor definition clutter (we should rely on #define > as little as humanly possible). > * Optimized to be easy to read and reason about. > > NEXT STEPS: > If we agree that this is the right thing to do, committers would ask > people to use `#pragma once` for new code when presented in code > reviews. For files that exist, I will take point on transitioning as > we complete the Windows integration work. I expect this work to > completely land before the new year. > > > Thanks, > > > [1] https://reviews.apache.org/r/39803/ > [2] https://www.marc.info/?t=142540100400015&r=1&w=2 > [3] https://issues.apache.org/jira/browse/MESOS-2211 > [4] https://reviews.apache.org/r/30100/ > [5] > http://stackoverflow.com/questions/228783/what-are-the-rules-about-using-an-underscore-in-a-c-identifier > > > -- > Alex > > Theory is the first term in the Taylor series of practice. -- Thomas M > Cover (1992) > > > -- Best Regards, Haosdent Huang
