RFR: 8254733: HotSpot Style Guide should permit using range-based for loops

[Kim Barrett]

On Sat, 28 Nov 2020 06:37:57 GMT, Kim Barrett <kbarrett at openjdk.org> wrote:

> Please review and vote on this change to the HotSpot Style Guide to
> permit the use of range-based `for` loops in HotSpot code. Range-based
> `for` is a feature added in C++11.
> 
> This is a modification of the Style Guide, so rough consensus among
> the HotSpot Group members is required to make this change.  Only Group
> members should vote for approval (via the github PR), though reasoned
> objectsions or comments from anyone will be considered.  A decision on
> this proposal will not be made before Monday 7-Dec-2020 at 12h00 UTC.
> 
> Since we're piggybacking on github PRs here, please use the PR review
> process to approve (click on Review Changes > Approve), rather than
> sending a "vote: yes" email reply that would be normal for a CFV.
> Other responses can still use email of course.

The range-based for loop feature is currently of limited utility,
because HotSpot code mostly avoids using the Standard Library and
existing HotSpot code provides relatively little support for the
feature. However, GrowableArray and EnumRange both provide the
necessary begin and end member functions returning iterators. There's
a chicken and egg problem with the latter; there's no reason to
provide support for the feature if it can't be used. By permitting use
of the feature we encourage adding support.

There is at least one use of the feature already present.  It was used
in a recent PR; the reviewers noted this but decided to allow it, with
the expectation that the feature would be explicitly permitted soon.

Most uses of the new EnumIterator facility could use and benefit from
the feature.

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PR: https://git.openjdk.java.net/jdk/pull/1488