RFR: 8369186: HotSpot Style Guide should permit some uses of the C++ Standard Library

[Kim Barrett]

On Wed, 8 Oct 2025 18:12:44 GMT, John R Rose <jrose at openjdk.org> wrote:

>> Please review this change to the HotSpot Style Guide to suggest that C++
>> Standard Library components may be used, after appropriate vetting and
>> discussion, rather than just a blanket "no, don't use it" with a few very
>> narrow exceptions. It provides some guidance on that vetting process and
>> the criteria to use, along with usage patterns.
>> 
>> In particular, it proposes that Standard Library headers should not be
>> included directly, but instead through HotSpot-provided wrapper headers. This
>> gives us a place to document usage, provide workarounds for platform issues in
>> a single place, and so on.
>> 
>> Such wrapper headers are provided by this PR for `<cstddef>`, `<limits>`, and
>> `<type_traits>`, along with updates to use them. I have a separate change for
>> `<new>` that I plan to propose later, under JDK-8369187. There will be
>> additional followups for other C compatibility headers besides `<cstddef>`.
>> 
>> This PR also cleans up some nomenclature issues around forbid vs exclude and
>> the like.
>> 
>> Testing: mach5 tier1-5, GHA sanity tests
>
> doc/hotspot-style.md line 629:
> 
>> 627: occur. The C++98/03 `std::allocator` class is too limited to support our
>> 628: usage. But changes to the allocator concept in more recent Standards removed
>> 629: some of the limitations, supporting statefull allocators. HotSpot may, in the
> 
> s/statefull/stateful/

Fixed locally for next push.

> doc/hotspot-style.md line 647:
> 
>> 645: using those names easier, since the qualifiers don't need to be included. But
>> 646: there are reasons not to do that.
>> 647: 
> 
> s/reasons/several reasons/ (to better tee up the following bullet list)

Fixed locally for next push.

> doc/hotspot-style.md line 651:
> 
>> 649:     headers may be included, adding to the list of names being used. Also,
>> 650:     future versions of the Standard Library may add currently unknown names to
>> 651:     the headers already being included.
> 
> Maybe mention the `assert` macro at the end of the bullet, if there is no other example of a previous name collision:
> 
>> …already being included.  (We already have a conflict from the HotSpot `assert` macro, although in that case the `std::` prefix is not applicable.)

There aren't any previous examples because we don't have any examples of doing
this thing that we're warning against (`using` a namespace).  `assert` is a
different problem, unrelated to namespaces. (Remember that one of the usual
knocks against macros is that they don't respect namespaces.)

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PR Review Comment: https://git.openjdk.org/jdk/pull/27601#discussion_r2415313253
PR Review Comment: https://git.openjdk.org/jdk/pull/27601#discussion_r2415313425
PR Review Comment: https://git.openjdk.org/jdk/pull/27601#discussion_r2415313670