RFC: JEP JDK-8208089: Implement C++14 Language Features

[Volker Simonis]

On Mon, Oct 8, 2018 at 12:49 PM Thomas Stüfe <thomas.stuefe at gmail.com> wrote:
>
> Hi Kim,
>
> is this JEP only about C++14 features or shall we discuss older
> features too? The reason I am asking is that I would like us to
> officially endorse namespaces. Not inline namespaces, just plain old
> namespaces.
>
> <quote>HotSpot makes very limited use of namespaces.</quote>
>
> Not really true, we already use them. E.g. in metaspace coding, I used
> them to keep the global name space clean and to keep internals
> internal. This was met with positive reviews, and it works on all
> toolchains, so compiler support should not be a problem. Using
> namespaces, we could get slowly replace the "AllStatic" classes, which
> are namespaces in all but name. In contrast to classes, namespaces can
> be spread over multiple files and compilation units, and allow for
> cleaner separation of internal and external coding.
>
> It also would allow us to get rid the middle-of-header-platform-inclusions:
>

That's overdue since a long time!
It would allow us to finally get correct code navigation in IDEs.

> For example, today we have:
>
> [os.hpp]
> class os: AllStatic {
> ....
> (platform independent, outward facing os:: functions)
> #include "os_linux.hpp"
> >> (Inner class "Linux" with platform specific os functions)
> ...
> }
>
> Not only is the inclusion in the middle of a class terrifying, it also
> means the shared, outward facing os:: namespace contains class Linux
> and lots of platform specific internals.
>
> With namespaces one could:
>
> [os.hpp]
> namespace os {
> ....
> (platform independent, outward facing os:: functions)
> ....
> }
>
> [os_linux.hpp]
> namespace os {
> namespace Linux {
> (linux specific os functions)
> }
> }
>
> I think this is way cleaner, and keeps platform specifics from
> including files which only care for the shared os interface.
>
> --
>
> Note that I would prefer forbidding the "using" directive for callers
> of namespace functions, but rather force them to spell out the
> namespace:
>

As far as I saw, "using" directives are already on the "Exclude" list
in Kim's proposal.

> So, instead of this:
>
> using os;
> jlong m = available_memory();
>
> I would prefer this, which is our current practice with AllStatic childs:
>
> jlong m = os::available_memory();
>
> The latter form would keep the code grepable.
>
> Best Regards, Thomas
> On Wed, Oct 3, 2018 at 9:13 PM Kim Barrett <kim.barrett at oracle.com> wrote:
> >
> > I've submitted a JEP for
> >
> > (1) enabling the use of C++14 Language Features when building the JDK,
> >
> > (2) define a process for deciding and documenting which new features
> > can be used or are forbidden in HotSpot code,
> >
> > (3) provide an initial list of permitted and forbidden new features.
> >
> > https://bugs.openjdk.java.net/browse/JDK-8208089
> >