[cfe-dev] http://llvm.org/bugs/show_bug.cgi?id=11462

[Jeffrey Yasskin]

FWIW, I think a better solution for the library issue would be for
clang to provide an attribute saying "ignore the rule in
[intro.object]p6, and allow this member to have 0 size." Doing this
would allow possibly-empty types to be used directly as members,
rather than obfuscating the code by forcing them into base classes or
tuples. Perhaps spell the attribute [[may_be_empty]] or
[[may_have_same_address_as_other_members]].

As c++std-lib-31670 indicates, there are non-final classes that can't
be derived from:

  struct B // cannot be a base class
  {
      virtual ~B() final {}
  };

So __is_final is probably not the right trait even for use cases
beyond the empty-base-class optimization.

On Fri, Dec 2, 2011 at 12:00 PM, Howard Hinnant <hhinnant at apple.com> wrote:
> I would like to draw clang developer's attention to:
>
> http://llvm.org/bugs/show_bug.cgi?id=11462
>
> This is a feature request for a new intrinsic __is_final(T).
>
> This intrinsic has become necessary due to the recent C++ language addition "final".  The problem is that many libraries (such as libc++) will derive from types they know nothing about except that the type is a class and is eligible for the empty base optimization.
>
> libc++ uses this technique in no less than 5 places to optimize away space for allocators, deleters, predicates, etc.  It makes all containers smaller, and is what enables sizeof(unique_ptr) == sizeof(void*).  I.e. it is a critically important optimization.
>
> Now that our customers can write:
>
> class final A {};
>
> we find ourselves in the position that libc++ will attempt to derive from A in a surprising number of places (because it is empty), only to result in a compile time error because A is also final.  Thus libc++ needs a way to check classes for final.
>
> Ganesh has proposed a patch to clang to implement __is_final, complete with a __has_feature check.  libc++ really needs this so that our clients can write:
>
>   std::tuple<A> t;
>
> Comments?  Questions?
>
> Thanks,
> Howard
>
> _______________________________________________
> cfe-dev mailing list
> cfe-dev at cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/cfe-dev