[libc-commits] [PATCH] D129920: [libc] Trivial implementation of std::optional

[Jeff Bailey via Phabricator via libc-commits]

jeffbailey added a comment.

In D129920#3657048 <https://reviews.llvm.org/D129920#3657048>, @tschuett wrote:

> Do you need the union and the `Placeholder`? In `reset` you reset the `StoredValue`. This implies something about `T`.

I think I need the union because I don't want to construct the object if I'm not using it as in this case:

  std::optional<std::string> foo() {
    return std::nullopt;
  }

Does a union still have that property if it only has a single member?  Or is there a different approach?

> I would add asserts to catch misbehaviour.

Will do.  It's almost tempting to also implement std::variant and use that since we'll probably want it later anyway.

> The LLVM optional uses a union for storage.

Right.  They're using the same approach that I am with:

  union
  {
      char __null_state_;
      value_type __val_;
  };

I've just used the words Placeholder and StoredValue.  They're using __engaged where I'm using InUse.


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