[libcxx-commits] [PATCH] D81954: Remove the try/catch codepath if `swap` is `noexcept`.

Zoe Carver via Phabricator via libcxx-commits libcxx-commits at lists.llvm.org
Tue Jun 16 12:39:54 PDT 2020

zoecarver added inline comments.

Comment at: libcxx/include/variant:1046
   void __swap(__impl& __that)  {
     if (this->valueless_by_exception() && __that.valueless_by_exception()) {
ldionne wrote:
> mpark wrote:
> > mpark wrote:
> > > mclow.lists wrote:
> > > > ldionne wrote:
> > > > > zoecarver wrote:
> > > > > > I think it would be good to also mark this `noexcept(__all<(is_nothrow_move_constructible_v<_Types> && is_nothrow_swappable_v<_Types>)...>::value)`.
> > > > > Yeah, I was going to suggest the same thing. If it makes sense. If not, why?
> > > > You pretty much have to do that, otherwise you get the try/catch/terminate block in the caller
> > > I would actually prefer to not mark this `noexcept`, because trying to propagate `noexcept` down doesn't really scale.
> > > As in, if we mark this with the `noexcept` condition, we'd also, with the same principle, mark `__generic_construct`,
> > > `__visit_alt_at`, and essentially propagate it down to all of the helper functions like `__assign`, etc.
> > Hm, I don't think this is true based on https://gcc.godbolt.org/z/68_ZWp
> I'm satisfied with this argument.
Given the Godbolt link, I'm OK not marking this `noexcept`. But, I would still rather we mark it `noexcept` and, for what it's worth, I think we should probably also mark all those functions `noexcept`. IMO it's better to mark as many functions as possible with `noexcept` as long as the standard allows it (and that seems to be what we do in most places). For example, if we use `__swap` in another function/member internally and that function isn't marked `noexcept`, we have suddenly added a lot of bloat for _no reason at all_.

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