On Wed, 15 Apr 2026 at 19:17, Marcin Jaczewski <marcinjaczewski86_at_[hidden]>
wrote:

> śr., 15 kwi 2026 o 19:39 Jonathan Wakely via Std-Proposals
> <std-proposals_at_[hidden]> napisał(a):
> >
> >
> >
> > On Mon, 13 Apr 2026 at 21:32, Robert Baumgartner via Std-Proposals <
> std-proposals_at_[hidden]> wrote:
> >>
> >> I want to solve the following problem: I have an enum type, potentially
> a large one like a set of actions. A subset of that enum type satisfies
> certain conditions that I want to check at runtime. That would look like
> this
> >>
> >> if(someSet.contains(x)) {
> >> // do something
> >> }
> >>
> >> std::unordered_set<T> does this job efficiently, but has to be
> initialized during run-time.
> >
> >
> > Does it?
> >
> > Can't you do this in C++26?
> > constexpr std::unordered_set<E> set({x, y, z});
> >
>
> But will it be available at runtime? As it have memory buffer that
> can't escape `constexpr`.
>

 Ah yes, you're right of course.


> This could only work if we could "leak" memory from compile time to run
> time,
> like it would be this memory block converted to some region data
> section of exe, like:
>
> ```
> constexpr char* foo = std::leak_from_constexpr(new char[100]{});
> //converted to
> static char foo_dummy[100]{};
> char* foo = &foo_dummy;
> ```
>
>
> > And if you don't want it to be constant, use constinit so that it's
> guaranteed to be initialized at compile time (avoiding the static init
> order fiasco) but can still be modified later.
> >
> >
> >>
> >> This is suboptimal whenever the values are known at compile time,
> especially in ultra-low latency environments, where the run-time
> initialization adds overhead that can be avoided.
> >>
> >> The currently best compile solution that I am aware of is using a
> std::array like
> >>
> >> inline static constexpr array mySet{a, b, c, …};
> >>
> >> , which is O(log(n)) run-time at best for a lookup operation, but
> requires manual sorting for optimal performance and lacks the semantics of
> a set.
> >>
> >> A combined std::static_unordered_set<T, N> merges both solutions in
> one, combining run-time efficiency with compile-time initialization.
> >>
> >> I'm aware of third-party solutions like frozen, and of std::flat_map
> (C++23) and constexpr std::map (C++26), which suggest the committee has
> appetite for this direction. I'd welcome pointers to any prior proposals I
> may have missed, and feedback on whether this is worth developing into a
> formal paper.
> >>
> >> Best regards,
> >>
> >> Robert Baumgartner
> >>
> >> My website: https://robba.github.io
> >> --
> >> Std-Proposals mailing list
> >> Std-Proposals_at_[hidden]
> >> https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals
> >
> > --
> > Std-Proposals mailing list
> > Std-Proposals_at_[hidden]
> > https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals
>