P1222R4 `<flat_set>`

[StephanTLavavej]

WG21-P1222R4 <flat_set>
WG21-P3567R2 flat_meow Fixes
LWG-3751 Missing feature macro for flat_set
LWG-3774 <flat_set> should include <compare>
LWG-3786 Flat maps' deduction guides need to default Allocator to be useful
LWG-3803 flat_meow constructors taking KeyContainer lack KeyCompare parameter
LWG-3879 erase_if for flat_{,multi}set is incorrectly specified
LWG-3884 flat_meow is missing allocator-extended copy/move constructors
LWG-3987 Including <flat_meow> doesn't provide std::begin/end
LWG-4384 flat_set::erase(iterator) is underconstrained

Feature-test macro:

#define __cpp_lib_flat_set 202511L

Internal VS-PR-420058 has taught the IDE about this new extensionless header - thanks Casey Carter (@CaseyCarter)!

WG21-P3567R2 "flat_meow Fixes" was voted into C++26 at the November 2025 meeting; while it wasn't officially a C++23 DR, we should obviously implement it as part of the initial release.

[Saalvage]

I have just started on a branch implementing this feature. :)
It will need a lot more tests and a few improvements but I'd love to contribute it!

[StephanTLavavej]

We're starting a multi-collaborator feature branch, feature/flat_set, with Stefan (@Saalvage)'s implementation, so that it can be refined and test coverage can be added.

[vmichal]

I have checked our support of papers tracked by this issue (also by #2910 ).
I would recommend removing the implemented papers from the list in the description of this issue and #2910 (if not to close these issues completely) to keep track of what is still missing.

Fully supported:

• LWG-3751 "Missing feature macro for flat_set"
• LWG-3774 "<flat_set> should include <compare>"
• LWG-3786 "Flat map's deduction guides need to default Allocator to be useful."
• LWG-3803 "flat_meow constructors taking KeyContainer lack KeyCompare parameter."
• LWG-3987 "Including <flat_meow> doesn't provide std::begin/end"
• LWG-3879 "erase_if for flat_{,multi}set is incorrectly specified"

Comments needed:

• LWG-3816 "flat_map and flat_multimap should impose sequence container requirements" - primarily wording, nothing really needed. We may consider creating concepts.
• LWG-3884 "flat_meow is missing allocator-extended copy/move constructors" - yes, supported, however, they are wrapped by #if 1 // TRANSITION, P2582R1 (MSVC, Clang, EDG). Is it something relevant?
• LWG-4384 "flat_set::erase(iterator) is underconstrained" is not applicable, we have same_as<iterator, const_iterator>

The remaining papers are larger, so I don't dare to claim 100 % compliance, but I found no problems.

• WG21-P0429R9 "<flat_map>"
• WG21-P1222R4 "<flat_set>"
• WG21-P3567R2 "flat_meow Fixes"

[frederick-vs-ja]

yes, supported, however, they are wrapped by #if 1 // TRANSITION, P2582R1 (MSVC, Clang, EDG). Is it something relevant?

When WG21-P2582R1 gets implemented by compilers, we can simply provide these constructors in common base classes. Currently, these constructors are necessary to be provided in derived classes to support CTAD.

• WG21-P3567R2 "flat_meow Fixes"

I think I've analyzed this paper and implemented previously missed parts.