wro.cpp wro.cpp
short

WG21 Brno: with C++26 done, the committee turns to undefined behavior

· english · audience: working-cpp

C++26 is done. It shipped reflection, contracts, a hardened standard library, and import std. WG21 met in Brno, Czechia from June 8 to 13, 2026 — the first meeting since C++26 was finalized at Croydon in March — and the question hanging over it was simple: what now?

Herb Sutter’s Brno trip report answers it clearly. The committee has turned, with unusual focus, to one target: undefined behavior. Brno was not a reflection meeting or a big-new-feature meeting. It was the opening move of what is shaping up to be C++29, the safety release.

The meeting by the numbers

About 200 people (55% on-site at Mendel University, 45% online), 28 nations, 22 active subgroups across 11 parallel tracks, over six days, with 25 first-time guests. The next meeting is in Buzios, Brazil, in November 2026.

The headline: write down all the UB

The structural move of the meeting was P3596R3, “Undefined Behavior and IFNDR Annexes” — two new annexes that catalogue, in one place, every case of undefined behavior and every “ill-formed, no diagnostic required” (IFNDR) corner in the standard. You cannot systematically eliminate what you have not first enumerated.

Paired with it: P3100, “A framework for systematically addressing undefined behaviour in the C++ Standard.” The plan, per Sutter, is a series of summer and fall telecons doing a line-by-line review of that catalogue, targeting C++29. This is the committee committing to grind down UB case by case, in public, on a schedule.

For a language whose detractors reduce it to “undefined behavior everywhere,” this is the most direct possible answer: name every instance, then close them. The safety state-of-the-union from May laid out the regulatory pressure (CISA, the EU CRA) behind this; Brno is the committee answering it with process, not slogans.

Profiles keep marching

The other half of the safety story is profiles — opt-in bundles of restrictions a compiler enforces. Brno moved several: P4222, “An initialization profile” (Stroustrup), the profile that stamps out uninitialized-read UB; P3984, “A type-safety profile” (Stroustrup); and P3589R2, “C++ Profiles: The Framework” (Dos Reis). SG23 (Safety and Security) is building the formal specification, targeting C++29 or a concurrent white paper.

Contracts grow a feature back

One adoption stands out for anyone following the contracts story: P3097R3, “Contracts for C++: Virtual functions.”

Virtual-function contracts were cut from the C++26 contracts MVP at Hagenberg in early 2025 — the interaction with inheritance was not mature enough to ship. P3097R3 brings them back for the next standard, with both base-class and final-overrider assertions evaluated. The MVP shipped; the feature that was deferred is already on its way back.

Quality-of-life, adopted

Brno also cleared a stack of smaller papers that working programmers will feel:

  • P2287R6, “Designated-initializers for Base Classes” (Revzin) — designated-init syntax that finally reaches through a base class.
  • P3091, “Better Lookups for map, unordered_map, and flat_map” — a .get(key) that returns an optional<mapped_type&> instead of forcing an insertion the way operator[] does.
  • P3668R4, “Defaulting Postfix Increment and Decrement Operations” — =default your postfix ++/-- to defer to the prefix versions.
  • P3125R6, “constexpr pointer tagging” — a portable pointer_tag_pair<Pointer, Tag> for stashing bits in a pointer’s spare low bits.
  • P3248, “Require [u]intptr_t” — intptr_t and uintptr_t go from optional to mandatory.

A closer look at these lands in a companion post this week.

So where was reflection?

Here is the telling part: Sutter’s Brno report does not mention reflection, P2996, or token injection at all.

That is not a snub. It is what shipping looks like. Reflection was the marquee feature of C++26; once a feature is in the standard, the committee’s attention moves to the next frontier, and right now that frontier is safety. The reflection work that remains — token injection and the design space around code generation — continues as study-group work aimed at C++29, exactly as the “reflection is only half” post and the May mailing roundup already documented. It was not a Brno plenary headline.

For the wro.cpp series, that is good news twice over. The reflection you have been reading all month is final, standardized C++26 — not a moving target. And the next chapter is still being designed, which means there is time to master the part that already ships.

The bigger picture

C++26 was the features release: reflection, contracts, hardened stdlib. Brno suggests C++29 wants to be the safety release — a full UB catalogue with a schedule to close it, profiles with a target standard, and contracts filling in the pieces the MVP deferred.

This week, three companion posts go deeper: the undefined-behavior catalogue, the quality-of-life adoptions, and the profiles-and-contracts safety track. Reflection did not make the Brno headlines, and that is exactly the sign you want after a feature ships: the committee trusts it enough to look past it.

Sources: Herb Sutter, Brno trip report (June 13, 2026) — primary source for the adoptions and numbers above. Papers referenced: P3596R3 (Undefined Behavior and IFNDR Annexes), P3100 (A framework for systematically addressing undefined behaviour), P4222 (An initialization profile), P3984 (A type-safety profile), P3589R2 (C++ Profiles: The Framework), P3097R3 (Contracts for C++: Virtual functions), P2287R6 (Designated-initializers for Base Classes), P3091 (Better Lookups for map, unordered_map, and flat_map), P3668R4 (Defaulting Postfix Increment and Decrement Operations), P3125R6 (constexpr pointer tagging), P3248 (Require [u]intptr_t). Additional trip reports (Inbal Levi, JeanHeyd Meneide, Barry Revzin) may add detail as they publish.