#cpp29
11 posts tagged #cpp29.
GCC trunk accepts a coroutine promise with return_value and return_void
GCC 16.2 rejects a coroutine promise type that declares both return_value and return_void. GCC trunk accepted it from 3 October 2026, implementing P3950R1 as a defect report, and now reports __cpp_impl_coroutine as 202606L. The same program on Compiler Explorer shows both results.
Quantities and units on the road to C++29
Everything this series showed is also a proposal: P3045, Quantities and units library, targets C++29 with mp-units as its reference implementation, led by Mateusz Pusz of Gdansk, an ISO C++ committee voting member since 2017. The series finale looks at what standardization takes, and at the Gdansk author behind the proposal.
C++29 mode is open, and the compilers already disagree
LLVM 23.1 shipped with -std=c++2d, a mode for the standard after C++26, and GCC trunk has one too. Both report __cplusplus as 202700. They do not agree about C++26 itself: GCC says 202603, which is the ratified value, and clang still says 202400.
Pattern matching did not make C++26
Enough people assumed pattern matching shipped in C++26 that it is worth stating plainly: it did not. P2688 missed the feature freeze and is now aimed at C++29, no shipping compiler implements it, and until then std::visit with an overload set remains the idiom. Here is what the proposal would actually give you.
Profiles take shape, and the contract C++26 left behind comes back
The other half of the Brno safety story: a profiles framework with real syntax (attributes, not pragmas) plus two concrete profiles from Stroustrup, and P3097 bringing virtual-function contracts back after they were cut from the C++26 MVP. All target C++29.
Five small C++29 papers from Brno you will actually use
Underneath the safety headlines, WG21 Brno cleared a stack of small papers that working programmers will feel every day: designated-initializers that reach through a base class, a map .get() that does not insert, defaultable postfix operators, constexpr pointer tagging (which quietly leans on reflection), and mandatory intptr_t. All target C++29.
C++ is writing down all of its undefined behavior
The structural move at WG21 Brno was not a feature -- it was a catalogue. P3596 adds two annexes that enumerate every case of undefined behavior and every ill-formed-no-diagnostic-required corner in the standard, and P3100 turns that list into a case-by-case program to close each one for C++29. There are exactly 80 core-language UB cases, and 70% of them are memory-safety bugs.
WG21 Brno: with C++26 done, the committee turns to undefined behavior
The WG21 Brno meeting (June 8-13, 2026) was the first since C++26 was finalized, and per Herb Sutter's trip report the theme is unmistakable: catalogue and eliminate undefined behavior, push memory-safety profiles toward C++29, and finish the pieces the C++26 MVP deferred. Reflection, the headline of C++26, was not on the marquee -- and that is exactly what shipping looks like.
The May 2026 WG21 mailing in five papers (pre-Brno, 116 total)
WG21 dropped the pre-Brno mailing in early May -- 116 papers, the largest pre-meeting mailing since the C++20 design crunch. Five papers are doing the heavy lifting for C++29's safety story (the headline axis named in P5000R1 'Direction for ISO C++29'), plus the coroutines-for-I/O work that finally got first LEWG review on the schedule. This is the table of contents for what wro.cpp will be tracking through to Brno (8-13 June).
C++ Safety State of the Union: May 2026
C++ in May 2026 has four conversations running at once: regulators (CISA, EU CRA) demanding memory-safety roadmaps; the committee fighting over how to respond (Hagenberg vote 19 profiles / 9 Safe C++ / 11 both); what actually shipped in C++26 (P2900 contracts, P3471 hardened stdlib, P2996 reflection); and what the industry actually deployed (Chrome MiraclePtr -57% UAF, Google's 0.3% perf-cost data, Apple libc++ safe-buffers). They don't talk to each other in public. This essay wires them. Pre-Brno (8-13 June) reading.
Revzin at C++Now: 'Reflection Is Only Half the Story' -- what generation looks like next
Barry Revzin's C++Now 2026 keynote (Mon 4 May, Aspen) landed the line the C++ reflection community has been circling for two years: C++26 reflection lets you OBSERVE -- the next question is what it looks like to GENERATE. The 90-minute talk is a tour of source-code-generation design space (macros, templates, Rust proc-macros, Swift macros, D mixins) and reads as the natural sequel to wro.cpp's whole 'Where this is heading' triptych framing.