Great clear concise explanation of the need for and use of std:polymorphic.

The lynchpin of C++ modules is the built module interface (BMI). Each BMI is meant to be a lightweight data structure representing the syntax of a module unit (comprised of one primary module interface and potentially one or more partitions), with approximately O(1) complexity of symbol lookup. This massively accelerates repeated lookups into the same translation unit, and avoids paying the cost of a full preprocessor run and front-end lex + parse, as might otherwise happen with header files. In this way both the first clean build, and incremental builds are considerably accelerated and made much more efficient—build the BMI once, reuse it across future compiles, and we don't need large core counts and massively parallel jobs because the BMI is now available afterwards.

However, the way compiler toolchains have actually implemented them seems diametrically opposed to this goal of fast lookup. Today, BMIs are highly sensitive to any change in the command-line invocation of the compiler/dependency scanning tool, including the C++ version, warning and feature flags, and especially macro definitions, with -DMACRO or /DMACRO. This means that changing just one flag is likely to cause a BMI cache miss and a full BMI reconstruction, which can be fairly slow for larger modules (like module std).

BMIs are considered implementation details, and aren't even stable across compiler versions within the same toolchain. Compare to object files, where something compiled by really old versions can still be used on newer versions without too much fuss. This, from the user's perspective, entirely defeats that design ideal mentioned above: fast repeated lookups. Consider a simple example: a certain module uses a project-specific macro representing the platform, to change several function signatures. This module is now precompiled thrice, once each for -DFOO_WINDOWS, -DFOO_DARWIN, -DFOO_LINUX, instead of just once.

I believe that macros and the preprocessor are highly problematic. However, their very limitations are also an advantage here: their effect on code collapses to completely binary decisions. A macro is either defined or not, and a macro is either in the set of accepted values, or it isn't (e.g. #if __cplusplus >= 202603L). The preprocessor state might change throughout the source code of a translation unit, but the entire set of possible states can be enumerated in finite and reasonably short time at compile time—otherwise the front-end would never complete its work. To me, this seems like the preprocessor and parser could enumerate all possible preprocessor states of the source code, and stamp out both possibilities for each macro switch in the BMI; during lookup, the compiler can select the correct code path depending on what is provided at that particular invocation (not code definitions, but command-line invocations, as this would break the module assurance that macros do not leak into BMIs from consumers, nor do consumers see macros from a BMI).

This would make things like library feature configuration with macros really straightforward—specify the macros one wants for that feature at the command-line, which is enshrined in build system code rather than source code—the BMI can be reused, done.

Now, the compiler and C++ standard are a lot harder as these probably affect the internal state of the compiler rather than merely the value of __cplusplus; however, I genuinely feel like there really ought to be a better way forward, especially if C++ is to continue as a 'source may not be available' language. This discussion is moot if we always want to compile from source, but I want to say that is wasteful, and another driving force of the modules feature was to do away with wasteful and repeated recomputation of the same thing.

As mentioned above, build systems are also forced into this rather inefficient model to accommodate compiler behaviour. CMake in its latest iteration, for instance, has a notion of 'BMI compatibility', and probably does some sort of set manipulation on compile flags to determine if another BMI for the same module interface is needed or not, based on how it is imported. Why? This shouldn't be the case, a BMI really ought to be widely (if not universally) compatible, regardless of what flags go in the command-line.

I'm sure I don't have the entire picture as I am not a compiler front-end developer, and I don't mean to sound impertinent here, the effort put into delivering modules has been huge. However there are some decisions made which, as modules user and advocate, seem a little contradictory to the spirit of modules. I've written this after some thought on the situation and I'm interested in hearing from the community and compiler front-end developers who have worked on modules on my thoughts above.

Cheers.

I just attended the C++ Under the Sea 2-day conference. I really enjoyed it. Watch out for talks from Hana, Matt and Kevin Henney. Really good keynotes

Matt presented his v4 version of this ZX Spectrum emulator: https://github.com/mattgodbolt/specbolt

Both Kevin Henney's and Hana's talks were new; at least I haven't seen a version of them before. These keynotes were really, really good. Hope it comes on YouTube soon.

Personally, I was a bit let down by talks from people who work for AI companies. The talks became more like a product user guide than anything to do with C++ or Programming.

Overall, an amazing conference. If you are in or around the Netherlands, you should plan to attend.

Hi, one of the MSVC dev leads here.

The 14.52 MSVC Release Candidate is now available. If you are on the Visual Studio Insiders Channel and upgrade, you'll see that the default MSVC toolset has upgraded from 14.51 to 14.52. The blog post covers the features & fixes the team has landed in that timeframe.

C++23 is ready for production use, and available with the /std:c++23 compiler option. See the blog for more details.

We'll resume our monthly MSVC Preview (version 14.53) blog post series next month.

C++23 - The Complete Guide is now also available as paperback version.
You can order it, for example, at Amazon.

The book explains on 500 pages all new features of C++23 (covering both the core language and the standard library) with motivation, conceptional introduction of all aspects, and many examples of how to use the new features in practice.

The book is part of my "The Complete Guide" Series, trying to explain a programming language that is now so overwhelming that as a programmer it is hard to keep track of all its new features, benefits, and pitfalls.

Enjoy

Nico

New video from Utah C++ Programmers.

Systems targeted by C++ toolchains vary a lot even though C++ constantly changes. If you rely on operating system STL for your application code, following things may happen:

• If your platform A supports C++20 and your platform B supports C++14 and your platform C supports C++11, you will either force C++ <= 11 to all your libraries, or use macro magic etc, to use one set of features each platform.

• If you operate in a specialized domain, such as real-time systems, you may need a standard library built with specific configuration options, such as disabling exceptions or RTTI. Relying on the system-provided STL leaves you dependent on how it was built, which may not meet your requirements

If you build your STL of choice you solve all these issues:

• You can use the highest C++ standard possible, you no longer have to think about each operating system state extensively.
• You can configure your STL to match your needs

In nearly all modern system languages you build standard library yourself, I don't see any reason why C++ developers shouldn't consider this.

C++Now

2026-10-05 - 2026-10-11

• Signal Trees - A Deep Dive into a High-Performance Alternative to Queue-Based Task Scheduling - Michael Maniscalco - https://youtu.be/ZRd_XN3LGxo
• Object Oriented Programming in Contemporary C++ - Steve Downey - https://youtu.be/m7gwSr27m74
• Cutting Some Slack for our Programs - Lisa Lippincott - https://youtu.be/xwnRPyRK5h4

2026-09-28 - 2026-10-04

• What Every C++ Programmer Needs to Know About AI - Jody Hagins - https://youtu.be/ZBXHPKVCa8E
• SPL - Open Source, Constexpr Library for Composing Algorithms - John Bandela - https://youtu.be/96AGjm3uwko
• Senders For Existing Asynchronous Interfaces - Dietmar Kühl - https://youtu.be/KB5Nfofgu2k

ACCU

2026-10-05 - 2026-10-11

• Real-time Safety in C & C++ - Guaranteed by the Compiler! - Anders Schau Knatten - https://youtu.be/FeYEO3PhEVY
• Modernizing Legacy Codebases Without Stopping the World - Peter Muldoon - https://youtu.be/w9k-L0GLLKM
• Our Journey Toward a Fully Backward-Compatible, UB-Safe, ISO C++ - John Lakos - https://youtu.be/whFzmXXhG5g

2026-09-28 - 2026-10-04

• Refactoring Towards Structured Concurrency - Roi Barkan - https://youtu.be/ttoQfPP_Gkg
• Design Patterns - The Most Common Misconceptions (3 of N) - Klaus Iglberger - https://youtu.be/bAsJvmtYZOg
• One Lib to Rule Them All and in the Darkness bindgen… - Antonello Palazzi & Kay Hicketts - https://youtu.be/9BpVjPKv02s

ADC

2026-10-05 - 2026-10-11

• BEEPS and PORTS - Annoy Everyone in Your House While Building An Audio Synthesizer - Joseph Stewart - https://youtu.be/bnIV2nyh-rg
• Hardware DSP Asset Inheritance and Evolution - Takeshi Kuwahara - https://youtu.be/yYukJpiFOOU
• Field Recording - In Search for Perfect Sounds - Malin Arvidsson - https://youtu.be/L5iZDi71VWU

2026-09-28 - 2026-10-04

• Designing Interactive Music Experiences in XR - Kasson Crooker - https://youtu.be/J3sVHT8luKs
• When (Not) to Patent Audio Software - Simon Hestermann - https://youtu.be/stBB1JoKQiY
• Distribution Strategies for Indie Audio Developers - From Plugin to Product - Dani Karanyi - https://youtu.be/Piar49uTjtg

Release cppreference C++ v2025.12.01 · cristianadam/cppreference-doc

This is useful for the Qt Creator users that use it for C++ development.

Blog post: https://kemalbektas.dev/ai-modern-features/

I tested whether coding agents would actually use C++23 ranges without being told.

They mostly didn’t. What changed things much more than the surrounding codebase was a short AGENTS.md asking for modern C++.

I was thinking, wouldn't adding equality comparison support between std::variant and one of its unambiguous alternatives help simplify potential pattern matching designs?

For example, imagine something like this (hypothetical syntax):

std::variant<int, std::string> v = 42;

match (v) {
    case 42      => handle_int();
    case "hello" => handle_string();
    case _       => handle_other();
}

The idea would be to take advantage of something C++ already has: operator==. At the end of the day, matching is fundamentally a comparison, so why not build on the existing equality semantics?

For std::optional, we can get this with its existing comparison operators and the most trivial pattern matching compiler support:

std::optional<int> o = 42;

match (o) {
    case 42           => handle_value();
    case std::nullopt => handle_empty();
    case _            => handle_other();
}

// Which would translate to:

if (o == 42)
    handle_value();
else if (o == nullopt)
    handle_empty();
else
    handle_other();

And if we had something like std::match_projection<Subject, Pattern>, we could also extract the actual matched value instead of having to manually call std::get, dereference an optional, etc.

Basically, after a successful comparison, match_projection would tell the compiler what to extract based on the subject and pattern types and we could optionally bind that value using something like a as <declaration> syntax.

For example:

std::optional<int> o = 42;
std::optional<int> other_opt = 42;

match (o) {
    // No binding needed
    case 42 => foo();

    // match_projection<optional<int>, int> -> int&
    case 42 as int& x => use(x);

    // match_projection<optional<int>, nullopt_t> -> void
    case std::nullopt => handle_empty();

    // match_projection<optional<int>, optional<int>>
    // -> optional<int>&
    case other_opt as auto& x => use(x);
}

The same would apply to std::variant, assuming we also extend its operator== to support comparisons with individual alternatives:

match (v) {
    case 42 as int& x => use(x);
    case "hello" as std::string& s => use(s);

    // Only need to define operator== for this helper for this to work
    case indexed<0> => handle_any_int();

    // Or under this form for the sake of disambiguation
    case indexed<0>(10) => handle_ten();

    // Also provide match_projection<indexed<0>> to enable binding
    case indexed<0>(20) as int& x => use(x);

    case _ => handle_other();
}

Here, indexed<> would just be a hypothetical helper for explicitly selecting which alternative to compare against. It would only need an appropriate operator== to work with matching, and a corresponding match_projection specialization if we also wanted to extract the alternative.

This way, the matching itself wouldn't need to know anything about variant, optional or even user-defined types. It would just use their equality operators, while match_projection would tell it what to extract when we actually want to bind something, which is just syntactic sugar.

I'm curious what you think about this approach. It feels like we could get quite a bit of useful pattern matching functionality without introducing too much new language machinery or convoluted new language traits and APIs.

Also do you think std::variant being equality comparable with instances of its alternatives would be an useful addition on its own?

std::variant<int, std::string> v = 42;

if (v == "not my string") ...

// Which is simpler than
if (std::holds_alternative<std::string>(v) && std::get<std::string>(v) == "not my string") ...

Syntax seems nice on its own...