C++26: Concept and variable-template template-parameters -- Sandor Dargo

SANDOR_DARGO_ROUND.JPGLast week, we discussed why we should sometimes use remove_cvref_t on our template parameters before applying concepts to them. We also saw that the solution is not super readable because we lose access to the terse, shorthand syntax.

C++26: Concept and variable-template template-parameters

by Sandor Dargo

From the article:

C++ already allows passing templates as template parameters, but only if they are class templates. A common reason for doing this is to allow higher-level abstractions. For instance, you may want to pass in a container template like std::vector, without specifying the type it contains.

Jason Turner explains this well in C++ Weekly - Ep 368 - The Power of template-template Parameters: A Basic Guide, but here’s his example for quick reference:

template<template <typename Contained, typename Alloc = std::allocator<Contained>>
		 typename ResultType>
auto get_data() {
	ResultType<double> result;
	// ...
	return result;
}

int main() {
	auto data = get_data<std::vector>();
}

Highlighting the student and support tickets for Meeting C++ 2025

Meeting C++ is offering online and onsite student and support tickets for this years conference!

Highlighting the student and support tickets for Meeting C++ 2025

by Jens Weller

From the article:

I'd like to point towards the programs for those that can't afford to pay for a ticket for Meeting C++ 2025: the programs for the student and support tickets.

And let me start with thanking those that enable these programs through their ticket buying: the attendees and sponsors of Meeting C++ 2025! With the schedule published, I'd like to highlight the student and support tickets for Meeting C++ 2025. For a few years now Meeting C++ has hosted programs to give students, underrepresented folks and those who can't afford a ticket access to the conference.

 

Use concepts with std::remove_cvref_t -- Sandor Dargo

SANDOR_DARGO_ROUND.JPGLet’s talk about templates, constraints, and concepts. We’ll start with a quick reminder of why concepts are essential when working with templates. Then we’ll dive into the challenge posed by reference-qualified types and finish with a practical solution.

Use concepts with std::remove_cvref_t

by Sandor Dargo

From the article:

By now, it’s well known that using unconstrained templates is discouraged. Even the C++ Core Guidelines strongly recommend against it.

T.47 only advises avoiding highly visible unconstrained templates with common names due to the risks of argument-dependent lookup going wrong. T.10 goes further, recommending that we specify concepts for every template argument to improve both simplicity and readability.

The same idea appears in I.9, which suggests documenting template parameters using concepts.

It’s hard to argue with these guidelines. Concepts make code more readable — just by looking at a function, class, or variable template, the reader can immediately tell what kinds of types are accepted.

If you want to learn more about concepts, check out my concepts-related articles or my book on concepts.

But what makes a good concept? That’s a more complex topic — and one we can’t fully cover in a single article.

Simple Compile-Time Dynamic Programming in Modern C++ -- Andrew Drakeford

logo.pngCompile time code can be very efficient. Andrew Drakeford demonstrates how to write efficient chains of matrix multiplication.

Simple Compile-Time Dynamic Programming in Modern C++

by Andrew Drakeford

From the article:

Modern C++ enables us to solve mathematical optimisation problems at compile time. With the expanded constexpr capabilities [Fertig21Turner18Turner19Wu24], we can now write clear and efficient optimisation logic that runs during compilation. Fixed-size containers such as std::array fit naturally into these routines. Even standard algorithms, such as std::sort and std::lower_bound, are now constexpr, enabling more straightforward code and more powerful compile-time computations. Additionally, compile-time optimisation generates constant results, which enables the compiler to create even more efficient code. We will use the matrix chain multiplication problem as our worked example.

Matrix chain multiplication

Matrix chain multiplication is a classic dynamic programming problem [Corman22Das19Mount]. It aims to determine the most efficient method for multiplying a sequence of matrices. Since matrix multiplication is associative, the order of grouping does not affect the result. However, the number of scalar multiplications involved can vary depending on the grouping.

Consider the three matrices A₁ (10×100), A₂ (100×5), and A₃ (5×50), multiplied in a chain, A₁ × A₂ × A₃.

There are two ways to multiply them:

  1. Grouping as (A₁ × A₂) × A₃ first computes a 10×5 matrix, then multiplies that with A₃. This results in 5,000 operations for the first multiplication, and another 2,500 for the second – a total of 7,500 scalar multiplications.
  2. Grouping as A₁ × (A₂ × A₃) first multiplies A₂ and A₃, then A₁. This results in 25,000 operations for the first step and 50,000 for the second – a total of 75,000, which is clearly worse.

C++26 reflection at compile-time -- Andreas Fertig

797f4c8c0b89b22b.pngIn today's post, I like to talk about C++26 and one of the probably most impactful features that have been added to the working draft. While C++26 is still some months away from official completion, since the WG21 summer meeting in June we all now know what will be in C++26.

C++26 reflection at compile-time

by Andreas Fertig

From the article:

While the new standard will have plenty of great improvements the one that will most likely change a lot is reflection at compile-time! In Sofia we voted seven reflection papers into C++26:

  • P1306R5 Expansion statements
  • P2996R13 Reflection for C++26
  • P3096R12Function parameter reflection in reflection for C++26
  • P3293R3Splicing a base class subobject
  • P3394R4Annotations for reflection
  • P3491R3define_static_
  • P3560R2Error handling in reflection

The papers above should give you enough to read for your vacation. I'll leave that theoretical study up to you for now.

Let's talk practical

The main question is, what can you do with that new feature? Well, I'm not the first one who published their ideas.

Format your own type (Part 2) -- Sandor Dargo

SANDOR_DARGO_ROUND.JPGPreviously, we discussed how to write our own formatter and finished with a relatively simple solution for printing a struct called ProgrammingLanguage. Today, we’ll take it to the next level.

Format your own type (Part 2)

by Sandor Dargo

From the article:

Add more options for semantic versioning

Let’s dream big. Instead of only handling major and minor versions (like Python 3.12), let’s aim to fully support semantic versioning.

Semantic versioning (SemVer) is a versioning scheme that conveys meaning about the underlying changes in a release. It typically consists of three parts: MAJOR.MINOR.PATCH.

We should be able to print all these correctly:

ProgrammingLanguage cpp{"C++", 20};
ProgrammingLanguage python312{"Python", 3, 12};
ProgrammingLanguage python31211{"Python", 3, 12, 11};


std::cout << std::format("{:%n%v} is fun", cpp) << '\n';  // C++20 is fun
std::cout << std::format("{:%n %v} is fun", python312) << '\n';  // Python 3.12 is fun
std::cout << std::format("{:%n %v} is fun", python31211) << '\n';  // Python 3.12.11 is fun

CppCon 2025 The Programmer CEO -- Greg Law

Registration is now open for CppCon 2025! The conference starts on September 13 and will be held in person in Aurora, CO. To whet your appetite for this year’s conference, we’re posting some upcoming talks that you will be able to attend this year. Here’s another CppCon future talk we hope you will enjoy – and register today for CppCon 2025!

The Programmer CEO

Wednesday, September 17 16:45 - 17:45 MDT

by Greg Law

Summary of the talk:

Many programmers think about starting a company. It’s often not about getting rich so much as to pursue a vision for a computer program that is much bigger than one person could write alone. Like most programmers who start up, I had no formal training and little experience outside of software development. I was naively confident, and didn’t know what I didn’t know (it turned out that that was a LOT!)

The talk includes some of the lessons I’ve learned along the way, many of which were a complete surprise. I’ll cover getting investment, building the product, building a team, and getting and keeping customers. Little of this talk is directly about programming, but it is aimed at programmers who want to create code in order to create a business, or who want to create a business so that they can create the code they want.

Much of the content is also relevant for programmers who find themselves doing non-programming tasks, such as managing people or customer-facing roles, and anyone working at a start-up. Contains candid, warts-and-all war stories, and because it’s for programmers, comes with a no adverts and no business-talk BS guarantee.

Greg is co-founder and CEO at Undo. He is a programmer at heart, but likes to keep one foot in the software world and one in the business world. Greg finds it particularly rewarding to turn innovative software technology into a real business. Greg has over 25 years' experience in innovative start-up software companies.

CppCon 2025 Engineers Are Users Too: Case Study in Design Thinking for Infrastructure -- Grace Alwan

Registration is now open for CppCon 2025! The conference starts on September 13 and will be held in person in Aurora, CO. To whet your appetite for this year’s conference, we’re posting some upcoming talks that you will be able to attend this year. Here’s another CppCon future talk we hope you will enjoy – and register today for CppCon 2025!

Engineers Are Users Too: A Case Study in Design Thinking for Infrastructure

Tuesday, September 16 15:15 - 16:15 MDT

by Grace Alwan

Summary of the talk:

Bringing together strong engineering skills with a foundation in UX Design can open unexpected doors—especially in fields like infrastructure, where there are few engineers trained in design. As a software engineer on an infrastructure team at a fintech company, I work on low-level systems that power how engineers manage compute at scale. But it’s my background in UX and human-computer interaction that’s made me an invaluable asset. By applying design thinking to infrastructure – through prototyping, user interviews, and iteration – I transformed a complex internal workflow and quickly became a subject matter expert.

This talk will walk you through that journey and give you the tools to be a design trailblazer in your own career. You'll leave with practical techniques for integrating empathy and user-centric thinking into deeply technical work—and insights on how expanding your skillset can accelerate your growth as a C++ or systems engineer.

Grace Alwan graduated from Stanford University in 2023, where she got her BS and MS in Computer Science specializing in Human-Computer Interaction. She is now a software engineer in the Technology Infrastructure org at Bloomberg, where she works on cluster and host management.

2025-09 Mailing Available

The 2025-09 mailing of new standards papers is now available.

 

WG21 Number Title Author Document Date Mailing Date Previous Version Subgroup
N5020 2026-11 Rio De Janeiro, Brazil Matheus Izvekov 2025-09-10 2025-09 All of WG21
P2953R2 Forbid defaulting operator=(X&&) && Arthur O'Dwyer 2025-09-02 2025-09 P2953R1 EWGI SG17: EWG Incubator
P3347R5 Invalid/Prospective Pointer Operations Paul E. McKenney 2025-08-27 2025-09 P3347R4 CWG Core
P3567R1 `flat_meow` Fixes Hui Xie 2025-09-06 2025-09 P3567R0 LWG Library
P3579R2 Fix matching of constant template parameters when matching template template parameters Matheus Izvekov 2025-09-10 2025-09 P3579R1 CWG Core
P3612R0 Harmonize proxy-reference operations (LWG 3638 and 4187) Arthur O'Dwyer 2025-09-02 2025-09 LWG Library
P3666r0 Bit-precise integers Jan Schultke 2025-09-11 2025-09 SG6 Numerics,SG22 Compatibility,EWG Evolution,LEWG Library Evolution
P3688R3 ASCII character utilities Jan Schultke 2025-09-09 2025-09 P3688R2 SG16 Unicode
P3695R1 Deprecate implicit conversions between Unicode character types Jan Schultke 2025-09-09 2025-09 P3695R0 SG16 Unicode,EWG Evolution
P3702R2 Stricter requirements for document submissions (SD-7) Jan Schultke 2025-09-10 2025-09 P3702R1 All of WG21
P3754R1 Slides for P3100R2 presentation to EWG Timur Doumler 2025-08-21 2025-09 P3754R0 EWG Evolution
P3776R0 More trailing commas Jan Schultke 2025-08-26 2025-09 EWG Evolution
P3776R1 More trailing commas Jan Schultke 2025-09-09 2025-09 P3776R0 EWG Evolution
P3784R1 range-if Michael Florian Hava 2025-09-10 2025-09 P3784R0 EWGI SG17: EWG Incubator
P3786R0 Tuple protocol for fixed-size spans Michael Florian Hava 2025-08-21 2025-09 LEWG Library Evolution
P3811R0 default comparison memory safety Jarrad J Waterloo 2025-08-15 2025-09 SG23 Safety and Security
P3812R0 const and & in default member functions Jarrad J Waterloo 2025-08-16 2025-09 EWG Evolution,CWG Core
P3813R0 execution::task::valueless() Michael Florian Hava 2025-09-10 2025-09 LEWG Library Evolution
P3815R0 Add scope_association concept to P3149 Jessica Wong 2025-09-01 2025-09 All of WG21
P3816R0 Hashing meta::info Matt Cummins 2025-09-01 2025-09 SG7 Reflection,LEWG Library Evolution,LWG Library,All of WG21
P3818R0 constexpr exception fix for potentially constant initialization Hana Dusíková 2025-08-31 2025-09 LEWG Library Evolution
P3818R1 constexpr exception fix for potentially constant initialization Hana Dusíková 2025-09-09 2025-09 P3818R0 LEWG Library Evolution
P3819R0 Remove evaluation_exception() from contract-violation handling for C++26 Peter Bindels 2025-09-05 2025-09 LEWG Library Evolution
P3820R0 Split constexpr uncaught_exceptions into distinct runtime and consteval functions Lénárd Szolnoki 2025-08-31 2025-09 LEWG Library Evolution
P3820R1 Fix constexpr uncaught_exceptions and current_exception Lénárd Szolnoki 2025-09-06 2025-09 P3820R0 LEWG Library Evolution
P3822R0 Conditional noexcept specifiers in compound requirements Viacheslav Luchkin 2025-09-01 2025-09 EWGI SG17: EWG Incubator,EWG Evolution
P3823R0 Wording for US NB comment 10 Arthur O'Dwyer 2025-09-10 2025-09 EWG Evolution
P3824R0 Static storage for braced initializers NBC examples Jarrad J Waterloo 2025-09-06 2025-09 SG23 Safety and Security,CWG Core
P3827R0 Wording for US NB comment 9 Arthur O'Dwyer 2025-09-10 2025-09 EWG Evolution
P3829R0 Contracts do not belong in the language David Chisnall 2025-09-03 2025-09 EWG Evolution,CWG Core,LWG Library
P3830R0 NB-Commenting is Not a Vehicle for Redesigning inplace_vector Nevin Liber 2025-09-03 2025-09 LEWG Library Evolution
P3831R0 Contract Labels Should Use Annotation Syntax Yihe Li 2025-09-06 2025-09 SG21 Contracts
P3832R0 Timed lock algorithms for multiple lockables Ted Lyngmo 2025-09-06 2025-09 LEWGI SG18: LEWG Incubator
P3834R0 Defaulting the Compound Assignment Operators Matthew Taylor 2025-09-10 2025-09 EWGI SG17: EWG Incubator
P3835R0 Contracts make C++ less safe -- full stop John Spicer 2025-09-08 2025-09 EWG Evolution,CWG Core,LWG Library
P3836R0 Make optional<T&> trivially copyable Jan Schultke 2025-09-11 2025-09 LEWG Library Evolution
P3838R0 Restoring Private Module Fragments Alisdair Meredith 2025-09-10 2025-09 CWG Core

Planning the next Meeting C++ job fairs

Meeting C++ is hosting a job fair in October online and planning a job fair in November in Berlin at Meeting C++ 2025!

Planning the next Meeting C++ job fairs

by Jens Weller

From the article:

The next Meeting C++ online job fair is planned for October 14th & 15th, also I'd like to talk about the onsite job fair plans for Meeting C++ 2025!

If you have open positions you should advertise them in the bi-weekly Meeting C++ Jobs Newsletter, which now also powers the candidate listing of Meeting C++ with 80+ international candidates at the moment.