hsutter/cppfront

cppfront

Copyright (c) Herb Sutter • See License

Cppfront is a compiler from an experimental C++ 'syntax 2' (Cpp2) to today's 'syntax 1' (Cpp1), to prove out some concepts, share some ideas, and prototype features that can also be proposed for evolving today's C++.

You can find the current status summary here: Cppfront status summary (summer 2026)

Documentation: available here

• Installation

What's different about this project?

In short, it aims to help evolve C++ itself, not to be a "C++ successor."

What it isn't. Cpp2 is not a successor or alternate language with its own divergent or incompatible ecosystem. For example, it does not have its own nonstandard incompatible modules/concepts/etc. that compete with the Standard C++ features; it does not replace your Standard C++ compiler or other tools; and it does not require any changes to your Standard C++ compiler or standard library or other libraries or tools to keep fully using all of them.

What it is. Cpp2 aims to be another "skin" for C++ itself, just a simpler and safer way to write ordinary C++ types/functions/objects, and a faster way to experiment with proposals for future new Standard C++ features in a simpler compiler and syntax flavor. It seamlessly uses Standard C++ modules, concept requirements, and other features.

It also works with all existing C++20 (or higher) compilers, libraries, and tools right out of the box with no changes required to use them all seamlessly and directly, with zero overhead.

For more, see What is Cpp2?.

Acknowledgments

Scores of people have given valuable feedback and many are listed below, but I especially want to thank Joe Duffy, Anders Hejlsberg, Bjarne Stroustrup, Andrew Sutton, Tim Sweeney, and Mads Torgersen for their insights and valuable feedback on this work over the years — especially when they disagreed with me. I'd also like to thank Dave Abrahams, Andrei Alexandrescu, Walter Bright, Lee Howes, Chris McKinsey, Scott Meyers, Gor Nishanov, Andrew Pardoe, Sean Parent, Jared Parsons, David Sankel, Nathan Sidwell, JC van Winkel, and Ville Voutilainen for broad feedback on the design.

Many more people are listed below for their help with specific parts of the design and those proposals/prototypes. I apologize for the names I have forgotten.

Further information

To learn more, please see:

• 👀 The documentation 👀
• My CppCon 2022 talk, "Can C++ be 10x simpler and safer ...?"
• My CppCon 2023 talk, "Cooperative C++ Evolution: Toward a TypeScript for C++"
• Update on cppfront at ACCU 2024
• This repo's wiki
• The list of papers and talks below

Papers and talks derived from this work (presented in current syntax as contributions toward ISO C++'s evolution itself)

Here are the ISO C++ papers and CppCon conference talks I've given since 2015 that have been derived from this work, in the order that I brought each piece forward. Most of the details in the materials below are still current with only incremental updates, apart from the specific syntax of course.

2015: Lifetime safety

• CppCon 2015: "Writing good C++14... by default" particularly from 29:00 onward shows the Lifetime analysis with live demos in a Visual Studio prototype.
• CppCon 2018: "Thoughts on a more powerful and simpler C++ (#5 of N):
  • The section starting at 18:00 is an update on the Lifetime status with live demos in a Clang prototype.
  • The final part starting at 1:28:00 shows the Lifetime and Metaclasses proposals working hand-in-hand. This is one of the few places before cppfront where the same compiler has contained prototypes of multiple 'syntax 2'-derived features so I could show how they build on each other when used together.
• C++ Core Guidelines: Lifetime safety profile is this static analysis adopted by the C++ Core Guidelines.
• P1179: Lifetime Safety: Preventing common dangling is the same analysis in the WG 21 paper list.

This is not yet implemented in cppfront. Implementations are shipping in Visual Studio and in CLion, and initial parts have been upstreamed in Clang. I want to especially thank Matthias Gehre, Gabor Horvath, Neil MacIntosh, and Kyle Reed for their help in implementing the Lifetime static analysis design in Visual Studio and a Clang fork. Thanks also to the following for their input and feedback on the specification: Andrei Alexandrescu, Steve Carroll, Pavel Curtis, Gabriel Dos Reis, Joe Duffy, Daniel Frampton, Anna Gringauze, Chris Hawblitzel, Nicolai Josuttis, Ellie Kornstaedt, Aaron Lahman, Ryan McDougall, Nathan Myers, Gor Nishanov, Andrew Pardoe, Jared Parsons, Dave Sielaff, Richard Smith, Jim Springfield, and Bjarne Stroustrup.

2016: Garbage-collected memory arena

• CppCon 2016: "Leak-freedom in C++... by default" particularly from 59:00 onward where I show the strawman prototype I wrote of a tracing garbage-collection memory arena.
• Github.com/hsutter/gcpp: "gcpp: Deferred and unordered destruction" is the GitHub prototype I wrote.

This is not yet implemented in cppfront. I welcome a real GC expert to collaborate with on bringing this forward to become a "real" usable tracing GC memory arena that C++ code can opt into, with real C++ zero-overhead costing (don't pay anything if you don't do a gc.new, and if you do use it then have the costs be proportional to the number of gc.new allocations).

2017: Spaceship operator for comparisons, <=>

• CppCon 2017 (just the intro, first 6 minutes): "Meta: Thoughts on generative C++".
• P0515: Consistent comparison is the proposal in today's syntax that I proposed, and was adopted, for C++20.

This is part of ISO C++20 and C++23, except only for chained comparisons which is implemented in cppfront. Thank you again to everyone who helped land this in the Standard in C++20 and improve it in C++23, including especially Walter Brown, Lawrence Crowl, Cameron DaCamara, Gabriel Dos Reis, Jens Maurer, Barry Revzin, Richard Smith, and David Stone.

2017: Reflection, generation, and metaclasses

• ACCU 2017: "Thoughts on metaclasses" is the first talk I gave about this.
• CppCon 2017: "Meta: Thoughts on generative C++" from after the intro, from 6:00 onward.
• CppCon 2018: "Thoughts on a more powerful and simpler C++ ("Simplifying C++" #5 of N):
  • The section starting at 51:00 is an update on the Metaclasses status with live demos in a Clang prototype.
  • (repeating the Lifetime section bullet above) The final part starting at 1:28 shows the Lifetime and Metaclasses proposals working hand-in-hand. This is one of the few places before cppfront where the same compiler has contained prototypes of multiple 'syntax 2'-derived features so I could show how they build on each other when used together.
• P0707: Metaclass functions: Generative C++ is the paper I brought to the ISO C++ committee.

This is mostly implemented in cppfront, except for the ability to write your own metafunctions (that's coming). Thanks again to Andrew Sutton and his colleagues Wyatt Childers and Jennifer Yao for their help in implementing the Clang-based prototypes of this proposal, and everyone else who contributed feedback on the design including Louis Brandy, Chandler Carruth, Casey Carter, Matúš Chochlík, Lawrence Crowl, Pavel Curtis, Louis Dionne, Gabriel Dos Reis, Joe Duffy, Kenny Kerr, Nicolai Josuttis, Aaron Lahman, Scott Meyers, Axel Naumann, Gor Nishanov, Stephan T. Lavavej, Andrew Pardoe, Sean Parent, Jared Parsons, David Sankel, Richard Smith, Jeff Snyder, Mike Spertus, Mads Torgersen, Daveed Vandevoorde, Tony Van Eerd, JC van Winkel, Ville Voutilainen, and Titus Winters, and many more WG 21 / SG 7 participants.

2018: Updates to Lifetime and Metaclasses (see above)

2019: Zero-overhead deterministic exceptions: Throwing values

• ACCU 2019: "De-fragmenting C++: Making exceptions more affordable and usable.
• CppCon 2019: "De-fragmenting C++: Making exceptions and RTTI more affordable and usable ("Simplifying C++" #6 of N).
• P0709: Zero-overhead deterministic exceptions: Throwing values.

This is not yet implemented in cppfront.

2020: Parameter passing

• ACCU autumn 2019: "Quantifying accidental complexity: An empirical look at teaching and using C++" was my first public talk about this, but a "beta" version that was not recorded; you can find the description here.
• CppCon 2020: "Quantifying accidental complexity: An empirical look at teaching and using C++":
  • The first half of the talk is about how to be rigorous and actually measure that we're making improvements, including to measure the percentage of today's C++ guidance that is about parameter passing and initialization.
  • The second half of the talk is about in, inout, out, move, and forward.
• d0708: "Parameter passing -> guaranteed unified initialization and value setting goes into additional detail I didn't have time for in the talk.
• Github.com/hsutter/708 is a repo with the paper and demo examples as used in the talk.
• P2064: "Assumptions" is also related to this 'syntax 2' work, because this work includes a contracts design, and assumptions ought to be separate from that. This paper was making the technical argument why assumptions and assertions (contracts) are different things.

This is implemented in cppfront, including the unified operator= for user-defined types. Thanks to Andrew Sutton for an initial Clang-based implementation.

2020: "Bridge to NewThingia"

In 2020 I also started socializing the ideas of:

• How do you answer "why is your thing different when others that look like it have all failed"?
• What does it take to be adoptable, including to enable incremental adoption?

I had specifically in mind a major C++ evolution's success when many attempts to make C or C++ safer have failed, and the importance of seamless compatibility. The talk was "Bridge to NewThingia," presented at:

• DevAroundTheSun: "Bridge to Newthingia", an initial 26-minute version.
• C++ on Sea: "Bridge to NewThingia" which especially at the end starting near 48:00 had a slide that directly tackled the "C++ major evolution" scenario, and laid out what I think it would take to have credible answers to the key questions.

2021: is, as, and pattern matching

• CppCon 2021: "Extending and simplifying C++: Thoughts on pattern matching using is and as".
• P2392: Pattern matching using is and as is the ISO C++ committee paper.

This is mostly implemented in cppfront. There is support for is, as, and basic inspect expressions.

2022: CppCon 2022 talk and cppfront

• CppCon 2022: "Can C++ be 10x simpler and safer ...?"
• This repo.

Epilog: 2016 roadmap diagram

Finally, here is a roadmap diagram I made in 2016 that is still recognizably a roadmap of Cpp2's design approach, although a few additions like <=> came later. I think this is important to show design decisions are related and support each other, so that they are not a gaggle of point fixes but a coordinated refactoring of C++ into a smaller number of regular and combinable features. As Bjarne Stroustrup put it in the ACM History of Programming Languages III (among other places):

"10% the size of C++ in definition and similar in front-end compiler size. ... Most of the simplification would come from generalization." (B. Stroustrup, ACM HOPL-III, 2007; emphasis added)

I haven't updated this roadmap diagram since 2016, but it shows many of the talks and papers that have come since then from this work, and it's still a pretty up-to-date roadmap of the major parts of Cpp2. As of spring 2023, cppfront implements most of this roadmap.

I hope you enjoy reading about this personal experiment, and I hope that it might at least start a conversation about what could be possible within C++'s own evolution to make C++ 10x simpler, safer, and more toolable.