Carbon Language: An experimental successor to C++

See our announcement video from CppNorth. Note that Carbon is not ready for use.

Fast and works with C++

• Performance matching C++ using LLVM, with low-level access to bits and addresses
• Interoperate with your existing C++ code, from inheritance to templates
• Fast and scalable builds that work with your existing C++ build systems

Modern and evolving

• Solid language foundations that are easy to learn, especially if you have used C++
• Easy, tool-based upgrades between Carbon versions
• Safer fundamentals, and an incremental path towards a memory-safe subset

Welcoming open-source community

• Clear goals and priorities with robust governance
• Community that works to be welcoming, inclusive, and friendly
• Batteries-included approach: compiler, libraries, docs, tools, package manager, and more

Why build Carbon?

C++ remains the dominant programming language for performance-critical software, with massive and growing codebases and investments. However, it is struggling to improve and meet developers' needs, as outlined above, in no small part due to accumulating decades of technical debt. Incrementally improving C++ is extremely difficult, both due to the technical debt itself and challenges with its evolution process. The best way to address these problems is to avoid inheriting the legacy of C or C++ directly, and instead start with solid language foundations like modern generics system, modular code organization, and consistent, simple syntax.

Existing modern languages already provide an excellent developer experience: Go, Swift, Kotlin, Rust, and many more. Developers that can use one of these existing languages should. Unfortunately, the designs of these languages present significant barriers to adoption and migration from C++. These barriers range from changes in the idiomatic design of software to performance overhead.

Carbon is fundamentally a successor language approach, rather than an attempt to incrementally evolve C++. It is designed around interoperability with C++ as well as large-scale adoption and migration for existing C++ codebases and developers. A successor language for C++ requires:

• Performance matching C++, an essential property for our developers.
• Seamless, bidirectional interoperability with C++, such that a library anywhere in an existing C++ stack can adopt Carbon without porting the rest.
• A gentle learning curve with reasonable familiarity for C++ developers.
• Comparable expressivity and support for existing software's design and architecture.
• Scalable migration, with some level of source-to-source translation for idiomatic C++ code.

With this approach, we can build on top of C++'s existing ecosystem, and bring along existing investments, codebases, and developer populations. There are a few languages that have followed this model for other ecosystems, and Carbon aims to fill an analogous role for C++:

• JavaScript → TypeScript
• Java → Kotlin
• C++ → Carbon

Language Goals

We are designing Carbon to support:

• Performance-critical software
• Software and language evolution
• Code that is easy to read, understand, and write
• Practical safety and testing mechanisms
• Fast and scalable development
• Modern OS platforms, hardware architectures, and environments
• Interoperability with and migration from existing C++ code

While many languages share subsets of these goals, what distinguishes Carbon is their combination.

We also have explicit non-goals for Carbon, notably including:

• A stable application binary interface (ABI) for the entire language and library
• Perfect backwards or forwards compatibility

Our detailed goals document fleshes out these ideas and provides a deeper view into our goals for the Carbon project and language.

Project status

Carbon Language is currently an experimental project. We are hard at work on a toolchain implementation with compiler and linker. You can try out the current state at compiler-explorer.com.

We want to better understand whether we can build a language that meets our successor language criteria, and whether the resulting language can gather a critical mass of interest within the larger C++ industry and community.

Currently, we have fleshed out several core aspects of both Carbon the project and the language:

• The strategy of the Carbon Language and project.
• An open-source project structure, governance model, and evolution process.
• Critical and foundational aspects of the language design informed by our experience with C++ and the most difficult challenges we anticipate. This includes designs for:
  • Generics
  • Class types
  • Inheritance
  • Operator overloading
  • Lexical and syntactic structure
  • Code organization and modular structure
• An under-development compiler and toolchain that will compile Carbon (and eventually C++ code as well) into standard executable code. This is where most of our current implementation efforts are directed.
  • Historically, there was also a prototype explorer interpreter that implemented an older version of the Carbon language design, but is no longer under development and has been archived.

If you're interested in contributing, we're currently focused on developing the Carbon toolchain until it can support Carbon ↔ C++ interop. Beyond that, we plan to continue developing the design and toolchain until we can ship the 0.1 language and support evaluating Carbon in more detail.

You can see our full roadmap for more details.

Carbon and C++

If you're already a C++ developer, Carbon should have a gentle learning curve. It is built out of a consistent set of language constructs that should feel familiar and be easy to read and understand.

The Carbon code here is hypothetical and meant to show the look and feel of the language.

C++ code like this:

corresponds to this Carbon code:

You can call Carbon from C++ without overhead and the other way around. This means you migrate a single C++ library to Carbon within an application, or write new Carbon on top of your existing C++ investment. For example:

Read more about C++ interop in Carbon.

Beyond interoperability between Carbon and C++, we're also planning to support migration tools that will mechanically translate idiomatic C++ code into Carbon code to help you switch an existing C++ codebase to Carbon.

Generics

Carbon provides a modern generics system with checked definitions, while still supporting opt-in templates for seamless C++ interop. Checked generics provide several advantages compared to C++ templates:

• Generic definitions are fully type-checked, removing the need to instantiate to check for errors and giving greater confidence in code.
  • Avoids the compile-time cost of re-checking the definition for every instantiation.
  • When using a definition-checked generic, usage error messages are clearer, directly showing which requirements are not met.
• Enables automatic, opt-in type erasure and dynamic dispatch without a separate implementation. This can reduce the binary size and enables constructs like heterogeneous containers.
• Strong, checked interfaces mean fewer accidental dependencies on implementation details and a clearer contract for consumers.

Without sacrificing these advantages, Carbon generics support specialization, ensuring it can fully address performance-critical use cases of C++ templates. For more details about Carbon's generics, see their design.

In addition to easy and powerful interop with C++, Carbon templates can be constrained and incrementally migrated to checked generics at a fine granularity and with a smooth evolutionary path.

Memory safety

Safety, and especially memory safety, remains a key challenge for C++ and something a successor language needs to address.

We plan to support a two step migration process:

1. Highly automated, minimal supervision migration from C++ to a dialect of Carbon designed for C++ interop and migration.
2. Incremental refactoring of the Carbon code to adopt memory-safe designs, patterns, and APIs.

We also want to address important, low-hanging fruit in the safety space immediately when migrating into Carbon:

• Tracking uninitialized states better, increased enforcement of initialization, and hardening against initialization bugs when needed.
• Designing fundamental APIs and idioms to support dynamic bounds checking.
• Switching from undefined behavior to erroneous behavior wherever possible, and marking the remaining undefined behavior with visible unsafe syntax.
• Having a default debug build mode that has less runtime overhead while being more comprehensive than existing C++ debug build modes combined with Address Sanitizer.

For more details, see our safety design.

Getting started

To try out Carbon immediately in your browser, you can use the toolchain at: carbon.compiler-explorer.com.

We are developing a traditional toolchain for Carbon th