Carbon Language:
An experimental successor to C++

Why? | Goals | Getting started | Join us

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 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 a 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 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 is currently an experimental project. 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
• A prototype interpreter demo that can both run isolated examples and gives a detailed analysis of the specific semantic model and abstract machine of Carbon. We call this the Carbon Explorer.

We are currently focused on getting more broad feedback and participation from the C++ community, completing the 0.1 language design, and completing the Carbon Explorer implementation of this design. Beyond that, we plan to prioritize C++ interoperability and a realistic toolchain that implements the 0.1 language and can be used to evaluate 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.

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 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 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, remain key challenges for C++ and something a successor language needs to address. Our initial priority and focus is on immediately addressing important, low-hanging fruit in the safety space:

• Tracking uninitialized states better, increased enforcement of initialization, and systematically providing hardening against initialization bugs when desired.
• Designing fundamental APIs and idioms to support dynamic bounds checks in debug and hardened builds.
• Having a default debug build mode that is both cheaper and more comprehensive than existing C++ build modes even when combined with Address Sanitizer.

Once we can migrate code into Carbon, we will have a simplified language with room in the design space to add any necessary annotations or features, and infrastructure like generics to support safer design patterns. Longer term, we will build on this to introduce a safe Carbon subset. This will be a large and complex undertaking, and won't be in the 0.1 design. Meanwhile, we are closely watching and learning from efforts to add memory safe semantics onto C++ such as Rust-inspired lifetime annotations.

Getting started

You can get started playing with Carbon by checking out the codebase and using the Carbon explorer:

# Install bazelisk using Homebrew.
$ brew install bazelisk

# Install Clang/LLVM using Homebrew.
# Many Clang/LLVM releases aren't built with options we rely on.
$ brew install llvm
$ export PATH="$(brew --prefix llvm)/bin:${PATH}"

# Download Carbon's code.
$ git clone https://github.com/carbon-language/carbon-lang
$ cd carbon-lang

# Build and run the explorer.
$ bazel run //explorer -- ./explorer/testdata/print/format_only.carbon

These instructions assume Homebrew; see our contribution tools documentation for more extensive tooling instructions.

Learn more about the Carbon project:

• Project goals
• Language design overview
• Carbon Explorer
• FAQ

Join us

Carbon is committed to a welcoming and inclusive environment where everyone can contribute.

• To watch for major release announcements, subscribe to our Carbon release post on GitHub and star carbon-lang.
• To join the design discussion, join our our Github forum.
• See our code of conduct and contributing guidelines for information about the Carbon development community.
• We discuss Carbon on Discord.