Abstract:

This webinar provides an introduction to building C++ projects with Bazel, targeting developers who are new to the build system. The presentation argues that Bazel is superior to traditional tools like CMake and Make, not just for scale or multi-language support, but for C++ development in general due to its robust features.

The session walks through a hands-on demonstration, starting with a simple "Hello World" application to introduce basic concepts. It then progresses to managing third-party dependencies hermetically using libmagic from the Bazel Central Registry (BCR), highlighting the benefits over system-installed libraries. A key focus is on developer experience, showing how to set up editor integration with clangd by generating a compilation database. The webinar culminates in a detailed example of cross-compilation, demonstrating how to build a Linux binary on a macOS host by using a sysroot for full hermeticity, Bazel platforms, and transitions. The final output is packaged into an OCI container and executed to verify the process. The talk also briefly covers linting with clang-tidy and automatic BUILD file generation with Gazelle.

A Developer's Guide to Building C++ with Bazel

• 0:03:17 Why Use Bazel for C++?: Unlike other languages where Bazel is chosen for scale or polyglot repositories, for C++, it is often chosen simply because it is a better build tool than alternatives like CMake or Makefiles. It provides superior reproducibility, hermeticity, and maintainability.
• 0:10:25 Scaffolding a Project: The demonstration begins by using the aspect init command to create a new, pre-configured C++ project. This setup includes rules_cc, the modern Starlark implementation of C++ rules, and a hermetic LLVM/Clang toolchain, which Bazel downloads automatically.
• 0:14:45 Declarative vs. Procedural Builds: A simple C program is created. Instead of writing a procedural Makefile with specific compiler commands, the developer uses a declarative BUILD file with a cc_binary rule. This abstracts away the command-line flags, making the build easier to maintain and scale.
• 0:26:17 Managing Dependencies Hermetically: The webinar demonstrates how to add a third-party dependency (libmagic) hermetically by fetching it from the Bazel Central Registry (BCR). This is contrasted with the non-portable, non-hermetic approach of using a library installed on the host machine (e.g., via Homebrew).
• 0:36:39 Editor Integration with clangd: To enable features like code completion and go-to-definition, a compilation database (compile_commands.json) is required. A helper tool is run with Bazel to generate this file, which allows the clangd language server in VS Code to understand the project structure and include paths managed by Bazel.
• 0:43:56 Cross-Compiling with Platforms and Sysroots: The most powerful feature demonstrated is cross-compilation. A Linux-specific program is built on a macOS host by:
  • Specifying a target platform using the --platforms flag (e.g., linux_aarch64).
  • Using a sysroot, a filesystem containing the necessary headers and libraries for the target platform, to achieve a fully hermetic build that is independent of the host machine's SDK.
• 0:49:37 Packaging Binaries into OCI Containers: The cross-compiled Linux binary is packaged into a minimal OCI container using rules_oci. The container is then loaded into a local runtime (Podman) and executed, confirming that the binary built on macOS runs correctly in a Linux environment.
• 0:54:06 Using Transitions for Cleaner Builds: Bazel's "transitions" are introduced as a way to specify a target platform directly in the BUILD file. This allows developers to build for a different platform without needing to remember and type the --platforms command-line flag every time.
• 0:57:46 Linting and Code Generation: The webinar briefly showcases how to run linters like clang-tidy consistently across the codebase using a custom bazel lint command. It also mentions the use of Gazelle for automatically generating and maintaining C++ BUILD files.