Introduction to Bazel: Building a C++ Project

In this tutorial, you’ll learn the basics of building C++ applications with Bazel. You will set up your workspace and build a simple C++ project that illustrates key Bazel concepts, such as targets and BUILD files. After completing this tutorial, take a look at Common C++ Build Use Cases for information on more advanced concepts such as writing and running C++ tests.

Estimated completion time: 30 minutes.

What you’ll learn

In this tutorial you’ll learn how to:

  • Build a target
  • Visualize the project’s dependencies
  • Split the project into multiple targets and packages
  • Control target visibility across packages
  • Reference targets through labels

Contents

Before you begin

To prepare for the tutorial, first Install Bazel if you don’t have it installed already. Then, retrieve the sample project from Bazel’s GitHub repository:

git clone https://github.com/bazelbuild/examples

The sample project for this tutorial is in the examples/cpp-tutorial directory and is structured as follows:

examples
└── cpp-tutorial
    ├──stage1
    │  ├── main
    │  │   ├── BUILD
    │  │   └── hello-world.cc
    │  └── WORKSPACE
    ├──stage2
    │  ├── main
    │  │   ├── BUILD
    │  │   ├── hello-world.cc
    │  │   ├── hello-greet.cc
    │  │   └── hello-greet.h
    │  └── WORKSPACE
    └──stage3
       ├── main
       │   ├── BUILD
       │   ├── hello-world.cc
       │   ├── hello-greet.cc
       │   └── hello-greet.h
       ├── lib
       │   ├── BUILD
       │   ├── hello-time.cc
       │   └── hello-time.h
       └── WORKSPACE

As you can see, there are three sets of files, each set representing a stage in this tutorial. In the first stage, you will build a single target residing in a single package. In the second stage, you will split your project into multiple targets but keep it in a single package. In the third and final stage, you will split your project into multiple packages and build it with multiple targets.

Build with Bazel

Set up the workspace

Before you can build a project, you need to set up its workspace. A workspace is a directory that holds your project’s source files and Bazel’s build outputs. It also contains files that Bazel recognizes as special:

  • The WORKSPACE file, which identifies the directory and its contents as a Bazel workspace and lives at the root of the project’s directory structure,

  • One or more BUILD files, which tell Bazel how to build different parts of the project. (A directory within the workspace that contains a BUILD file is a package. You will learn about packages later in this tutorial.)

To designate a directory as a Bazel workspace, create an empty file named WORKSPACE in that directory.

When Bazel builds the project, all inputs and dependencies must be in the same workspace. Files residing in different workspaces are independent of one another unless linked, which is beyond the scope of this tutorial.

Understand the BUILD file

A BUILD file contains several different types of instructions for Bazel. The most important type is the build rule, which tells Bazel how to build the desired outputs, such as executable binaries or libraries. Each instance of a build rule in the BUILD file is called a target and points to a specific set of source files and dependencies. A target can also point to other targets.

Take a look at the BUILD file in the cpp-tutorial/stage1/main directory:

cc_binary(
    name = "hello-world",
    srcs = ["hello-world.cc"],
)

In our example, the hello-world target instantiates Bazel’s built-in cc_binary rule. The rule tells Bazel to build a self-contained executable binary from the hello-world.cc source file with no dependencies.

The attributes in the target explicitly state its dependencies and options. While the name attribute is mandatory, many are optional. For example, in the hello-world target, name is self-explanatory, and srcs specifies the source file(s) from which Bazel builds the target.

Build the project

Let’s build your sample project. Change into the cpp-tutorial/stage1 directory and run the following command:

bazel build //main:hello-world

Notice the target label - the //main: part is the location of our BUILD file relative to the root of the workspace, and hello-world is what we named that target in the BUILD file. (You will learn about target labels in more detail at the end of this tutorial.)

Bazel produces output similar to the following:

INFO: Found 1 target...
Target //main:hello-world up-to-date:
  bazel-bin/main/hello-world
INFO: Elapsed time: 2.267s, Critical Path: 0.25s

Congratulations, you just built your first Bazel target! Bazel places build outputs in the bazel-bin directory at the root of the workspace. Browse through its contents to get an idea for Bazel’s output structure.

Now test your freshly built binary:

bazel-bin/main/hello-world

Review the dependency graph

A successful build has all of its dependencies explicitly stated in the BUILD file. Bazel uses those statements to create the project’s dependency graph, which enables accurate incremental builds.

Let’s visualize our sample project’s dependencies. First, generate a text representation of the dependency graph (run the command at the workspace root):

bazel query --notool_deps --noimplicit_deps "deps(//main:hello-world)" \
  --output graph

The above command tells Bazel to look for all dependencies for the target //main:hello-world (excluding host and implicit dependencies) and format the output as a graph.

Then, paste the text into GraphViz.

On Ubuntu, you can view the graph locally by installing GraphViz and the xdot Dot Viewer:

sudo apt update && sudo apt install graphviz xdot

Then you can generate and view the graph by piping the text output above straight to xdot:

xdot <(bazel query --notool_deps --noimplicit_deps "deps(//main:hello-world)" \
  --output graph)

As you can see, the first stage of the sample project has a single target that builds a single source file with no additional dependencies:

Dependency graph for 'hello-world'

Now that you have set up your workspace, built your project, and examined its dependencies, let’s add some complexity.

Refine your Bazel build

While a single target is sufficient for small projects, you may want to split larger projects into multiple targets and packages to allow for fast incremental builds (that is, only rebuild what’s changed) and to speed up your builds by building multiple parts of a project at once.

Specify multiple build targets

Let’s split our sample project build into two targets. Take a look at the BUILD file in the cpp-tutorial/stage2/main directory:

cc_library(
    name = "hello-greet",
    srcs = ["hello-greet.cc"],
    hdrs = ["hello-greet.h"],
)

cc_binary(
    name = "hello-world",
    srcs = ["hello-world.cc"],
    deps = [
        ":hello-greet",
    ],
)

With this BUILD file, Bazel first builds the hello-greet library (using Bazel’s built-in cc_library rule), then the hello-world binary. The deps attribute in the hello-world target tells Bazel that the hello-greet library is required to build the hello-world binary.

Let’s build this new version of our project. Change into the cpp-tutorial/stage2 directory and run the following command:

bazel build //main:hello-world

Bazel produces output similar to the following:

INFO: Found 1 target...
Target //main:hello-world up-to-date:
  bazel-bin/main/hello-world
INFO: Elapsed time: 2.399s, Critical Path: 0.30s

Now test your freshly built binary:

bazel-bin/main/hello-world

If you now modify hello-greet.cc and rebuild the project, Bazel will only recompile that file.

Looking at the dependency graph, you can see that hello-world depends on the same inputs as it did before, but the structure of the build is different:

Dependency graph for 'hello-world'

You’ve now built the project with two targets. The hello-world target builds one source file and depends on one other target (//main:hello-greet), which builds two additional source files.

Use multiple packages

Let’s now split the project into multiple packages. Take a look at the contents of the cpp-tutorial/stage3 directory:

└──stage3
   ├── main
   │   ├── BUILD
   │   ├── hello-world.cc
   │   ├── hello-greet.cc
   │   └── hello-greet.h
   ├── lib
   │   ├── BUILD
   │   ├── hello-time.cc
   │   └── hello-time.h
   └── WORKSPACE

Notice that we now have two sub-directories, and each contains a BUILD file. Therefore, to Bazel, the workspace now contains two packages, lib and main.

Take a look at the lib/BUILD file:

cc_library(
    name = "hello-time",
    srcs = ["hello-time.cc"],
    hdrs = ["hello-time.h"],
    visibility = ["//main:__pkg__"],
)

And at the main/BUILD file:

cc_library(
    name = "hello-greet",
    srcs = ["hello-greet.cc"],
    hdrs = ["hello-greet.h"],
)

cc_binary(
    name = "hello-world",
    srcs = ["hello-world.cc"],
    deps = [
        ":hello-greet",
        "//lib:hello-time",
    ],
)

As you can see, the hello-world target in the main package depends on the hello-time target in the lib package (hence the target label //lib:hello-time) - Bazel knows this through the deps attribute. Take a look at the dependency graph:

Dependency graph for 'hello-world'

Notice that for the build to succeed, we make the //lib:hello-time target in lib/BUILD explicitly visible to targets in main/BUILD using the visibility attribute. This is because by default targets are only visible to other targets in the same BUILD file. (Bazel uses target visibility to prevent issues such as libraries containing implementation details leaking into public APIs.)

Let’s build this final version of our project. Change into the cpp-tutorial/stage3 directory and run the following command:

bazel build //main:hello-world

Bazel produces output similar to the following:

INFO: Found 1 target...
Target //main:hello-world up-to-date:
  bazel-bin/main/hello-world
INFO: Elapsed time: 0.167s, Critical Path: 0.00s

Now test the freshly built binary:

bazel-bin/main/hello-world

You’ve now built the project as two packages with three targets and understand the dependencies between them.

Use labels to reference targets

In BUILD files and at the command line, Bazel uses labels to reference targets - for example, //main:hello-world or //lib:hello-time. Their syntax is:

//path/to/package:target-name

If the target is a rule target, then path/to/package is the path to the directory containing the BUILD file, and target-name is what you named the target in the BUILD file (the name attribute). If the target is a file target, then path/to/package is the path to the root of the package, and target-name is the name of the target file, including its full path.

When referencing targets at the repository root, the package path is empty, just use //:target-name. When referencing targets within the same BUILD file, you can even skip the // workspace root identifier and just use :target-name.

Further reading

Congratulations! You now know the basics of building a C++ project with Bazel. Next, read up on the most common C++ build use cases. Then, check out the following:

Happy building!