--- layout: documentation title: Build Tutorial - C++ --- 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](../cpp-use-cases.md) 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](#before-you-begin) * [Install Bazel](#install-bazel) * [Get the sample project](#get-the-sample-project) * [Build with Bazel](#build-with-bazel) * [Set up the workspace](#set-up-the-workspace) * [Understand the BUILD file](#understand-the-build-file) * [Build the project](#build-the-project) * [Review the dependency graph](#review-the-dependency-graph) * [Refine your Bazel build](#refine-your-bazel-build) * [Specify multiple build targets](#specify-multiple-build-targets) * [Use multiple packages](#use-multiple-packages) * [Use labels to reference targets](#use-labels-to-reference-targets) * [Further reading](#further-reading) ## Before you begin To prepare for the tutorial, first [Install Bazel](../install.md) 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](../be/c-cpp.html#cc_binary). 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: ```sh 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 --nohost_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](http://www.webgraphviz.com/). 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 --nohost_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'](/assets/cpp-tutorial-stage1.png) 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](../be/c-cpp.html#cc_library), 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'](/assets/cpp-tutorial-stage2.png) 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: ```python cc_library( name = "hello-time", srcs = ["hello-time.cc"], hdrs = ["hello-time.h"], visibility = ["//main:__pkg__"], ) ``` And at the `main/BUILD` file: ```python 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'](/assets/cpp-tutorial-stage3.png) 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 within the same package, you can skip the package path and 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](../cpp-use-cases.md). Then, check out the following: * [External Dependencies](../external.html) to learn more about working with local and remote repositories. * The [Build Encyclopedia](../be/overview.html) to learn more about Bazel. * The [Java build tutorial](java.md) to get started with building Java applications with Bazel. * The [Android application tutorial](android-app.md) to get started with building mobile applications for Android with Bazel. * The [iOS application tutorial](ios-app.md) to get started with building mobile applications for iOS with Bazel. Happy building!