site/docs/tutorial/cpp.md - bazel - Git at Google

 ---
 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/).

 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 [mobile application tutorial](app.md) to get started with
    building mobile applications for Android and iOS with Bazel.

 Happy building!
	---
	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/).

	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 [mobile application tutorial](app.md) to get started with
	building mobile applications for Android and iOS with Bazel.

	Happy building!