path: root/site/docs/cpp-use-cases.md

---
layout: documentation
title: Common C++ Build Use Cases
---

Introduction to Bazel: Common C++ Build Use Cases
==========

Here you will find some of the most common use cases for building C++ projects
with Bazel. If you have not done so already, get started with building C++
projects with Bazel by completing the tutorial
[Introduction to Bazel: Build a C++ Project](tutorial/cpp.html).

## Contents

*  [Including multiple files in a target](#including-multiple-files-in-a-target)
*  [Using transitive includes](#using-transitive-includes)
*  [Adding include paths](#adding-include-paths)
*  [Including external libraries](#including-external-libraries)
*  [Writing and running C++ tests](#writing-and-running-c-tests)
*  [Adding dependencies on precompiled libraries](#adding-dependencies-on-precompiled-libraries)

## Including multiple files in a target

You can include multiple files in a single target with
[glob](../be/functions.html#glob).
For example:

```python
cc_library(
    name = "build-all-the-files",
    srcs = glob(["*.cc"]),
    hdrs = glob(["*.h"]),
)
```

With this target, Bazel will build all the `.cc` and `.h` files it finds in the
same directory as the `BUILD` file that contains this target (excluding
subdirectories).

## Using transitive includes

If a file includes a header, then the file's rule should depend on that header's
library.  Conversely, only direct dependencies need to be specified as
dependencies.  For example, suppose `sandwich.h` includes `bread.h` and
`bread.h` includes `flour.h`.  `sandwich.h` doesn't include `flour.h` (who wants
flour in their sandwich?), so the `BUILD` file would look like this:

```python
cc_library(
    name = "sandwich",
    srcs = ["sandwich.cc"],
    hdrs = ["sandwich.h"],
    deps = [":bread"],
)

cc_library(
    name = "bread",
    srcs = ["bread.cc"],
    hdrs = ["bread.h"],
    deps = [":flour"],
)

cc_library(
    name = "flour",
    srcs = ["flour.cc"],
    hdrs = ["flour.h"],
)
```

Here, the `sandwich` library depends on the `bread` library, which depends
on the `flour` library.

## Adding include paths

Sometimes you cannot (or do not want to) root include paths at the workspace
root. Existing libraries might already have an include directory that doesn't
match its path in your workspace.  For example, suppose you have the following
directory structure:

```
└── my-project
    ├── legacy
    │   └── some_lib
    │       ├── BUILD
    │       ├── include
    │       │   └── some_lib.h
    │       └── some_lib.cc
    └── WORKSPACE
```

Bazel will expect `some_lib.h` to be included as
`legacy/some_lib/include/some_lib.h`, but suppose `some_lib.cc` includes
`"include/some_lib.h"`.  To make that include path valid,
`legacy/some_lib/BUILD` will need to specify that the `some_lib/`
directory is an include directory:

```python
cc_library(
    name = "some_lib",
    srcs = ["some_lib.cc"],
    hdrs = ["include/some_lib.h"],
    copts = ["-Ilegacy/some_lib/include"],
)
```

This is especially useful for external dependencies, as their header files
must otherwise be included with a `/` prefix.

## Including external libraries

Suppose you are using [Google Test](https://github.com/google/googletest). You
can use one of the `new_` repository functions in the `WORKSPACE` file to
download Google Test and make it available in your repository:

```python
new_http_archive(
    name = "gtest",
    url = "https://github.com/google/googletest/archive/release-1.7.0.zip",
    sha256 = "b58cb7547a28b2c718d1e38aee18a3659c9e3ff52440297e965f5edffe34b6d0",
    build_file = "gtest.BUILD",
)
```

**NOTE:** If the destination already contains a `BUILD` file, you can use one of
the `non-new_` functions.

Then create `gtest.BUILD`, a `BUILD` file used to compile Google Test.
Google Test has several "special" requirements that make its `cc_library` rule
more complicated:

*  `googletest-release-1.7.0/src/gtest-all.cc` `#include`s all of the other
   files in `googletest-release-1.7.0/src/`, so we need to exclude it from the
   compile or we'll get link errors for duplicate symbols.

*  It uses header files that are relative to the
`googletest-release-1.7.0/include/` directory  (`"gtest/gtest.h"`), so we must
add that directory to the include paths.

*  It needs to link in `pthread`, so we add that as a `linkopt`.

The final rule therefore looks like this:

```python
cc_library(
    name = "main",
    srcs = glob(
        ["googletest-release-1.7.0/src/*.cc"],
        exclude = ["googletest-release-1.7.0/src/gtest-all.cc"]
    ),
    hdrs = glob([
        "googletest-release-1.7.0/include/**/*.h",
        "googletest-release-1.7.0/src/*.h"
    ]),
    copts = [
        "-Iexternal/gtest/googletest-release-1.7.0/include"
    ],
    linkopts = ["-pthread"],
    visibility = ["//visibility:public"],
)
```

This is somewhat messy: everything is prefixed with `googletest-release-1.7.0`
as a byproduct of the archive's structure. You can make `new_http_archive` strip
this prefix by adding the `strip_prefix` attribute:

```python
new_http_archive(
    name = "gtest",
    url = "https://github.com/google/googletest/archive/release-1.7.0.zip",
    sha256 = "b58cb7547a28b2c718d1e38aee18a3659c9e3ff52440297e965f5edffe34b6d0",
    build_file = "gtest.BUILD",
    strip_prefix = "googletest-release-1.7.0",
)
```

Then `gtest.BUILD` would look like this:

```python
cc_library(
    name = "main",
    srcs = glob(
        ["src/*.cc"],
        exclude = ["src/gtest-all.cc"]
    ),
    hdrs = glob([
        "include/**/*.h",
        "src/*.h"
    ]),
    copts = ["-Iexternal/gtest/include"],
    linkopts = ["-pthread"],
    visibility = ["//visibility:public"],
)
```

Now `cc_` rules can depend on `@gtest//:main`.

## Writing and running C++ tests

For example, we could create a test `./test/hello-test.cc` such as:

```cpp
#include "gtest/gtest.h"
#include "lib/hello-greet.h"

TEST(HelloTest, GetGreet) {
  EXPECT_EQ(get_greet("Bazel"), "Hello Bazel");
}
```

Then create `./test/BUILD` file for your tests:

```python
cc_test(
    name = "hello-test",
    srcs = ["hello-test.cc"],
    copts = ["-Iexternal/gtest/include"],
    deps = [
        "@gtest//:main",
        "//lib:hello-greet",
    ],
)
```

Note that in order to make `hello-greet` visible to `hello-test`, we have to add
`"//test:__pkg__",` to the `visibility` attribute in `./lib/BUILD`.

Now you can use `bazel test` to run the test.

```
bazel test test:hello-test
```

This produces the following output:

```
INFO: Found 1 test target...
Target //test:hello-test up-to-date:
  bazel-bin/test/hello-test
INFO: Elapsed time: 4.497s, Critical Path: 2.53s
//test:hello-test PASSED in 0.3s

Executed 1 out of 1 tests: 1 test passes.
```


## Adding dependencies on precompiled libraries

If you want to use a library of which you only have a compiled version (for
example, headers and a `.so` file) wrap it in a `cc_library` rule:

```python
cc_library(
    name = "mylib",
    srcs = ["mylib.so"],
    hdrs = ["mylib.h"],
)
```

This way, other C++ targets in your workspace can depend on this rule.