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.

Contents

Including multiple files in a target

You can include multiple files in a single target with glob. For example:

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:

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:

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. You can use one of the repository functions in the WORKSPACE file to download Google Test and make it available in your repository:

load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")

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 leave out the build_file attribute.

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 #includes 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:

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 http_archive strip this prefix by adding the strip_prefix attribute:

load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")

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:

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:

#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:

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

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

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