--- 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.