You can use Bazel to build your C++ application. In this tutorial you'll learn how to:
Suppose that you have an existing project in a directory, say, ~/gitroot/my-project/. Create an empty file at ~/gitroot/my-project/WORKSPACE to show Bazel where your project's root is. We are going to create a small hello world project with the following directory structure: {% highlight bash %} └── my-project ├── lib │ ├── BUILD │ ├── hello-greet.cc │ └── hello-greet.h ├── main │ ├── BUILD │ ├── hello-time.cc │ ├── hello-time.h │ └── hello-world.cc └── WORKSPACE {% endhighlight %}
Using the following commands to create the necessary source files: {% highlight bash %} $ # If you're not already there, move to your workspace directory. $ cd ~/gitroot/my-project $ mkdir ./main $ cat > main/hello-world.cc <<‘EOF’ #include “lib/hello-greet.h” #include “main/hello-time.h” #include #include
int main(int argc, char** argv) { std::string who = “world”; if (argc > 1) { who = argv[1]; } std::cout << get_greet(who) <<std::endl; print_localtime(); return 0; } EOF $ cat > main/hello-time.h <<‘EOF’ #ifndef MAIN_HELLO_TIME_H_ #define MAIN_HELLO_TIME_H_
void print_localtime();
#endif EOF $ cat > main/hello-time.cc <<‘EOF’ #include “main/hello-time.h” #include #include
void print_localtime() { std::time_t result = std::time(nullptr); std::cout << std::asctime(std::localtime(&result)); } EOF $ mkdir ./lib $ cat > lib/hello-greet.h <<‘EOF’ #ifndef LIB_HELLO_GREET_H_ #define LIB_HELLO_GREET_H_
#include
std::string get_greet(const std::string &thing);
#endif EOF $ cat > lib/hello-greet.cc <<‘EOF’ #include “lib/hello-greet.h” #include
std::string get_greet(const std::string& who) { return "Hello " + who; } EOF {% endhighlight %}
As you can see from the source code, main/hello-world.cc needs to include both lib/hello-greet.h and main/hello-time.h. First we create lib/BUILD for hello-greet.cc:
{% highlight python %} cc_library( name = “hello-greet”, srcs = [“hello-greet.cc”], hdrs = [“hello-greet.h”], visibility = [“//main:pkg”], ) {% endhighlight %}
Note that visibility = ["//main:__pkg__"] indicates hello-greet is visible from main/BUILD. Then we'd create the following main/BUILD file:
{% highlight python %} cc_library( name = “hello-time”, srcs = [“hello-time.cc”], hdrs = [“hello-time.h”], )
cc_binary( name = “hello-world”, srcs = [“hello-world.cc”], deps = [ “:hello-time”, “//lib:hello-greet”, ], ) {% endhighlight %}
Note when depending on a target in the same package, we can just use :hello-time. When the target is in other package, a full path from root should be used, like //lib:hello-greet.
Now you are ready to build your hello world C++ binary:
{% highlight bash %} $ bazel build main:hello-world INFO: Found 1 target... Target //main:hello-world up-to-date: bazel-bin/main/hello-world INFO: Elapsed time: 2.869s, Critical Path: 1.00s $ ./bazel-bin/main/hello-world Hello world Thu Jun 23 18:51:46 2016 $ ./bazel-bin/main/hello-world Bazel Hello Bazel Thu Jun 23 18:52:10 2016 {% endhighlight %}
Congratulations, you've just built your first Bazel target!
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:
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.
Sometimes you cannot (or do not want to) base include paths at the workspace root. Existing libraries might already have a include directory that doesn't match its path in your workspace. For example, suppose you have the following directory structure:
└── my-project
├── third_party
│ └── some_lib
│ ├── BUILD
│ ├── include
│ │ └── some_lib.h
│ └── some_lib.cc
└── WORKSPACE
Bazel will expect some_lib.h to be included as third_party/some_lib/include/some_lib.h, but suppose some_lib.cc includes "include/some_lib.h". To make that include path valid, third_party/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 = ["some_lib.h"], copts = ["-Ithird_party/some_lib"], )
This is especially useful for external dependencies, as their header files must otherwise be included with an external/[repository-name]/ prefix.
Suppose you are using Google Test. You can use one of the new_ repository functions in the WORKSPACE file to download Google Test and make it available in your repository:
new_http_archive( name = "gtest", url = "https://github.com/google/googletest/archive/release-1.7.0.zip", sha256 = "b58cb7547a28b2c718d1e38aee18a3659c9e3ff52440297e965f5edffe34b6d0", build_file = "gtest.BUILD", )
Then create gtest.BUILD, a BUILD file to use 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.googletest-release-1.7.0/include/ directory ("gtest/gtest.h"), so we must add that directory the include paths.linkopt.The final rule 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 new_http_archive strip this prefix by adding the strip_prefix attribute:
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:
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.
For example, we could create a test ./test/hello-test.cc such as:
#include "gtest/gtest.h" #include "lib/hello-greet.h" TEST(FactorialTest, Negative) { 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", "//lib:hello-greet", ], )
Note in order to make hello-greet visible to hello-test, we have to add "//test:__pkg__", to visibility attribute in ./lib/BUILD.
Now you can use bazel test to run the test.
{% highlight bash %} $ bazel test test:hello-test 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. {% endhighlight %}
If you want to use a library that you only have a compiled version of (e.g., headers and a .so) wrap it in a cc_library rule:
cc_library( name = "mylib", srcs = ["mylib.so"], hdrs = ["mylib.h"], )
Then other C++ targets in your workspace can depend on this rule.