Add icu4c (and other libs) to bazel project - c++

Question

I have a project that builds via CMake and requires a lot of manual installations of additional deps. I want to migrate this project to Bazel and make these libs automatically downloadable. I found a solution for Boost, but I can't understand how to add icu4c and other libs which builds via other tools.

Answer 1

There are many ways to make use of third-party libraries using Bazel. The chosen approach depends on different properties of the third-party library, e.g.: Does the third-party library already support Bazel? Is the library available only as a pre-build package? Does the library use code generators, or any other tools, or transitive dependencies?
Given the example of {fmt} which uses CMake as build system you can proceed as the following:
First approach: Inject a BUILD file
In your WORKSPACE file you can do something like:
maybe(
new_git_repository,
name = "fmt",
branch = "master",
remote = "https://github.com/fmtlib/fmt",
build_file = "//third_party:fmt.BUILD",
)
The corresponding fmt.BUILD file can look like this:
cc_library(
name = "fmt",
srcs = [
#"src/fmt.cc", # No C++ module support
"src/format.cc",
"src/os.cc",
],
hdrs = [
"include/fmt/args.h",
"include/fmt/chrono.h",
"include/fmt/color.h",
"include/fmt/compile.h",
"include/fmt/core.h",
"include/fmt/format.h",
"include/fmt/format-inl.h",
"include/fmt/locale.h",
"include/fmt/os.h",
"include/fmt/ostream.h",
"include/fmt/printf.h",
"include/fmt/ranges.h",
"include/fmt/xchar.h",
],
includes = [
"include",
"src",
],
strip_include_prefix = "include",
visibility = ["//visibility:public"],
)
Advantages:
fmt-8.01 does not have out-of-the-box support for {fmt}. This way Bazel can make use of {fmt} without the need that {fmt} knows anything about Bazel
fmt-8.0.1 needs not to be modified
Disadvantages:
Reinvent the wheel: Every Bazel project that wants to use {fmt} has to reinvent this fmt.BUILD file.
Maintenance costs: If different Bazel projects want to adapt to future versions of {fmt} every single project has to do this maintenance by its own. Maybe new files will be introduced.
Missing Knowledge: Maybe for some reason, it makes sense to define some special defines upfront, etc. It also takes some time and knowledge of {fmt} to set up such a BUILD file. What is the best practice to build this lib?
Second approach: Bazelize {fmt}
Add a WORKSPACE file and BUILD file to the {fmt} repository.
This way {fmt} gets bazelized and can be used in your Bazel builds.
You could use it then this way:
Example
Create a WORKSPACE.bazel file with the following content:
load("#bazel_tools//tools/build_defs/repo:git.bzl", "git_repository")
# Fetch bazelized fmt
git_repository(
name = "fmt",
branch = "bazel-support", # A copy of master where BUILD.bazel, WORKSPACE.bazel, .bazelrc and .bazelversion are moved to root
remote = "https://github.com/<user_or_organisation>/fmt", # replace <user_or_organisation> by a valid account
)
Create a BUILD.bazel file and add a dependency to {fmt} (wit the content of fmt.BUILD).
In favor of keeping the {fmt} project directory clean, those files were not added to the project root directory (see here for details).
Third approach: Using the {fmt} repository with Bazel
Even though the {fmt} repository does not contain a WORKSPACE file in its root directory, there is an easy approach to use the {fmt} repository with Bazel out of the box. This is demonstrated in the following example.
Add to your WORKSPACE file:
load("#bazel_tools//tools/build_defs/repo:git.bzl", "new_git_repository")
# Fetch all files from fmt including the BUILD file `support/bazel/BUILD.bazel`
new_git_repository(
name = "fmt_workaround",
branch = "master",
remote = "https://github.com/fmtlib/fmt/",
build_file_content = "# Empty build file on purpose"
)
# Now the BUILD file `support/bazel/BUILD.bazel` can be used:
new_git_repository(
name = "fmt",
branch = "master",
remote = "https://github.com/fmtlib/fmt/",
build_file = "#fmt_workaround//:support/bazel/BUILD.bazel"
)
Create a BUILD.bazel file and add a dependency to {fmt}:
cc_binary( # Build a binary
name = "Demo", # Name of the binary
srcs = ["main.cpp"], # List of files - we only have main.cpp
deps = ["#fmt//:fmt"], # Depend on fmt
)
Make use of {fmt} in main.cpp:
#include "fmt/core.h"C
int main() {
fmt::print("The answer is {}.\n", 42);
}
The expected output of this example is The answer is 42.
Forth approach: Make use of patch_cmd
load("#bazel_tools//tools/build_defs/repo:git.bzl", "git_repository")
git_repository(
name = "fmt",
branch = "master",
patch_cmds = [
"mv support/bazel/.bazelrc .bazelrc",
"mv support/bazel/.bazelversion .bazelversion",
"mv support/bazel/BUILD.bazel BUILD.bazel",
"mv support/bazel/WORKSPACE.bazel WORKSPACE.bazel",
],
# Windows related patch commands are only needed in the case MSYS2 is not installed
patch_cmds_win = [
"Move-Item -Path support/bazel/.bazelrc -Destination .bazelrc",
"Move-Item -Path support/bazel/.bazelversion -Destination .bazelversion",
"Move-Item -Path support/bazel/BUILD.bazel -Destination BUILD.bazel",
"Move-Item -Path support/bazel/WORKSPACE.bazel -Destination WORKSPACE.bazel",
],
remote = "https://github.com/fmtlib/fmt",
)
More details here.
Other libraries
I have written a few blog posts about Bazelizing different libs:
Bazel: Bazelizing Qt5 for macOS
Bazel: Bazelizing Embree 3.13.0
Bazel: Bazelizing Qt5 & Qt6
Bazel: Handling external dependencies in OpenEXR
Bazel: Offical support for OpenEXR
Bazel: Bazelizing OpenEXR
Bazel: Bazelizing Embree 3.12.1