I'm working on a C++ project and I need Numpy like arrays and functionalities in C++. I found some alternatives like xtensor, NumCpp etc. These are header only libraries. The problem is I'm experimenting with Bazel for the first time so, I don't have any idea about how do I add header only library to Bazel workspace. There are some suggestions like genrule-environment, rules-foreign-cc suggested on other questions around Bazel. I've added http_archive to WORKSPACE file, but I'm not sure what to add in BUILD file.
WORKSPACE file
load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")
all_content = """filegroup(name = "all", srcs = glob(["**"]), visibility = ["//visibility:public"])"""
http_archive(
name = "xtensor",
build_file_content = all_content,
strip_prefix = "xtensor-master",
urls = ["https://github.com/xtensor-stack/xtensor/archive/refs/heads/master.zip"],
)
http_archive(
name = "NumCpp",
build_file_content = all_content,
strip_prefix = "NumCpp-master",
urls = ["https://github.com/dpilger26/NumCpp/archive/refs/heads/master.zip"],
)
http_archive(
name = "rules_foreign_cc",
sha256 = "c2cdcf55ffaf49366725639e45dedd449b8c3fe22b54e31625eb80ce3a240f1e",
strip_prefix = "rules_foreign_cc-0.1.0",
url = "https://github.com/bazelbuild/rules_foreign_cc/archive/0.1.0.zip",
)
load("@rules_foreign_cc//:workspace_definitions.bzl", "rules_foreign_cc_dependencies")
rules_foreign_cc_dependencies()
I've tried to follow documentation, but failed every time. Some help would be appreciated.
For simple things like header-only libraries, I would write BUILD files yourself, without using rules_foreign_cc. Just write a cc_library
with no srcs
. Something like this:
http_archive(
name = "xtensor",
build_file_content = all_content,
strip_prefix = "xtensor-master",
urls = ["https://github.com/xtensor-stack/xtensor/archive/refs/heads/master.zip"],
build_file_content = """
cc_library(
name = "xtensor",
visibility = ["//visibility:public"],
hdrs = glob(["xtensor/*.hpp"]),
defines = [
"XTENSOR_ENABLE_ASSERT",
],
deps = [
"@tbb",
],
)
""",
)
@xtensor
will be your Bazel repository. The build_file_content
will be used to create a BUILD.bazel
file in it. That means your code can depend on it via @xtensor//:xtensor
, which can be shortened to just @xtensor
. You can also put that in a separate file (say build/BUILD.xtensor.bazel
) and then use build_file = "@build//:BUILD.xtensor.bazel"
instead of putting it inline in WORKSPACE
via build_file_content
.
rules_foreign_cc is going to generate something equivalent if you get it set up, but that seems like a lot more trouble to me than just writing it yourself. I set defines
and deps
based on a scan through the CMakeLists.txt, you'll want to customize based on how you want it configured.
The only other thing I see that CMakeLists doing (beyond finding dependencies and setting some -D
flags, which translate as shown above) is generating a single file that #includes all the others. If you want to do that, I'd do something like this (in the BUILD file for @xtensor
):
genrule(
name = "gen_single_include",
outs = ["xtensor.hpp"],
cmd = "\n".join([
"cat > $@ <<'END'",
"#ifndef XTENSOR",
"#define XTENSOR",
] + ["#include \"%s\"" % h[len("xtensor/"):]
for h in glob(["xtensor/*.hpp"])
if h not in [
"xtensor/xexpression_holder.hpp",
"xtensor/xjson.hpp",
"xtensor/xmime.hpp",
"xtensor/xnpy.hpp",
]] + [
"#endif",
"END",
]),
I may not have gotten everything perfect here, but the idea is to build up the file using cat
to copy stdin to it, a bash here document to feed the content in, and Python-style list comprehensions to actually build up the content.