Bazel 20 中 C 20 的具体应用场景有哪些？

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C++20 使用 Bazel 和 Clang 进行构建的步骤如下：

1. Bazel 构建系统的安装： - 下载并安装 Bazel。 - 配置 Bazel 工作区。

2. LLVM 编译器的安装： - 下载并安装 LLVM。 - 确保 Clang 是 LLVM 的一部分。

3. Bazel 和 Clang 的配置： - 配置 Bazel 以使用 Clang 作为编译器。 - 设置 C++20 库和工具链。

4. C++20 库与应用的构建： - 编写 Bazel 工程文件（如 `BUILD`）。 - 指定 C++20 库和应用的依赖。 - 运行 Bazel 命令进行构建。

示例：

plaintext安装 Bazel安装 LLVM配置 Bazel 使用 Clang设置 C++20 库编写 BUILD 文件运行 bazel build //path/to:target

C++20 如何以 Bazel & Clang 进行构建呢？

本文将介绍：

• Bazel 构建系统的安装
• LLVM 编译系统的安装
  • Clang is an "LLVM native" C/C++/Objective-C compiler
• Bazel Clang 工具链的配置
• C++20 库与应用的构建

本文示例可见： github.com/ikuokuo/start-cpp20

本文是于 Ubuntu 20 上进行的实践，Windows 可以用 WSL 准备环境。

安装 Bazel，以二进制方式

Bazelisk 是安装 Bazel 的推荐方式，我们安装它的二进制发布即可：

cd ~ wget github.com/bazelbuild/bazelisk/releases/download/v1.12.0/bazelisk-linux-amd64 -O bazelisk-1.12.0-linux-amd64 chmod a+x bazelisk-* sudo ln -s $(pwd)/bazelisk-1.12.0-linux-amd64 /usr/local/bin/bazel touch WORKSPACE # 国内下载 Bazel 可能遇到如下问题，配置 .bazeliskrc 解决 # could not resolve the version 'latest' to an actual version number # github.com/bazelbuild/bazelisk/issues/220 cat <<-EOF > .bazeliskrc BAZELISK_BASE_URL=github.com/bazelbuild/bazel/releases/download USE_BAZEL_VERSION=5.2.0 EOF bazel version

更多方式，可见官方文档。进一步，推荐安装 buildtools，下载后软链一下：

sudo ln -s $(pwd)/buildifier-5.1.0-linux-amd64 /usr/local/bin/buildifier sudo ln -s $(pwd)/buildozer-5.1.0-linux-amd64 /usr/local/bin/buildozer

Bazel 如何构建 C++ 项目，可见我的 Start Bazel 笔记。

安装 LLVM，以源码方式

Clang 有关 std::fromat 文本格式化的特性，默认未开启：

The paper is implemented but still marked as an incomplete feature (the feature-test macro is not set and the libary is only available when built with LIBCXX_ENABLE_INCOMPLETE_FEATURES). Not yet implemented LWG-issues will cause API and ABI breakage.

C++20 特性，编译器支持情况：

• C++ compiler support
• libc++ C++20 Status

因此，这里以源码方式安装 LLVM，需要构建 Clang & libc++：

• Building Clang
• Building libc++

git clone -b llvmorg-14.0.6 --depth 1 github.com/llvm/llvm-project.git cd llvm-project mkdir _build cd _build # llvm install path, such as /usr/local/llvm LLVM_PREFIX=$HOME/Apps/llvm-14.0.6 cmake -DCMAKE_BUILD_TYPE=Release \ -DCMAKE_INSTALL_PREFIX=$LLVM_PREFIX \ -DLLVM_ENABLE_PROJECTS=clang \ -DLLVM_ENABLE_RUNTIMES="libcxx;libcxxabi" \ -DLIBCXX_ENABLE_INCOMPLETE_FEATURES=ON \ ../llvm make -j`nproc` make install sudo ln -s $LLVM_PREFIX /usr/local/llvm cat <<-EOF >> ~/.bashrc # llvm export LLVM_HOME=/usr/local/llvm export PATH=\$LLVM_HOME/bin:\$PATH export LD_LIBRARY_PATH=\$LLVM_HOME/lib/x86_64-unknown-linux-gnu:\$LD_LIBRARY_PATH EOF llvm-config --version clang --version

LLVM_PREFIX 安装路径自己决定。最后，编译测试：

cat <<-EOF > hello.cc #include <format> #include <iostream> int main() { std::string message = std::format("The answer is {}.", 42); std::cout << message << std::endl; } EOF clang++ -std=c++20 -stdlib=libc++ hello.cc -o hello ./hello 安装 LLVM，以二进制方式

可省略该节。本文实践未用此方式，因为想开启更多 C++20 特性。这里仅作记录，有需要可参考。

方式 1. 安装二进制发布：

cd ~ wget github.com/llvm/llvm-project/releases/download/llvmorg-13.0.0/clang+llvm-13.0.0-x86_64-linux-gnu-ubuntu-20.04.tar.xz tar -xf clang+llvm-*.tar.xz sudo ln -s $(pwd)/clang+llvm-13.0.0-x86_64-linux-gnu-ubuntu-20.04 /usr/local/llvm cat <<-EOF >> ~/.bashrc # llvm export LLVM_HOME=/usr/local/llvm export PATH=\$LLVM_HOME/bin:\$PATH EOF llvm-config --version clang --version

方式 2. 用 apt 进行安装： apt.llvm.org/

方式 3. 用已配好的工具链： LLVM toolchain for Bazel

配置 Clang 工具链

本文依照 Bazel Tutorial: Configure C++ Toolchains 步骤配置的 Clang 工具链，最后项目根目录会多如下文件：

• WORKSPACE
• .bazelrc
• toolchain/BUILD
• toolchain/cc_toolchain_config.bzl

WORKSPACE 表示 Bazel 工作区，内容空。

.bazelrc 允许 --config=clang_config 启用 Clang 工具链：

# Use our custom-configured c++ toolchain. build:clang_config --crosstool_top=//toolchain:clang_suite # Use --cpu as a differentiator. build:clang_config --cpu=linux_x86_64 # Use the default Bazel C++ toolchain to build the tools used during the build. build:clang_config --host_crosstool_top=@bazel_tools//tools/cpp:toolchain

toolchain/BUILD 配置 Clang 工具链信息：

load(":cc_toolchain_config.bzl", "cc_toolchain_config") package(default_visibility = ["//visibility:public"]) #filegroup(name = "clang_suite") cc_toolchain_suite( name = "clang_suite", toolchains = { "linux_x86_64": ":linux_x86_64_toolchain", }, ) filegroup(name = "empty") cc_toolchain( name = "linux_x86_64_toolchain", toolchain_identifier = "linux_x86_64-toolchain", toolchain_config = ":linux_x86_64_toolchain_config", all_files = ":empty", compiler_files = ":empty", dwp_files = ":empty", linker_files = ":empty", objcopy_files = ":empty", strip_files = ":empty", supports_param_files = 0, ) #filegroup(name = "linux_x86_64_toolchain_config") cc_toolchain_config(name = "linux_x86_64_toolchain_config")

toolchain/cc_toolchain_config.bzl 配置 Clang 工具链规则：

# C++ Toolchain Configuration # bazel.build/docs/cc-toolchain-config-reference # github.com/bazelbuild/bazel/blob/master/tools/build_defs/cc/action_names.bzl load("@bazel_tools//tools/build_defs/cc:action_names.bzl", "ACTION_NAMES") load( "@bazel_tools//tools/cpp:cc_toolchain_config_lib.bzl", "feature", "flag_group", "flag_set", "tool_path", ) all_compile_actions = [ ACTION_NAMES.c_compile, ACTION_NAMES.cpp_compile, ACTION_NAMES.linkstamp_compile, ACTION_NAMES.assemble, ACTION_NAMES.preprocess_assemble, ACTION_NAMES.cpp_header_parsing, ACTION_NAMES.cpp_module_compile, ACTION_NAMES.cpp_module_codegen, ] all_link_actions = [ ACTION_NAMES.cpp_link_executable, ACTION_NAMES.cpp_link_dynamic_library, ACTION_NAMES.cpp_link_nodeps_dynamic_library, ] def _impl(ctx): llvm_version = "14.0.6" llvm_prefix = "/home/john/Apps/llvm-{}".format(llvm_version) llvm_bindir = llvm_prefix + "/bin" tool_paths = [ tool_path( name = "gcc", path = llvm_bindir + "/clang", ), tool_path( name = "ld", path = llvm_bindir + "/ld.lld", ), tool_path( name = "ar", path = llvm_bindir + "/llvm-ar", ), tool_path( name = "cpp", path = llvm_bindir + "/clang-cpp", ), tool_path( name = "gcov", path = llvm_bindir + "/llvm-cov", ), tool_path( name = "nm", path = llvm_bindir + "/llvm-nm", ), tool_path( name = "objdump", path = llvm_bindir + "/llvm-objdump", ), tool_path( name = "strip", path = llvm_bindir + "/llvm-strip", ), ] features = [ feature( name = "default_compiler_flags", enabled = True, flag_sets = [ flag_set( actions = all_compile_actions, flag_groups = ([ flag_group( flags = [ "-O2", "-DNDEBUG", "-Wall", "-Wextra", "-Wpedantic", "-fPIC", "-std=c++20", "-stdlib=libc++", ], ), ]), ), ], ), feature( name = "default_linker_flags", enabled = True, flag_sets = [ flag_set( actions = all_link_actions, flag_groups = ([ flag_group( flags = [ "-lc++", "-lc++abi", "-lm", "-ldl", "-lpthread", ], ), ]), ), ], ), ] return cc_common.create_cc_toolchain_config_info( ctx = ctx, features = features, cxx_builtin_include_directories = [ llvm_prefix + "/lib/clang/{}/include".format(llvm_version), llvm_prefix + "/include/x86_64-unknown-linux-gnu/c++/v1", llvm_prefix + "/include/c++/v1", "/usr/local/include", "/usr/include/x86_64-linux-gnu", "/usr/include", ], toolchain_identifier = "local", host_system_name = "local", target_system_name = "local", target_cpu = "linux_x86_64", target_libc = "unknown", compiler = "clang", abi_version = "unknown", abi_libc_version = "unknown", tool_paths = tool_paths, ) cc_toolchain_config = rule( implementation = _impl, attrs = {}, provides = [CcToolchainConfigInfo], )

llvm_prefix 给到自己的 LLVM 安装路径。

构建 C++20 库与应用

本文示例的 code/00/ 路径下准备了 C++20 的库与应用：

code/00/ ├── BUILD ├── greet │ ├── BUILD │ ├── greet.cc │ └── greet.h └── main.cc 编写 binary

main.cc:

#include <format> #include <iostream> #include <string> #include <string_view> #include "greet/greet.h" template <typename... Args> std::string dyna_print(std::string_view rt_fmt_str, Args&&... args) { return std::vformat(rt_fmt_str, std::make_format_args(args...)); } int main() { std::cout << greet::hello("world") << std::endl; std::string fmt; for (int i{}; i != 3; ++i) { fmt += "{} "; // constructs the formatting string std::cout << fmt << " : "; std::cout << dyna_print(fmt, "alpha", 'Z', 3.14, "unused"); std::cout << '\n'; } }

BUILD:

load("@rules_cc//cc:defs.bzl", "cc_binary") cc_binary( name = "main", srcs = ["main.cc"], deps = [ "//code/00/greet:greet", ], ) 编写 library

greet.h:

#pragma once #include <string> #include <string_view> namespace greet { std::string hello(std::string_view who); } // namespace greet

greet.cc:

#include "greet.h" #include <format> #include <utility> namespace greet { std::string hello(std::string_view who) { return std::format("Hello {}!", std::move(who)); } } // namespace greet

BUILD:

load("@rules_cc//cc:defs.bzl", "cc_library") package(default_visibility = ["//visibility:public"]) cc_library( name = "greet", srcs = [ "greet.cc", ], hdrs = [ "greet.h", ], ) Bazel 构建

bazel build --config=clang_config //code/00:main 运行测试

$ bazel-bin/code/00/main Hello world! {} : alpha {} {} : alpha Z {} {} {} : alpha Z 3.14 查看依赖

sudo apt update && sudo apt install graphviz xdot -y # view xdot <(bazel query --notool_deps --noimplicit_deps "deps(//code/00:main)" --output graph) # to svg dot -Tsvg <(bazel query --notool_deps --noimplicit_deps "deps(//code/00:main)" --output graph) -o 00_main.svg

更多参考

• Bazel Tutorial
  • Configure C++ Toolchains
  • Build a C++ Project
• Bazel Issue
  • Support C++20 modules #4005
• Project Example
  • How to Use C++20 Modules with Bazel and Clang
  • bazel-cpp20: Template for bazel with C++20
  • Clang toolchain

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