Build a simple Bazel project using Conan¶

In this example, we are going to create a Hello World program that uses one of the most popular C++ libraries: fmt.

Note

This example is based on the main Build a simple CMake project using Conan tutorial. So we highly recommend reading it before trying out this one.

We’ll use Bazel as the build system and helper tool in this case, so you should get it installed before going forward with this example. See how to install Bazel.

Please, first clone the sources to recreate this project. You can find them in the examples2 repository in GitHub:

$ git clone https://github.com/conan-io/examples2.git
$ cd examples2/examples/tools/google/bazeltoolchain/string_formatter

We start from a very simple C++ language project with this structure:

.
├── WORKSPACE
├── conanfile.txt
└── main
    ├── BUILD
    └── demo.cpp

This project contains a WORKSPACE file loading the Conan dependencies (in this case only fmt) and a main/BUILD file which defines the demo bazel target and it’s in charge of using fmt to build a simple Hello World program.

Let’s have a look at each file’s content:

main/demo.cpp¶

#include <cstdlib>
#include <fmt/core.h>

int main() {
    fmt::print("{} - The C++ Package Manager!\n", "Conan");
    return EXIT_SUCCESS;
}

WORKSPACE¶

load("@//conan:dependencies.bzl", "load_conan_dependencies")
load_conan_dependencies()

main/BUILD¶

load("@rules_cc//cc:defs.bzl", "cc_binary")

cc_binary(
    name = "demo",
    srcs = ["demo.cpp"],
    deps = [
        "@fmt//:fmt"
    ],
)

conanfile.txt¶

[requires]
fmt/10.1.1

[generators]
BazelDeps
BazelToolchain

[layout]
bazel_layout

Conan uses the BazelToolchain to generate a conan_bzl.rc file which defines the conan-config bazel-build configuration. This file and the configuration are passed as parameters to the bazel build command. Apart from that, Conan uses the BazelDeps generator to create all the bazel files ([DEP]/BUILD.bazel and dependencies.bzl) which define all the dependencies as public bazel targets. The WORKSPACE above is already ready to load the dependencies.bzl which will tell the main/BUILD all the information about the @fmt//:fmt bazel target.

As the first step, we should install all the dependencies listed in the conanfile.txt. The command conan install does not only install the fmt package, it also builds it from sources in case your profile does not match with a pre-built binary in your remotes. Furthermore, it will save all the files created by the generators listed in the conanfile.txt in a folder named conan/ (default folder defined by the bazel_layout).

$ conan install . --build=missing
# ...
======== Finalizing install (deploy, generators) ========
conanfile.txt: Writing generators to /Users/franchuti/develop/examples2/examples/tools/google/bazeltoolchain/string_formatter/conan
conanfile.txt: Generator 'BazelDeps' calling 'generate()'
conanfile.txt: Generator 'BazelToolchain' calling 'generate()'
conanfile.txt: Generating aggregated env files
conanfile.txt: Generated aggregated env files: ['conanbuild.sh', 'conanrun.sh']
Install finished successfully

Now we are ready to build and run our application:

$ bazel --bazelrc=./conan/conan_bzl.rc build --config=conan-config //main:demo
Starting local Bazel server and connecting to it...
INFO: Analyzed target //main:demo (38 packages loaded, 272 targets configured).
INFO: Found 1 target...
INFO: From Linking main/demo:
ld: warning: ignoring duplicate libraries: '-lc++'
Target //main:demo up-to-date:
  bazel-bin/main/demo
INFO: Elapsed time: 60.180s, Critical Path: 7.68s
INFO: 6 processes: 4 internal, 2 darwin-sandbox.
INFO: Build completed successfully, 6 total actions

$ ./bazel-bin/main/demo
Conan - The C++ Package Manager!