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Describe a better approach of how to use {fmt} as a depency in a Bazel project (#2516)
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# Bazel support
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Bazel is an open-source build tool.
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More information about Bazel can be found [here](https://bazel.build/).
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To get [Bazel](https://bazel.build/) working with {fmt} you can copy the files `BUILD.bazel`, `WORKSPACE.bazel`, `.bazelrc`, and `.bazelversion` from this folder (`support/bazel`) to the root folder of this project. This way {fmt} gets bazelized and can be used with Bazel (e.g. doing a `bazel build //...` on {fmt}).
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## Using the fmt repository with Bazel
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## Using {fmt} as a dependency
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Even though the {fmt} repository does not contain a `WORKSPACE` file in its root directory,
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there is an easy approach to use the {fmt} repository with Bazel out of the box.
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This is demonstrated in the following example.
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The following minimal example shows how to use {fmt} as a dependency within a Bazel project.
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Add to your `WORKSPACE` file:
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The following file structure is assumed:
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```python
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load("@bazel_tools//tools/build_defs/repo:git.bzl", "new_git_repository")
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# Fetch all files from fmt including the BUILD file `support/bazel/BUILD.bazel`
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new_git_repository(
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name = "fmt_workaround",
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branch = "master",
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remote = "https://github.com/fmtlib/fmt/",
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build_file_content = "# Empty build file on purpose"
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)
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# Now the BUILD file `support/bazel/BUILD.bazel` can be used:
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new_git_repository(
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name = "fmt",
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branch = "master",
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remote = "https://github.com/fmtlib/fmt/",
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build_file = "@fmt_workaround//:support/bazel/BUILD.bazel"
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)
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```
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example
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├── BUILD.bazel
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├── main.cpp
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└── WORKSPACE.bazel
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```
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Create a `BUILD.bazel` file and add a dependency to {fmt}:
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*main.cpp*:
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```python
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cc_binary( # Build a binary
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name = "Demo", # Name of the binary
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srcs = ["main.cpp"], # List of files - we only have main.cpp
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deps = ["@fmt//:fmt"], # Depend on fmt
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)
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```
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Make use of {fmt} in `main.cpp`:
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```C++
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```c++
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#include "fmt/core.h"
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int main() {
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fmt::print("The answer is {}.\n", 42);
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fmt::print("The answer is {}\n", 42);
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}
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```
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The expected output of this example is `The answer is 42`.
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## Bazelize fmt
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First downloading a build file and then making use of it can be considered as a bit unclean, nevertheless, it works.
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A cleaner Bazel solution would be to move the `WORKSPACE` and `BUILD` files to the root folder of the {fmt} Git repository.
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In favor of keeping the {fmt} project directory clean, those files were not added to the project root directory.
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If you do not like this, you can fork this repository and move the files `BUILD.bazel`, `WORKSPACE.bazel`, `.bazelrc`, and `.bazelversion` from this folder (`support/bazel`) to the root folder of this project.
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This way {fmt} gets bazelized and can be used in your Bazel builds.
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**Example**
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Create a `WORKSPACE.bazel` file with the following content:
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*WORKSPACE.bazel*:
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```python
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load("@bazel_tools//tools/build_defs/repo:git.bzl", "git_repository")
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# Fetch bazelized fmt
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git_repository(
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name = "fmt",
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branch = "bazel-support", # A copy of master where BUILD.bazel, WORKSPACE.bazel, .bazelrc and .bazelversion are moved to root
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remote = "https://github.com/<user_or_organisation>/fmt", # replace <user_or_organisation> by a valid account
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branch = "master",
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remote = "https://github.com/fmtlib/fmt",
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patch_cmds = [
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"mv support/bazel/.bazelrc .bazelrc",
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"mv support/bazel/.bazelversion .bazelversion",
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"mv support/bazel/BUILD.bazel BUILD.bazel",
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"mv support/bazel/WORKSPACE.bazel WORKSPACE.bazel",
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],
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# Windows-related patch commands are only needed in the case MSYS2 is not installed.
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# More details about the installation process of MSYS2 on Windows systems can be found here:
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# https://docs.bazel.build/versions/main/install-windows.html#installing-compilers-and-language-runtimes
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# Even if MSYS2 is installed the Windows related patch commands can still be used.
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patch_cmds_win = [
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"Move-Item -Path support/bazel/.bazelrc -Destination .bazelrc",
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"Move-Item -Path support/bazel/.bazelversion -Destination .bazelversion",
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"Move-Item -Path support/bazel/BUILD.bazel -Destination BUILD.bazel",
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"Move-Item -Path support/bazel/WORKSPACE.bazel -Destination WORKSPACE.bazel",
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],
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)
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```
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Create a `BUILD.bazel` file and add a dependency to {fmt} (same as above).
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In the *WORKSPACE* file, the {fmt} GitHub repository is fetched. Using the attribute `patch_cmds` the files `BUILD.bazel`, `WORKSPACE.bazel`, `.bazelrc`, and `.bazelversion` are moved to the root of the {fmt} repository. This way the {fmt} repository is recognized as a bazelized workspace.
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*BUILD.bazel*:
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```python
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cc_binary(
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name = "Demo",
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srcs = ["main.cpp"],
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deps = ["@fmt"],
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)
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```
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The *BUILD* file defines a binary named `Demo` that has a dependency to {fmt}.
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To execute the binary you can run `bazel run //:Demo`.
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Make use of {fmt} in `main.cpp` (same as above).
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