Monthly Archives: April 2021

The tradeoffs in using -Weverything

In addition to well-known -Wall, clang offers another interesting warning flag: -Weverything. While it sounds like a “strictly better” option (the more warnings, the better?) it’s not.

This topic is already well covered by Arthur O’Dwyer’s blog post and the clang documentation.

Arthur makes a strong case against use of this flag. While the clang docs generally agree, though offering a slightly more lenient position:

If you do use -Weverything then we advise that you address all new compiler diagnostics as they get added to Clang, either by fixing everything they find or explicitly disabling that diagnostic with its corresponding Wno- option.

What I have not seen clearly expressed is how use of -Weverything is a tradeoff.

The downsides are:

  • Increased flag noise from having manually resolve contradicting flags (using  -Wno-c++98-compat and -Wno-missing-prototypes)
  • Friction in upgrading compilers. Upgrading may “break” your existing warning-clean code, because of new warnings that have been added. This means you need to resolve all of those issues (via fixing code, or adding new flags to disable warnings) before officially supporting the toolchain.
  • Exposure to “experimental” diagnostics that may be “lower quality”.

However, there are advantages:

  • Discovering useful new warnings that you may never have used otherwise
  • Finding and fixing bugs during compiler upgrades as a result of these new warnings

Whether you are working on a library or application will also affect your decision.

If you maintain a library (especially an open source one), it makes sense to avoid -Weverything. You have less control over which compiler versions your users use, and you want to avoid users encountering new warnings if they use a new compiler release to build the library. (Assuming you keep a warning-clean codebase).

However, if you are simply developing an application (especially a closed source one), it may make sense to use -Weverything. In this environment, you have complete control over the approved toolchain that can be used to build the project, and can have a transition period where new warnings are addressed. There will be more friction for toolchain upgrades, but you may find that the benefits in continually using the newest warnings outweigh that downside.

Thanks to Ryan Brown for teaching me this.

Surgical formatting with git-clang-format

If you’re already a 10x engineer, you probably won’t need this article. But for the rest of us, this is what I wish I knew about clang-format as an inexperienced C++ programmer: how to only format the changes in your pull request.

You may have already heard of clang-format. It auto-formats source files for languages including C and C++. You can aim it at a source file and format the entire thing using clang-format -i file.cpp.

If you’re contributing to a project that is already 100% clang-format clean, then this workflow works fine. But you’ll occasionally encounter a project that is not quite 100% formatted, such as LLVM, osquery, or Electron1.

For these projects, the “format entire files” workflow doesn’t work because you’ll incidentally format parts of the files that are unrelated to your contribution. This will add noise to your diff and make it harder for your reviewers.

In this case, you need a way to surgically format only the lines changed in your contribution. To do this, you can use the clang-format git extension. This article will cover the basics of git-clang-format, including a practical workflow that allows for messy development, and formatting at the end.

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