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  1. Darwin.imp

IWYU in Quickstep

Why use include-what-you-use (or IWYU)?

“Include what you use” means for every symbol (type, function, variable, or macro) that you use in foo.cpp, it should #include a .h file that exports the declaration of that symbol. This puts us in a state where every file includes the headers it needs to declare the symbols that it uses. When every file includes what it uses, then it is possible to edit any file and remove unused headers, without fear of accidentally breaking the upwards dependencies of that file. It also becomes easy to automatically track and update dependencies in the source code.

Additonally, IWYU suggests forward declaration wherever feasible, which in turn reduces code size and compile time.

See this page for more details.

Installing IWYU

Gentoo Linux

IWYU is available in portage at dev-util/include-what-you-use, but it may require keywording with ~amd64 (or equivalent for another architecture) to be installed. Note that IWYU version numbers in portage correspond to the LLVM/clang version they require. If you do not also have ~ keyworded LLVM and Clang, then you should keyword the specific version of IWYU that is compatible with your installed LLVM.

Edit /etc/portage/package.keywords (or, if you manage that as a directory, a sensibly-named file underneath it) if necessary, adding a line like:

dev-util/include-what-you-use ~amd64

Or, if you need to keyword a specific version to match your LLVM libraries, something like:

=dev-util/include-what-you-use-3.5 ~amd64

Then just emerge the package:

emerge --ask dev-util/include-what-you-use


If you are using Homebrew to manage packages on your Mac, the easiest way to install is by using using the recipe at jasonmp85/homebrew-iwyu

brew tap jasonmp85/homebrew-iwyu
brew install iwyu

If you're not using homebrew, there is a pre-built IWYU binary for Mac available at the project download page.

Other Operating Systems

There are prebuilt IWYU binaries available on the project download page for Ubuntu Linux, Mac OS X, and FreeBSD (all on x86-64) and Win32. If your OS/Arch doesn't have prebuilt binaries available, you will have to download the source from that page and compile it against LLVM yourself.

Running IWYU

We include a helper script (third_party/iwyu/ to simplify using IWYU in our code base in two ways:

  • Helper script understands the clang arguments (like defines) and user includes for the quickstep project, and invokes IWYU with correct arguments.
  • IWYU uses libclang to find all symbols in the given header or source file. However, libclang is quirky with respect to including the correct system include directories.

The following command will try to figure out the missing system include directories, construct the appropriate IWYU command to run, and invoke IWYU.

./third_party/iwyu/ <path-to-hpp-or-cpp-file(s)>

The helper script lets you provides additional overrides and specify new overrides. See next section for more details.

Custom IWYU configuration


IWYU is not perfect, and it has support for defining a mapping file to specify overrides for incorrectly deduced header files where symbols are defined. See the section named IWYU Mappings in IWYU README for more details about the format of IWYU mappings.

The helper script picks up .iwyu.imp in the quickstep root directory (if present), and uses the mappings in the file.

The helper also lets you set a custom configuration via a python source file. The python source is supposed to define a dictionary named CONFIG with optional entries for:

  • system includes (system-includes),
  • user includes (user-includes),
  • IWYU mappings files (mappings),
  • custom clang arguments (args),
  • option to ignore default configuration (ignore-defaults).

Sample configuration:

  'system-includes': ['/a', '/b'],
  'user-includes': ['./include', './build/include'],
  'mappings': ['sample.imp'],
  'args': ['-std=c++11', '-x', 'c++'],
  'ignore-defaults': True,

See third_party/iwyu/ for more detailed sample configuration.

Default IWYU configuration

The helper script uses the following default configuration:

  • system-includes: Runs clang ... -v /dev/null and iwyu ... -v /dev/null and figures out the missing system include directories in IWYU.
  • user-includes: Uses standard user include directories in the quickstep project. For example, ., ./build, ./third_party/glog/src, etc. See the helper script for more details.
  • args: Standard Clang args to compile C++ and a few Quickstep specific hash-defines.
  • mappings: Uses third_party/iwyu/{PLATFORM}.imp if present.