Include What You Use
For more in-depth documentation, see docs.
Instructions for users
"Include what you use" means this: for every symbol (type, function, variable, or macro) that you use in
foo.h should include a .h file that exports the declaration of that symbol. (Similarly, for
foo.h should do the including.) Obviously symbols defined in
foo.cc itself are excluded from this requirement.
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.
This is alpha quality software -- at best (as of July 2018). It was originally written to work specifically in the Google source tree, and may make assumptions, or have gaps, that are immediately and embarrassingly evident in other types of code.
While we work to get IWYU quality up, we will be stinting new features, and will prioritize reported bugs along with the many existing, known bugs. The best chance of getting a problem fixed is to submit a patch that fixes it (along with a test case that verifies the fix)!
Include-what-you-use makes heavy use of Clang internals, and will occasionally break when Clang is updated. We build IWYU regularly against Clang mainline to detect and fix such compatibility breaks as soon as possible.
NOTE: the IWYU master branch follows Clang main branch.
We also have convenience tags and branches for released versions of Clang (called
clang_5.0). To build against a Clang release, check out the corresponding branch in IWYU before configuring the build. You can use this mapping table to combine Clang and IWYU versions correctly:
|Clang||IWYU version||IWYU branch|
NOTE: If you use the Debian/Ubuntu packaging available from https://apt.llvm.org, you'll need the following packages installed:
Packaging for other platforms will likely be subtly different.
How to build standalone
This build mode assumes you already have compiled LLVM and Clang libraries on your system, either via packages for your platform or built from source. To set up an environment for building IWYU:
Create a directory for IWYU development, e.g.
Clone the IWYU Git repo:
iwyu$ git clone https://github.com/include-what-you-use/include-what-you-use.git
Presumably, you'll be building IWYU with a released version of LLVM and Clang, so check out the corresponding branch. For example, if you have Clang 6.0 installed, use the
mastertracks LLVM & Clang
iwyu$ cd include-what-you-use iwyu/include-what-you-use$ git checkout clang_6.0
Create a build root and use CMake to generate a build system linked with LLVM/Clang prebuilts:
# This example uses the Makefile generator, but anything should work. iwyu/include-what-you-use$ cd .. iwyu$ mkdir build && cd build # For IWYU 0.10/Clang 6 and earlier iwyu/build$ cmake -G "Unix Makefiles" -DIWYU_LLVM_ROOT_PATH=/usr/lib/llvm-6.0 ../include-what-you-use # For IWYU 0.11/Clang 7 and later iwyu/build$ cmake -G "Unix Makefiles" -DCMAKE_PREFIX_PATH=/usr/lib/llvm-7 ../include-what-you-use
llvm-7suffixes with the actual version compatible with your IWYU branch)
or, if you have a local LLVM and Clang build tree, you can specify that as
CMAKE_PREFIX_PATHfor IWYU 0.11 and later:
iwyu/build$ cmake -G "Unix Makefiles" -DCMAKE_PREFIX_PATH=~/llvm-project/build ../include-what-you-use
Once CMake has generated a build system, you can invoke it directly from
How to build as part of LLVM
Instructions for building LLVM and Clang are available at https://clang.llvm.org/get_started.html.
To include IWYU in the LLVM build, use the
LLVM_EXTERNAL_*_SOURCE_DIR CMake variables when configuring LLVM:
llvm-project/build$ cmake -G "Unix Makefiles" -DLLVM_ENABLE_PROJECTS=clang -DLLVM_EXTERNAL_PROJECTS=iwyu -DLLVM_EXTERNAL_IWYU_SOURCE_DIR=/path/to/iwyu /path/to/llvm-project/llvm llvm-project/build$ make
This builds all of LLVM, Clang and IWYU in a single tree.
How to install
If you're building IWYU out-of-tree or installing pre-built binaries, you need to make sure it can find Clang built-in headers (
stdarg.h and friends.)
Clang's default policy is to look in
path/to/clang-executable/../lib/clang/<clang ver>/include. So if Clang 3.5.0 is installed in
/usr/bin, it will search for built-ins in
Clang tools have the same policy by default, so in order for IWYU to analyze any non-trivial code, it needs to find Clang's built-ins in
3.5.0 is a stand-in for the version of Clang your IWYU was built against.
Note that some distributions/packages may have different defaults, you can use
clang -print-resource-dir to find the base path of the built-in headers on your system.
So for IWYU to function correctly, you need to copy the Clang
include directory to the expected location before running (similarly, use
include-what-you-use -print-resource-dir to learn exactly where IWYU wants the headers).
This weirdness is tracked in issue 100, hopefully we can make this more transparent over time.
How to run
The original design was built for Make, but a number of alternative run modes have come up over the years.
Running on single source file
The simplest way to use IWYU is to run it against a single source file:
include-what-you-use $CXXFLAGS myfile.cc
$CXXFLAGS are the flags you would normally pass to the compiler.
Plugging into existing build system
Typically there is already a build system containing the relevant compiler flags for all source files. Replace your compiler with
include-what-you-use to generate a large batch of IWYU advice. Depending on your build system/build tools, this can take many forms, but for a simple GNU Make system it might look like this:
make -k CXX=include-what-you-use CXXFLAGS="-Xiwyu --error_always"
-Xiwyu --error_always switch makes
include-what-you-use always exit with an error code, so the build system knows it didn't build a .o file. Hence the need for
In this mode
include-what-you-use only analyzes the .cc (or .cpp) files known to your build system, along with their corresponding .h files. If your project has a .h file with no corresponding .cc file, IWYU will ignore it unless you use the
--check_also switch to add it for analysis together with a .cc file. It is possible to run IWYU against individual header files, provided the compiler flags are carefully constructed to match all includers.
Using with CMake
CMake has grown native support for IWYU as of version 3.3. See their documentation for CMake-side details.
CMAKE_CXX_INCLUDE_WHAT_YOU_USE option enables a mode where CMake first compiles a source file, and then runs IWYU on it.
Use it like this:
mkdir build && cd build CC="clang" CXX="clang++" cmake -DCMAKE_CXX_INCLUDE_WHAT_YOU_USE=include-what-you-use ...
or, on Windows systems:
mkdir build && cd build cmake -DCMAKE_CXX_COMPILER="%VCINSTALLDIR%/bin/cl.exe" -DCMAKE_CXX_INCLUDE_WHAT_YOU_USE=include-what-you-use -G Ninja ...
These examples assume that
include-what-you-use is in the
PATH. If it isn't, consider changing the value to an absolute path. Arguments to IWYU can be added using CMake's semicolon-separated list syntax, e.g.:
... cmake -DCMAKE_CXX_INCLUDE_WHAT_YOU_USE="include-what-you-use;-w;-Xiwyu;--verbose=7" ...
The option appears to be separately supported for both C and C++, so use
CMAKE_C_INCLUDE_WHAT_YOU_USE for C code.
Note that with Microsoft's Visual C++ compiler, IWYU needs the
--driver-mode=cl argument to understand the MSVC options from CMake.
Using with a compilation database
iwyu_tool.py script pre-dates the native CMake support, and works off the compilation database format. For example, CMake generates such a database named
compile_commands.json with the
CMAKE_EXPORT_COMPILE_COMMANDS option enabled.
The script's command-line syntax is designed to mimic Clang's LibTooling, but they are otherwise unrelated. It can be used like this:
mkdir build && cd build CC="clang" CXX="clang++" cmake -DCMAKE_EXPORT_COMPILE_COMMANDS=ON ... iwyu_tool.py -p .
or, on Windows systems:
mkdir build && cd build cmake -DCMAKE_CXX_COMPILER="%VCINSTALLDIR%/bin/cl.exe" -DCMAKE_C_COMPILER="%VCINSTALLDIR%/VC/bin/cl.exe" -DCMAKE_EXPORT_COMPILE_COMMANDS=ON -G Ninja ... python iwyu_tool.py -p .
Unless a source filename is provided, all files in the project will be analyzed.
iwyu_tool.py --help for more options.
We also include a tool that automatically fixes up your source files based on the IWYU recommendations. This is also alpha-quality software! Here's how to use it (requires python):
make -k CXX=include-what-you-use CXXFLAGS="-Xiwyu --error_always" 2> /tmp/iwyu.out python fix_includes.py < /tmp/iwyu.out
If you don't like the way
fix_includes.py munges your
#include lines, you can control its behavior via flags.
fix_includes.py --help will give a full list, but these are some common ones:
-b: Put blank lines between system and Google includes
--nocomments: Don't add the 'why' comments next to includes
How to correct IWYU mistakes
fix_includes.pyhas removed an
#includeyou actually need, add it back in with the comment '
// IWYU pragma: keep' at the end of the
#includeline. Note that the comment is case-sensitive.
fix_includes.pyhas added an
#includeyou don't need, just take it out. We hope to come up with a more permanent way of fixing later.
fix_includes.pyhas wrongly added or removed a forward-declare, just fix it up manually.
fix_includes.pyhas suggested a private header file (such as
<bits/stl_vector.h>) instead of the proper public header file (
<vector>), you can fix this by inserting a specially crafted comment near top of the private file (assuming you can write to it): '
// IWYU pragma: private, include "the/public/file.h"'.
Current IWYU pragmas are described in IWYUPragmas.