# Include What You Use #

[![IWYU CI](https://github.com/include-what-you-use/include-what-you-use/actions/workflows/ci.yml/badge.svg)](https://github.com/include-what-you-use/include-what-you-use/actions/workflows/ci.yml)

For more in-depth documentation, see [docs](docs).


## Instructions for users ##

"Include what you use" means this: for every symbol (type, function, variable,
or macro) that you use in `foo.cc` (or `foo.cpp`), either `foo.cc` or `foo.h`
should include a .h file that exports the declaration of that
symbol. (Similarly, for `foo_test.cc`, either `foo_test.cc` or `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.


### CAVEAT ###

This is experimental software, as of June 2024.  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)!


### Clang compatibility ###

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_<version>`, e.g. `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    |
|-------|--------------|----------------|
| 3.6   | 0.4          | `clang_3.6`    |
| 3.7   | 0.5          | `clang_3.7`    |
| 3.8   | 0.6          | `clang_3.8`    |
| 3.9   | 0.7          | `clang_3.9`    |
| 4.0   | 0.8          | `clang_4.0-r2` |
| 5.0   | 0.9          | `clang_5.0`    |
| 6     | 0.10         | `clang_6.0`    |
| 7     | 0.11         | `clang_7.0`    |
| 8     | 0.12         | `clang_8.0`    |
| 9     | 0.13         | `clang_9.0`    |
| 10    | 0.14         | `clang_10`     |
| 11    | 0.15         | `clang_11`     |
| 12    | 0.16         | `clang_12`     |
| 13    | 0.17         | `clang_13`     |
| 14    | 0.18         | `clang_14`     |
| 15    | 0.19         | `clang_15`     |
| 16    | 0.20         | `clang_16`     |
| 17    | 0.21         | `clang_17`     |
| 18    | 0.22         | `clang_18`     |
| 19    | 0.23         | `clang_19`     |
| 20    | 0.24         | `clang_20`     |
| 21    | 0.25         | `clang_21`     |
| 22    | 0.26         | `clang_22`     |
| ...   | ...          | ...            |
| main  |              | `master`       |

> NOTE: If you use the Debian/Ubuntu packaging available from
> <https://apt.llvm.org>, you'll need the following packages installed:
>
> * `llvm-<version>-dev`
> * `libclang-<version>-dev`
> * `clang-<version>`
>
> 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. `iwyu`

* 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 `clang_6.0` branch. IWYU `master` tracks LLVM & Clang
  `main`:
  ```
  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
  ```
  (substitute the `llvm-6.0` or `llvm-7` suffixes 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_PATH` for 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
  `build`, e.g.
  ```
  iwyu/build$ make
  ```

### 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_PROJECTS` and
`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 ###

To install and use a pre-built IWYU, besides any dynamic library dependencies,
you need to make sure it can find the Clang built-in headers (`stdarg.h` and
friends).

This is a surprisingly complex problem, so it helps to first understand how
Clang locates the built-in headers.

The built-in headers live in what Clang calls the _resource directory_, which
contains various runtime resources for the compiler. The resource dir is
configurable at Clang build time, using the `CLANG_RESOURCE_DIR` CMake
variable. `CLANG_RESOURCE_DIR` is always a relative path, so the effective
absolute path can be computed at runtime relative to the `clang` executable.

The Clang build sets up a resource dir in the build tree and copies the relevant
resources there (the built-in headers among them), so it's possible to run
`clang` directly from the build tree. Furthermore, the Clang install target will
copy the resource dir to the install tree.

The Clang `Driver` library is responsible for computing the effective path at
runtime. It will look up the current executable path (typically `clang`), strip
off the filename and append `CLANG_RESOURCE_DIR` to form the resource dir
path. You can use `clang -print-resource-dir` to print the effective resource
dir for a particular Clang tree.

Phew! What does this mean for IWYU?

IWYU links to the Clang `Driver` library, and so would nominally get the exact
same policy by default: `CLANG_RESOURCE_DIR` relative to the
`include-what-you-use` executable. This means the IWYU build would have to
create the resource dir in its build tree, and also make sure it's available in
the install tree, using a custom install target. But `CLANG_RESOURCE_DIR` is not
exported from the Clang CMake system, so it's not possible to know at build-time
where the resources need to be.

Since Clang has all the knowledge about which resources need to go into the
resource dir, and also decides under the covers where it has to be, it's
difficult for IWYU to make any principled decisions. We side-step this conflict
by exposing our own set of CMake variables for the resource dir:

* `IWYU_RESOURCE_DIR`: same semantics as Clang's `CLANG_RESOURCE_DIR`
* `IWYU_RESOURCE_RELATIVE_TO`: which executable to serve as the anchor path for
  the resource directory (`clang` or `iwyu`)

First, `IWYU_RESOURCE_DIR` exists to supplement `CLANG_RESOURCE_DIR`. Packagers
for a platform where Clang has a custom `CLANG_RESOURCE_DIR` can repeat the same
customization for IWYU. By default it will use the same default pattern as
Clang, i.e. `../lib/clang/<clang-version>`.

Second, we can use `IWYU_RESOURCE_RELATIVE_TO` to decide which executable path
to use as the anchor for the relative `IWYU_RESOURCE_DIR`.

If it is `clang`, we resolve the path to the `clang` executable _at configure-
time_, and bake that absolute path into `include-what-you-use`.

If it is `iwyu`, the `include-what-you-use` executable resolves its own path _at
runtime_.

That means packagers can easily build:

* an `include-what-you-use` that has a package dependency on Clang, and relies
  entirely on its resource dir (`-DIWYU_RESOURCE_RELATIVE_TO=clang`)
* an `include-what-you-use` that has a package dependency on a Clang with a
  custom resource dir (`-DIWYU_RESOURCE_RELATIVE_TO=clang
  -DIWYU_RESOURCE_DIR=../what/clang/said`)
* an `include-what-you-use` that can be installed separate from Clang in its own
  prefix (`-DIWYU_RESOURCE_RELATIVE_TO=iwyu -DCMAKE_INSTALL_PREFIX=/usr/local`),
  assuming a custom install step to also copy the built-in headers to the
  default `IWYU_RESOURCE_DIR` in the same prefix.
* an `include-what-you-use` that can be installed separate from Clang in an
  arbitrary prefix with a custom resource dir (`-DIWYU_RESOURCE_RELATIVE_TO=iwyu
  -DIWYU_RESOURCE_DIR=../share/include-what-you-use`), assuming a custom install
  step to also copy the built-in headers to the custom `IWYU_RESOURCE_DIR`.

IWYU uses `IWYU_RESOURCE_RELATIVE_TO=clang` by default, because that produces a
runnable `include-what-you-use` in the build tree, which depends directly on the
Clang package it was configured for with `-DCMAKE_PREFIX_PATH`. It's also
suitable for packaging, in the sense that the IWYU package can be made to depend
on the Clang package, and will then automatically use the Clang resource dir on
the target system.

`IWYU_RESOURCE_RELATIVE_TO=iwyu` is more suitable to build a fully independent
IWYU package, but also requires some custom logic outside the IWYU build to
package and install relevant parts of the resource dir from Clang in a suitable
location.

Use `include-what-you-use -print-resource-dir` to learn exactly where IWYU
expects the resource dir to be installed.


### 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 $CXXFLAGS myfile.cc
```
where `$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"
```
(The additional `-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 `-k`.)

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](https://cmake.org/cmake/help/latest/prop_tgt/LANG_INCLUDE_WHAT_YOU_USE.html)
for CMake-side details.

The `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 ####

The `iwyu_tool.py` script pre-dates the native CMake support, and works off the
[compilation database
format](https://clang.llvm.org/docs/JSONCompilationDatabase.html). 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 ...
python3 iwyu_tool.py -p .
```

Unless a source filename is provided, all files in the project will be analyzed.

See `iwyu_tool.py --help` for more options.


#### Applying fixes ####

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 python3):
```
make -k CXX=include-what-you-use CXXFLAGS="-Xiwyu --error_always" 2> /tmp/iwyu.out
python3 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 ###

* If `fix_includes.py` has removed an `#include` you actually need, add it back
  in with the comment '`// IWYU pragma: keep`' at the end of the `#include`
  line.  Note that the comment is case-sensitive.
* If `fix_includes.py` has added an `#include` you don't need, just take it out.
  We hope to come up with a more permanent way of fixing later.
* If `fix_includes.py` has wrongly added or removed a forward-declare, just fix
  it up manually.
* If `fix_includes.py` has 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](docs/IWYUPragmas.md).


## More questions? ##

See our [FAQ](./docs/IWYUFAQ.md) for longer-form Q&A.
