"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.
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.
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:
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:
* 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`:
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 `/usr/lib/clang/3.5.0/include`.
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 `path/to/iwyu/../lib/clang/3.5.0/include` where `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](https://github.com/include-what-you-use/include-what-you-use/issues/100), hopefully we can make this more transparent over time.
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.
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.
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.:
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.
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 python):
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:
* 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"`'.
