# CCache and Utilities

Over the versions, common utilities that help you write good code have had support added to CMake. This is usually in the form of a property and matching `CMAKE_*` initialization variable. The feature is not meant to be tied to one special program, but rather any program that is somewhat similar in behavior.

All of these take `;` separated values (a standard list in CMake) that describe the program and options that you should run on the source files of this target.

## CCache

Set the `CMAKE_<LANG>_COMPILER_LAUNCHER` variable or the `<LANG>_COMPILER_LAUNCHER` property on a target to use something like CCache to "wrap" the compilation of the target. Support for CCache has been expanding in the latest versions of CMake. In practice, this tends to look like this:

```cmake
find_program(CCACHE_PROGRAM ccache)
if(CCACHE_PROGRAM)
    set(CMAKE_CXX_COMPILER_LAUNCHER "${CCACHE_PROGRAM}")
    set(CMAKE_CUDA_COMPILER_LAUNCHER "${CCACHE_PROGRAM}") # CMake 3.9+
endif()
```

## Utilities

Set the following properties or `CMAKE_*` initializer variables to the command line for the tools. Most of them are limited to C or CXX with make or ninja generators.

- `<LANG>_CLANG_TIDY`: CMake 3.6+
- `<LANG>_CPPCHECK`
- `<LANG>_CPPLINT`
- `<LANG>_INCLUDE_WHAT_YOU_USE`

## Clang tidy

This is the command line for running clang-tidy, as a list (remember, a semicolon separated string is a list).

Here is a simple example of using Clang-Tidy:

```term
~/package # cmake -S . -B build-tidy -DCMAKE_CXX_CLANG_TIDY="$(which clang-tidy);-fix"
~/package # cmake --build build -j 1
```

The `-fix` part is optional, and will modify your source files to try to fix
the tidy warning issued. If you are working in a git repository, this is fairly
safe as you can see what has changed. However, make sure you **do not run your
makefile/ninja build in parallel**! This will not work very well at all if it
tries to fix the same header twice.

If you want to explicitly use the target form to ensure you only call this on
your local targets, you can set a variable (maybe something like
`DO_CLANG_TIDY`) instead of the `CMAKE_CXX_CLANG_TIDY` variable, then add it to
your target properties as you create them. You can find clang-tidy in your path
like this:

```cmake
find_program(
    CLANG_TIDY_EXE
    NAMES "clang-tidy"
    DOC "Path to clang-tidy executable"
)
```

## Include what you use

This is an example for using include what you use. First, you'll need to have
the tool, such as in a docker container or with brew (macOS) with `brew install include-what-you-use`. Then, you can pass this into your build without
modifying the source:

```term
~/package # cmake -S . -B build-iwyu -DCMAKE_CXX_INCLUDE_WHAT_YOU_USE=include-what-you-use
```

Finally, you can collect the output and (optionally) apply the fixes:

```term
~/package # cmake --build build-iwyu 2> iwyu.out
~/package # fix_includes.py < iwyu.out
```

(You should check the fixes first, or touch them up after applying!)

## Link what you use

There is a boolean target property, `LINK_WHAT_YOU_USE`, that will check for extraneous files when linking.

## Clang-format

Clang-format doesn't really have an integration with CMake, unfortunately. You could make a custom target (See [this post](https://arcanis.me/en/2015/10/17/cppcheck-and-clang-format)), or you can run it manually. An interesting project that I have not really tried is [here](https://github.com/kbenzie/git-cmake-format); it adds a format target and even makes sure that you can't commit unformatted files.

The following two line would do that in a git repository in bash (assuming you have a `.clang-format` file):

```term
gitbook $ git ls-files -- '*.cpp' '*.h' | xargs clang-format -i -style=file
gitbook $ git diff --exit-code --color
```