ROOT is a C++ Toolkit for High Energy Physics. It is huge. There are really a lot of ways to use it in CMake, though many/most of the examples you'll find are probably wrong. Here's my recommendation.
Most importantly, there are _lots of improvements_ in CMake support in more recent versions of ROOT - Using 6.16+ is much, much easier! If you really must support 6.14 or earlier, see the section at the end. There were further improvements in 6.20, as well, it behaves much more like a proper CMake project, and exports C++ standard features for targets, etc.
to attempt to find ROOT. If you don't have your paths set up, you can pass `-DROOT_DIR=$ROOTSYS/cmake` to find ROOT. (But, really, you should source `thisroot.sh`).
ROOT 6.12 and earlier do not add the include directory for imported targets. ROOT 6.14+ has corrected this error, and required target properties have been getting better. This method is rapidly becoming easier to use (see the example at the end of this page for the older ROOT details).
ROOT [provides a utility](https://root.cern.ch/how/integrate-root-my-project-cmake) to set up a ROOT project, which you can activate using `include("${ROOT_USE_FILE}")`. This will automatically make ugly directory level and global variables for you. It will save you a little time setting up, and will waste massive amounts of time later if you try to do anything tricky. As long as you aren't making a library, it's probably fine for simple scripts. Includes and flags are set globally, but you'll still need to link to `${ROOT_LIBRARIES}` yourself, along with possibly `ROOT_EXE_LINKER_FLAGS` (You will have to `separate_arguments` first before linking or you will get an error if there are multiple flags, like on macOS). Also, before 6.16, you have to manually fix a bug in the spacing.
Find ROOT allows you to specify components. It will add anything you list to `${ROOT_LIBRARIES}`, so you might want to build your own target using that to avoid listing the components twice. This did not solve dependencies; it was an error to list `RooFit` but not `RooFitCore`. If you link to `ROOT::RooFit` instead of `${ROOT_LIBRARIES}`, then `RooFitCore` is not required.
Dictionary generation is ROOT's way of working around the missing reflection feature in C++. It allows ROOT to learn the details of your class so it can save it, show methods in the Cling interpreter, etc. Your source code will need the following modifications to support dictionary generation:
ROOT provides `rootcling` and `genreflex` (a legacy interface to `rootcling`) binaries which produce the source files required to build the dictionary. It also defines `root_generate_dictionary`, a CMake function to invoke `rootcling` during the build process.
The `if(...)` condition is added to fix a bug in the NewMacros file that causes dictionary generation to fail if there is not at least one global include directory or a `inc` folder. Here I'm including a non-existent directory just to make it work. There is no `ROOT_NONEXISTENT_DIRECTORY_HACK` directory.
`rootcling` uses a special header file with a [specific formula][linkdef-root] to determine which parts to generate dictionaries for. The name of this file may have any prefix, but **must** end with `LinkDef.h`. Once you have written this header file, the dictionary generation function can be invoked.
Sometimes, you might want to ask ROOT to generate the dictionary, and then add the source file to your library target yourself. You can call the `root_generate_dictionary` with the name of the dictionary, e.g. `G__Example`, any required header files, and finally the special `LinkDef.h` file, listed after `LINKDEF`:
-`${NAME}.cxx`: This file should be included in your sources when you make your library.
-`lib{NAME}.rootmap` (`G__` prefix removed): The rootmap file in plain text
-`lib{NAME}_rdict.pcm` (`G__` prefix removed): A [ROOT pre-compiled module file][]
The name (`${NAME}`) of the targetthat you must create is determined by the dictionary name; if the dictionary name starts with `G__`, it will be removed. Otherwise, the name is used directly.
The final two output files must sit next to the library output. This is done by checking `CMAKE_LIBRARY_OUTPUT_DIRECTORY` (it will not pick up local target settings). If you have a libdir set but you don't have (global) install locations set, you'll also need to set `ARG_NOINSTALL` to `TRUE`.
Instead of manually adding the generated to your library sources, you can ask ROOT to do this for you by passing a `MODULE` argument. This argument should specify the name of an existing build target:
```cmake
add_library(Example)
root_generate_dictionary(G__Example Example.h MODULE Example LINKDEF ExampleLinkDef.h)
