ROOT

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.

Finding ROOT

ROOT 6.10+ supports config file discovery, so you can just do:

import:'find_package', lang:'cmake'

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).

The right way (Targets)

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).

To link, just pick the libraries you want to use:

import:'add_and_link', lang:'cmake'

If you'd like to see the default list, run root-config --libs on the command line. In Homebrew ROOT 6.18 this would be:

ROOT::Core
ROOT::Gpad
ROOT::Graf3d
ROOT::Graf
ROOT::Hist
ROOT::Imt
ROOT::MathCore
ROOT::Matrix
ROOT::MultiProc
ROOT::Net
ROOT::Physics
ROOT::Postscript
ROOT::RIO
ROOT::ROOTDataFrame
ROOT::ROOTVecOps
ROOT::Rint
ROOT::Thread
ROOT::TreePlayer
ROOT::Tree

The old global way

ROOT provides a utility 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.

Here's what it would look like:

import:'core', lang:'cmake'

Components

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

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:

Your class definition should end with ClassDef(MyClassName, 1)
Your class implementation should have ClassImp(MyClassName) in it

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.

To load this function, first include the ROOT macros:

include("${ROOT_DIR}/modules/RootNewMacros.cmake")
# For ROOT versions than 6.16, things break 
# if nothing is in the global include list!
if (${ROOT_VERSION} VERSION_LESS "6.16")
    include_directories(ROOT_NONEXISTENT_DIRECTORY_HACK)
endif()

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 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.

Manually building the dictionary

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:

root_generate_dictionary(G__Example Example.h LINKDEF ExampleLinkDef.h)

This command will create three files:

${NAME}.cxx: This file should be included in your sources when you make your library.
lib{NAME}.rootmap (G__ prefix removed): The rootmap file in plain text
lib{NAME}_rdict.pcm (G__ prefix removed): A ROOT pre-compiled module file The name (${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.

Building the dictionary with an existing target

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:

add_library(Example)
root_generate_dictionary(G__Example Example.h MODULE Example LINKDEF ExampleLinkDef.h)

The full name of the dictionary (e.g. G__Example) should not be identical to the MODULE argument.

Using Old ROOT

If you really have to use older ROOT, you'll need something like this:

# ROOT targets are missing includes and flags in ROOT 6.10 and 6.12
set_property(TARGET ROOT::Core PROPERTY
    INTERFACE_INCLUDE_DIRECTORIES "${ROOT_INCLUDE_DIRS}")

# Early ROOT does not include the flags required on targets
add_library(ROOT::Flags_CXX IMPORTED INTERFACE)


# ROOT 6.14 and earlier have a spacing bug in the linker flags
string(REPLACE "-L " "-L" ROOT_EXE_LINKER_FLAGS "${ROOT_EXE_LINKER_FLAGS}")

# Fix for ROOT_CXX_FLAGS not actually being a CMake list
separate_arguments(ROOT_CXX_FLAGS)
set_property(TARGET ROOT::Flags_CXX APPEND PROPERTY
    INTERFACE_COMPILE_OPTIONS ${ROOT_CXX_FLAGS})

# Add definitions
separate_arguments(ROOT_DEFINITIONS)
foreach(_flag ${ROOT_EXE_LINKER_FLAG_LIST})
    # Remove -D or /D if present
    string(REGEX REPLACE [=[^[-//]D]=] "" _flag ${_flag})
    set_property(TARGET ROOT::Flags APPEND PROPERTY INTERFACE_LINK_LIBRARIES ${_flag})
endforeach()

# This also fixes a bug in the linker flags
separate_arguments(ROOT_EXE_LINKER_FLAGS)
set_property(TARGET ROOT::Flags_CXX APPEND PROPERTY
    INTERFACE_LINK_LIBRARIES ${ROOT_EXE_LINKER_FLAGS})

# Make sure you link with ROOT::Flags_CXX too!

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