LArSoft  v08_42_01
Liquid Argon Software toolkit -
An algorithm producing a new data product: `TotallyCheatTrackingAlg`
Example name: TotallyCheatTracks
Type: LArSoft algorithm with module
Author: Gianluca Petrillo (
Created on: December 26, 2017
Version: 1.0

This example shows an algorithm whose output is stored in a new data product.

Features of the algorithm and module:

  • most features are in common with the example RemoveIsolatedSpacePoints
  • an algorithm fills a new class, which a art module stores as data product
  • associations to input objects are also produced

Missing features that you need to go elsewhere if you need to implement:

  • complete unit test (some test is still present)
  • lar_ci integration test

Technical choices:

  • employs the module/algorithm factorisation model
  • uses validation of the configuration
  • it is fully documented via Doxygen interpreted comments

How to use this document

This document is pretty long. You are encouraged to read it all, but you might understandably opt out of that. To make it even longer, we assume that you are aware of the existence of the art introductory documentation (in particular, the art workbook). Having read it will imbue the present text with a lot more sense. And if you have no idea about what art is, or what a art module is, you definitely need to go there first. Also, most of the information that is already present in LArSoft example RemoveIsolatedSpacePoints is not repeated in its entirety here.

This document is organised in sections. We suggest that you read the section above to know what this example is about. Then, you can dig into the code and come back to the pertinent section when you have questions.

For every questions, answered or not here, you are strongly encouraged to contact the example's author (contact information is at the top of this file).

And, if you want to have a bit more printer friendly format, know this text file is written in markdown format and you can convert it to something else with:

pandoc -s -S --toc -o README.html
pandoc -s -S --toc -o README.pdf

et cetera.

The creation of data products

A "data product" is a class that can be saved in a art ROOT output file. In order for any class to become a data product, it needs to satisfy a few requirements. The hardest part about creating a good data product is its design: that class will be saved in the output of your experiment forever, and once it's saved it can be hard to change it without breaking the old data and code.

This is not a guide to data product design, and the choices made in this example may be not motivated in detail. More information is available in art wiki and some LArSoft code conventions are described in the LArSoft wiki.


The core example is split between two directories:

1 larexamples/Algoritmhs/TotallyCheatTracks/
2 |-- # this file
3 |-- CheatTrackData ## contains the data product class ##
4 | |-- CheatTrack.h # data product class header
5 | |-- classes.h # file for ROOT to generate class dictionary (I)
6 | `-- classes_def.xml # file for ROOT to generate class dictionary (II)
7 |-- TotallyCheatTrackingAlg.h # algorithm header
8 |-- TotallyCheatTrackingAlg.cxx # algorithm implementation
9 |-- cheat_tracks.fcl # algorithm standard configuration
10 |-- # art module writing the data products
11 `-- totallycheattracking.fcl # example job configuration

Each (sub)directory also has its own CMakeLists.txt file.

Tests are in test/Algoritmhs/TotallyCheatTracks directory.

Directory structure

The directory structure in this example is comprised of a main directory (TotallyChearTracks), and a subdirectory containing only the data product (CheatTrackData). This separation is not required but it is recommended, because it guarantees that the data product libraries have no unexpected dependencies. This is more important for data products because its code and library may end up in other code which does not know of LArSoft or of art (e.g. gallery).

While the factorisation model is still applied, respect to the example RemoveIsolatedSpacePoints, here the algorithm and module share the same directory. This choice should be avoided in normal code, but it is used here for simplicity, because factorisation is not the focus of this example.

The data product class

The data product we need represents a reconstructed track, and we choose to characterise it with:

  • a trajectory (position and momentum);
  • a particle identification code; we use PDG codes.

The most important part of the data product class is to explain, in the documentation, what it is meant to represent. Failure to do so has caused and still causes countless troubles in LArSoft.

The design that is recommended by art and LArSoft is of a class whose content can not be modified after construction. Therefore, the constructor arguments must fully define the final content of the object. While this may be tedious on the caller side, it is considered good practice. More important, the data product should expose a stable access interface, and hide the internal representation of the data. The implementation in this example goes almost all the way to it:

  • data members are private
  • access to the data is via member functions

We use a standard LArSoft data class for the interface exposing the trajectory concept, recob::Trajectory. In this first version, the interface returns that object directly from an internal data member, which is acceptable although it might make a future change of internal representation cumbersome. Also it is important to make sure that the concept represented by recob::Trajectory fits well with the concept in our class: each evolution of recob::Trajectory will be reflected in our object.

We choose to give a name to the particle ID type (PDGID_t). In the future, that might become a more complex object, but the change might be absorbed by the type name and by a proper design of the new class interface.

We write some method to print the content of the object to screen. This is very useful when debugging and for messages on screen (e.g. errors). We chose to go with a more powerful method dump() able to output on any stream (including the usual STL output streams like std::cout, but also the message facility loggers like mf::LogInfo() and LOG_DEBUG()), with control on the amount of detail printed and on the indentation of the output, and to have an inflexible output in the traditional operator<<, which must be implemented as a free function in the same namespace where the data product is defined.

We also need to provide a default constructor, because ROOT requires it to read objects of this type from ROOT trees. We let the compiler write the default constructor, but we specify that the default particle ID is an invalid one, InvalidParticleID (specifically, value is 0). This constructor needs to be public but it will be of little use to normal users, since the object can't be changed after construction.

We have added a name for the invalid particle ID and a method to check its validity in a easy way. Experience shows that checks with the plain value get confusing and if (!part.hasParticleId()) is less likely to be misinterpreted than if (part.particleId() == 0).

Normally, we should also provide a unit test. Unit tests for these simple data products are also simple, and may consist of constructing an object with the main constructor and checking that all the accessors give the expected result, and doing the same for the default constructor.

The style convention is drinking camel case for member functions (e.g. particleId(), with all words separated not by an underscore, but by the capitalised first letter of each word, except the first one), vigilant-camel-case for data types (e.g. PDGID_t, the first letter also capital), and data member names prepended by f (e.g. fTraj).

The class is defined in a header file (CheatTrack.h). All member functions are very simple and are (and should) be implemented inline, that is in the header. The only exception is the dump() method, which is not very simple but is a template and therefore also needs to be implemented in the header file. This means that there is no implementation file (no CheatTrack.cxx), and also that there will be no library CheatTrackData or similar (it would have been on Linux). So we don't need to add to the link lists that use lar::example::CheatTrack a larexamples_Algoritmhs_TotallyCheatTracks_CheatTrackData line in the link list: including the header in the C++ source files will be enough.

The algorithm and module

This section is succinct and does not describe the technical choices in the module and algorithm code. Nothing is new here with respect to the example RemoveIsolatedSpacePoints.

The (preposterous) algorithm TotallyCheatTrackingAlg accepts a single simulated particle (simb::MCParticle, like the ones produced by LArG4 module and in the simb::MCTruth objects produced by event generators), and "reconstructs" it into a new data product lar::example::CheatTrack. The module drives this algorithm and performs a minimal selection on the input collection of simulated particles: only particles with a minimum energy and path length will be processed. The module also saves an association between the input particle and the reconstructed CheatTrack.

Algorithm design

The design of the algorithm here is not explained in detail, and we refer to the usual example RemoveIsolatedSpacePoints for most of the explanations. A few notes follow about peculiarities of this design which are not present in that example.

The algorithm presents the complete interface as recommended for a stateless algorithm: a constructor with (validated) configuration, a setup phase, and a compact execution method which merges input, processing and output. Due to the simplicity of the algorithm, though, the configuration is empty and the setup is no operation. While these parts are unnecessary and not mandated, it's not a bad idea to leave them anyway for future extensions.

The most important difference in the design is that this algorithm processes a single input object rather than a collection of them. It is up to the caller to make sure that all the objects which need to be converted are processed. This has some consequences:

  1. the caller module gets to put together the data product
  2. input/output object associations must also be managed by the caller module
  3. the algorithm is independent of the way the input is stored

The last point is particularly relevant for this case in LArSoft: the module LArG4 produces a collection of simb::MCParticle, and the algorithm could have easily been written to accept a std::vector<simb::MCParticle> as input. Yet, simb::MCParticle objects are also stored as part of simb::MCTruth, and that object does not offer them as a collection but only individually. A similar situation may be found in other parts of the code, and it's often a good choice to have the algorithm process the smallest unit possible (simb::MCParticle in this case), or to accept as input a collection of pointers to the objects (std::vector<simb::MCParticle const*>), so that the conversion from other formats at least does not require to duplicate the content of the input.

Creation of the associations

The module is in charge of creating the association between the input and output data. This design choice is twofold. First, the algorithm is designed to deal with the single output, as described in the previous section. Second, it allows the algorithm to be more independent of framework concepts like the art pointers required to create associations.

An association is a template data product provided by art, which connects two elements from different data product collections, and with an optional "payload" of data describing each connection. The art::Assns class is a collection of such connections; in jargon, this collection may be referred to as "association" or "associations". In our example, the connection is between simb::MCParticle and lar::example::CheatTrack, with no payload, and this is represented by the associations class:

art::Assns<lar::example::CheatTrack, simb::MCParticle>

The first argument (here lar::example::CheatTrack) is sometimes called the "left" type, and the second argument (simb::MCParticle) the "right" type. The payload would be the third template argument; it is rarely used and its default is void, which means no payload at all. Associations are bidirectional, in that they store enough information to allow retrieving which simb::MCParticle object(s) are associated to a lar::example::CheatTrack, and which lar::example::CheatTrack object(s) are associated to a simb::MCParticle. Also, objects on both sides can be shared by different connections ("many-to-many" associations).

The basic way to add a connection to the associations object is via the method addSingle(), which takes as arguments the art pointer to the left object and one to the connected right object (and the payload data to be copied, when it applies). LArSoft used to provide some functions called util::CreateAssns(). These functions have been deprecated because they are so hard to use correctly, that they cause often unnecessary overhead and sometimes plain mistakes.

The example member function lar::example::TotallyCheatTracker::produce() shows how to use addSingle() to add a connection. It is as simple as:

trackToPart->addSingle(trackPtr, partPtr);

where the two arguments are art pointers to the objects to be connected. The complication is only how to create those two pointers. This depends on whether the data product already exists in the event (like simb::MCParticle) or if we are creating it now (like our output lar::example::CheatTrack). art provides a utility art::PtrMaker which almost uniforms the interface to create the pointers. Once a art::PtrMaker object is created, the only necessary information to create a pointer to an object is the index of that object in the data product collection, whether the object is already there, or it will put there at a later time. The method instantiates two of these objects:

art::PtrMaker<simb::MCParticle> makePartPtr(event,;
art::PtrMaker<lar::example::CheatTrack> makeTrackPtr(event, *this);

The art::PtrMaker class accepts as template parameter the class the pointers will be pointing to. Then:

  • if the target data product already exists, art::PtrMaker must be constructed with the ID of that data product; the ID can be obtained from the data product handle (as in the example);
  • if the target data product is going to be produced by the current module, the module itself needs to be passed to art::PtrMaker constructor (if the target data product also has a instance name, that name must be specified too as the third argument).

In the particle loop, the module keeps track of the index of the input particle. This may be achieved by a range for loop and a separate index (iParticle), which has potentially slightly better performance but is more error prone (remember to always increase the index, and do that at the right time!). An alternative is to go with a index-based for loop (as in the example); another one is to have a iterator loop with an index... whatever it takes. The track index is easier, because when we are adding the connection to a track, that track has just been appended to the track collection, and therefore its index is the size of that collection minus one. Note that the whole thing may have been written in a more compact way as:

  (makeTrackPtr(tracks->size() - 1), makePartPtr(iParticle));

Also note that the creation of a art pointer for an existing data product is very simple also without art::PtrMaker: in this case it would have been:

art::Ptr<simb::MCParticle> const partPtr { particleHandle, iParticle };

We use art::PtrMaker for uniformity (also, it's negligibly more performant).

The ROOT dictionary

The ROOT "dictionary" is a library produced by ROOT that enables introspection of the classes. Type introspection is a feature which allows to ask, at run time, which methods and fields are present in the class, to execute the former and ask the value of the latter. It is the key feature of the ROOT interpreter and of many other ROOT facilities, including the input and output of almost any C++ objects in a ROOT tree.

The build system is able to ask ROOT to create a dictionary when provided with a few instructions. And ROOT (rootcling) can do that when provided a lot more instructions.


The classes_def.xml contains the list of all the classes for which a dictionary is needed. It is quite tedious a list to compile, and on top of that its format is XML (almost!).

The XML format is almost standard, except that the characters <, > and & are allowed unescaped in the attribute values, which is self-evidently good when one has to add a class like:

std::pair<art::Ptr<lar::example::CheatTrack>, art::Ptr<simb::MCParticle>>

(as we do) without escaping all those angular parentheses.

The classes_def.xml file also takes note of the different versions of a class. The feature coping with the changes of data product classes is called schema evolution. For example, if we run for a while with the current version of lar::example::CheatTrack, and then we add a fType data member, the class changes and when ROOT is given an old file (where lar::example::CheatTrack has no fType) it needs to know that there is no fType in the data, and how to fill in a default value. While a remarkable level of complication may become involved, ROOT is able to correctly react to the simplest changes with no user action. The important thing is to inform ROOT that a class may change, by explicitly assigning a ClassVersion number (the first version number is strongly recommended to be 10). The resulting magic line we write in classes_def.xml is then:

<class name="lar::example::CheatTrack" ClassVersion="10" />

(this is a "empty element" XML tag, <TAG/>).

Note: when the build system realises the class is changed, it will update the classes_def.xml file, issue an error on screen and ask to rebuild again. For example:

INFO: adding version info for class 'lar::example::CheatTrack':<version ClassVersion="10" checksum="790383593"/>
WARNING: classes_def.xml files have been updated: rebuild dictionaries.

Rebuilding is expected to succeed. Also make sure that you don't overwrite the file after it has been changed by the build system (for example, if you have the file open in an editor and you save it again without reloading it first) and that you commit the updated classes_def.xml to your GIT repository. You may notice that the empty element tag has been replaced with a complete tag:

<class name="lar::example::CheatTrack" ClassVersion="10" >
 <version ClassVersion="10" checksum="790383593"/>

(<TAG> ... </TAG>); the tag contains now a list of versions, and for each a checksum extracted from the class at that version. The next time the class is changed, the build system will again add a tag <version ... /> to that list and ask to build again.

The definition of which classes we need to appear in this file is the most troublesome. The list starts simple: since we want the class lar::example::CheatTrack, we add the corresponding line:

<class name="lar::example::CheatTrack" ClassVersion="10" />

Then we need to specify all the classes it needs. lar::example::CheatTrack contains a recob::Trajectory, therefore that class needs to be known to ROOT. Fortunately, recob::Trajectory is already a data product on its own merit, and a dictionary for it is generated in lardataobj. But what we want to write in the art ROOT file is actually a std::vector<lar::example::CheatTrack>, and as a consequence that class also needs its entry in classes_def.xml:

<class name="std::vector<lar::example::CheatTrack>" />

(we don't expect the class std::vector to change, so we don't bother to specify a version for it). Furthermore, unbeknownst to us, what actually art writes is a wrapped version of that vector, and it's on us to ask ROOT to generate a dictionary for it too:

<class name="art::Wrapper<std::vector<lar::example::CheatTrack>>" />

It's also good practice to generate a dictionary for the class art pointer, art::Ptr<lar::example::CheatTrack>. This takes finally care of the data product itself.

Things go wild with associations. The relevant object is art::Assns<lar::example::CheatTrack, simb::MCParticle> (1 entry), and the data product is wrapped in art::Wrapper<art::Assns<lar::example::CheatTrack, simb::MCParticle, void>> (2 entries so far). But the art::Assns internally uses a std::pair of pointers: std::pair<art::Ptr<lar::example::CheatTrack>, art::Ptr<simb::MCParticle>> (3 entries so far). Ugh. And the story about the bidirectionality of art::Assns also implies that dictionaries should be available when swapping left and right class: art::Assns<simb::MCParticle, lar::example::CheatTrack> and related classes (double everything: 6 entries!). Now, did we say art::Ptr? well, those too need to have a dictionary. We already requested one for art::Ptr<lar::example::CheatTrack> above, while the one for simb::MCParticle is already provided in nusimdata. And this concludes the list. Take a look at the classes_def.xml in lardataobj/RecoBase for guaranteed headache.


The C++ header classes.h is expected to include all the class and type declarations that are needed by the classes we want the dictionary of. The included headers need to be enough to declare all the classes involved in classes_def.xml. That usually includes the class headers (here, one for lar::example::CheatTrack and one for simb::MCParticle), plus Assns.h and Wrapper.h from art (art::Ptr is used by art::Assns and therefore indirectly included already, but it would not hurt to include it explicitly).


The art build system will create a ROOT dictionary automatically when the macro art_make() is used in CMakeLists.txt, if the files classes.h and classes_def.xml are present in the same directory. In our example, we needed to tell it about two additional libraries needed for linking, since we ask for the dictionary of associations to simb::MCParticle (from nusimdata_SimulationBase library in nusimdata). We specify the extra library after the DICT_LIBRARIES keyword of art_make(). It is also possible to create only the dictionary by using art_dictionary().

Unit tests

A minimal unit test is provided to prove that the dictionary generation works properly. The test, in larexamples/test/Algorithms/TotallyCheatTracks, is shortly described here.

The test is an art test using Boost unit test, so it is run with lar_ut. The configuration, test_totallycheattracker.fcl, first creates some simulated particles with a custom test module, then runs the tracker TotallyCheatTracker and finally runs a custom dumper which prints the particles and its associations. The output file can be used to verify that the data product was written correctly. For example, we can have a run in gallery:

setup larsoftobj v1_34_00 -q prof:e14
root -l <<EOC
gallery::Event event({ "TotallyCheatTracker_test.d/CheatTrkTest.root" });
auto const& tracks = *(event.getValidHandle<std::vector<lar::example::CheatTrack>>("cheattrk"));
for (auto const& track: tracks) std::cout << track << std::endl;

will print something like:

Successfully opened file TotallyCheatTracker_test.d/CheatTrkTest.root
particle: mu+ (ID=-13); momentum: 2 GeV/c; trajectory with 5 points at ( 1 ; 2 ; 3 ) cm toward ( -1 ; 0 ; 0 ) with momentum 2 GeV/c
ends at ( -3 ; 2 ; 3 ) cm toward ( -1 ; 0 ; 0 ) with momentum 2 GeV/c
particle: pi+ (ID=211); momentum: 3 GeV/c; trajectory with 6 points at ( -3 ; 2 ; 3 ) cm toward ( 0 ; -1 ; 0 ) with momentum 3 GeV/c
ends at ( -3 ; -3 ; 3 ) cm toward ( 0 ; -1 ; 0 ) with momentum 3 GeV/c
particle: pi0 (ID=111); momentum: 4 GeV/c; trajectory with 7 points at ( -3 ; -3 ; 3 ) cm toward ( 0 ; 0 ; -1 ) with momentum 4 GeV/c
ends at ( -3 ; -3 ; -3 ) cm toward ( 0 ; 0 ; -1 ) with momentum 4 GeV/c
particle: pi- (ID=-211); momentum: 3 GeV/c; trajectory with 8 points at ( -3 ; -3 ; -3 ) cm toward ( 1 ; 0 ; 0 ) with momentum 3 GeV/c
ends at ( 4 ; -3 ; -3 ) cm toward ( 1 ; 0 ; 0 ) with momentum 3 GeV/c
particle: neutron (ID=2112); momentum: 2 GeV/c; trajectory with 9 points at ( 4 ; -3 ; -3 ) cm toward ( 0 ; 1 ; 0 ) with momentum 2 GeV/c
ends at ( 4 ; 5 ; -3 ) cm toward ( 0 ; 1 ; 0 ) with momentum 2 GeV/c

One remarkable feature of FHiCL configuration is employed in the module ParticleMaker, which allows reading a sequence of structures from the configuration file (test_totallycheattracker.fcl shows that).


If you have any question about the example, please contact its author. This section will be populated with questions and their answers.

Change log

Version 1.0: December 26, 2017 ( original version