Athena Developer Guide

Athena Chapter 5 Writing algorithms Version/Issue: 2.0.0
5.3.1 Creation (and algorithm factories)
As mentioned before, a concrete algorithm class must specify a single constructor with the same
parameter signature as the constructor of the base class.
In addition to this a concrete algorithm factory must be provided. This is a technical matter which
permits the application manager to create new algorithm objects without having to include all of the
concrete algorithm header files. From the point of view of an algorithm developer it implies adding two
lines into the implementation file, of the form:
static const AlgFactory s_factory;
const IAlgFactory& ConcreteAlgorithmFactory = s_factory;
where “ConcreteAlgorithm” should be replaced by the name of the derived algorithm class (see
for example lines 10 and 11 in Listing 5.2 below).
5.3.2 Declaring properties
In general a concrete algorithm class will have several data members which are used in the execution of
the algorithm proper. These data members should of course be initialized in the constructor, but if this
was the only mechanism available to set their value it would be necessary to recompile the code every
time you wanted to run with different settings. In order to avoid this, the framework provides a
mechanism for setting the values of member variables at run time.
The mechanism comes in two parts: the declaration of properties and the setting of their values. As an
example consider the class TriggerDecision in Listing 5.2 which has a number of variables whose
value we would like to set at run time.
The default values for the variables are set within the constructor (within an initialiser list) as per
normal. To declare them as properties it suffices to call the declareProperty() method. This
method is overloaded to take an std::string as the first parameter and a variety of different types
for the second parameter. The first parameter is the name by which this member variable shall be
referred to, and the second parameter is a reference to the member variable itself.
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In the example we associate the name “PassAllMode” to the member variable m_passAllMode,
and the name “MuonCandidateCut” to m_muonCandidateCut. The first is of type boolean and
the second an integer. If the job options service (described in Chapter 13) finds an option in the job
options file belonging to this algorithm and whose name matches one of the names associated with a
member variable, then that member variable will be set to the value specified in the job options file.