Athena Developer Guide

Athena
Chapter 11 Histogram facilities Version/Issue: 2.0.0
Chapter 11
Histogram facilities
11.1 Overview
The histogram data store is one of the data stores discussed in Chapter 2. Its purpose is to store statistics
based data and user created objects that have a lifetime of more than a single event (e.g. histograms).
As with the other data stores, all access to data is via a service interface. In this case it is via the
IHistogramSvc interface, which is derived from the IDataProviderSvc interface discussed in
Chapter 7. The user asks the Histogram Service to book a histogram and register it in the histogram data
store. The service returns a pointer to the histogram, which can then be used to fill and manipulate the
histogram
The histograms themselves are booked and manipulated using four interfaces as defined by the AIDA
(Abstract Interfaces for Data Analysis) project. These interfaces are documented on the AIDA web
pages: http://wwwinfo.cern.ch/asd/lhc++/AIDA/. The Athena implementation uses the transient part of
HTL (Histogram Template Library, http://wwwinfo.cern.ch/asd/lhc++/HTL/), provided by LHC++.
The histogram data model is shown in Figure 11.1. The interface IHistogram is a base class, which
is used for management purposes. It is not a complete histogram interface, it should not be used by the
users. Both interfaces IHistogram1D and IHistogram2D are derived from IHistogram, and
use by reference the IAxis interface. Users can book their 1D or 2D histograms in the histogram data
store in the same type of tree structure as the event data. Concrete 1D and 2D histograms derive from
the DataObject in order to be storable.
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