Athena Developer Guide

Athena
Chapter 7 Accessing data Version/Issue: 2.0.0
The thing to note here is that if the appropriate converter is supplied, as discussed in Chapter 15, then
this class may also be saved into a persistent store (e.g. a ROOT file or an Objectivity database) and
read back at a later date. In order for the persistency to work the following are required: the unique class
identifier number (CLID_UDO in the example), and the clID() and classID() methods which
return this identifier.
The procedure for allocating unique class identifiers is, for the time being, experiment specific.
Types which are derived from ContainedObject are implemented in the same way, and must have
a CLID in the range of an unsigned short. Contained objects may only reside in the store when
they belong to a container, e.g. an ObjectVector which is registered into the store. The class
identifier of a concrete object container class is calculated (at run time) from the type of the objects
which it contains, by setting bit 16. The static classID() method is required because the container
may be empty.
7.8 The SmartDataPtr/SmartDataLocator utilities
The usage of the data services is simple, but extensive status checking and other things tend to make the
code difficult to read. It would be more convenient to access data items in the store in a similar way to
accessing objects with a C++ pointer. This is achieved with smart pointers, which hide the internals of
the data services.
7.8.1 Using SmartDataPtr/SmartDataLocator objects
The SmartDataPtr and a SmartDataLocator are smart pointers that differ by the access to the
data store. SmartDataPtr first checks whether the requested object is present in the transient store
and loads it if necessary (similar to the retrieveObject method of IDataProviderSvc).
SmartDataLocator only checks for the presence of the object but does not attempt to load it
(similar to findObject).
Both SmartDataPtr and SmartDataLocator objects use the data service to get hold of the
requested object and deliver it to the user. Since both objects have similar behaviour and the same user
interface, in the following only the SmartDataPtr is discussed.
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