Athena Developer Guide

Athena
Chapter 7 Accessing data Version/Issue: 2.0.0
7.4 Object containers
As mentioned before, all objects which can be placed directly within one of the stores must be derived
from the DataObject class. There is, however, another (indirect) way to store objects within a store.
This is by putting a set of objects (themselves not derived from DataObject and thus not directly
storable) into an object which is derived from DataObject and which may thus be registered into a
store.
An object container base class is implemented within the framework and a number of templated object
container classes may be implemented in the future. For the moment, two “concrete” container classes
are implemented: ObjectVector and ObjectList. These classes are based upon the
STL classes and provide mostly the same interface. Unlike the STL containers which are essentially
designed to hold objects, the container classes within the framework contain only pointers to objects,
thus avoiding a lot of memory to memory copying.
A further difference with the STL containers is that the type T cannot be anything you like. It must be a
type derived from the ContainedObject base class, see Figure 7.1. In this way all “contained”
objects have a pointer back to their containing object. This is required, in particular, by the converters
for dealing with links between objects. A ramification of this is that container objects may not contain
other container objects (without the use of multiple inheritance).
DataObject
ObjectContainerBase
parent
ContainedObject
T
ObjectVector
T
Figure 7.1 The relationship between the DataObject, ObjectVector and ContainedObject classes.
As mentioned above, objects which are contained within one of these container objects may not be
located, or registered, individually within the store. Only the container object may be located via a call
to findObject() or retrieveObject(). Thus with regard to interaction with the data stores a
container object and the objects that it contains behave as a single object.
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