Athena Developer Guide

Athena
Chapter 7 Accessing data Version/Issue: 2.0.0
The first four methods are for retrieving a pointer to an object that is already in the store. How the
object got into the store, whether it has been read in from a persistent store or added to the store by an
algorithm, is irrelevant.
The find and retrieve methods come in two versions: one version uses a full path name as an
object identifier, the other takes a pointer to a previously retrieved object and the name of the object to
look for below that node in the tree.
Additionally the find and retrieve methods differ in one important respect: the find method will
look in the store to see if the object is present (i.e. in memory) and if it is not will return a null pointer.
The retrieve method, however, will attempt to load the object from a persistent store (database or
file) if it is not found in memory. Only if it is not found in the persistent data store will the method
return a null pointer (and a bad status code of course).
7.3 Using data objects
Whatever the concrete type of the object you have retrieved from the store the pointer which you have
is a pointer to a DataObject, so before you can do anything useful with that object you must cast it to
the correct type, for example:
1: typedef ObjectVector MyTrackVector;
2: DataObject *pObject;
3:
4: StatusCode sc = eventSvc()->retrieveObject(“/Event/MyTracks”,pObject);
5: if( sc.isFailure() )
6: return sc;
7:
8: MyTrackVector *tv = 0;
9: try {
10: tv = dynamic_cast (pObject);
11: } catch(...) {
12: // Print out an error message and return
13: }
14: // tv may now be manipulated.
The typedef on line 1 is just to save typing: in what follows we will use the two syntaxes
interchangeably. After the dynamic_cast on line 10 all of the methods of the MyTrackVector
class become available. If the object which is returned from the store does not match the type to which
you try to cast it, an exception will be thrown. If you do not catch this exception it will be caught by the
algorithm base class, and the program will stop, probably with an obscure message. A more elegant
way to retrieve the data involves the use of Smart Pointers - this is discussed in section 7.8
As mentioned earlier a certain amount of run-time investigation may be done into what data is available
in the store. For example, suppose that we have various sets of testbeam data and each data set was
taken with a different number of detectors. If the raw data is saved on a per-detector basis the number of
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