Athena Developer Guide

Athena
Chapter 7 Accessing data Version/Issue: 2.0.0
trackContainer.release(h1);
Since the fate of a contained object is so closely tied to that of its container life would become more
complex if objects could belong to more than one container. Suppose that an object belonged to two
containers, one of which was deleted. Should the object be deleted and removed from the second
container, or not deleted? To avoid such issues an object is allowed to belong to a single container only.
If you wish to move an object from one container to another, you must first remove it from one and then
add to the other. However, the first operation is done implicitly for you when you try to add an object to
a second container:
container1.push_back(h1); // Add to fist container
container2.push_back(h1); // Move to second container
// Internally invokes release().
Since the object h1 has a link back to its container, the push_back() method is able to first follow
this link and invoke the release() method to remove the object from the first container, before
adding it into the second.
In general your first exposure to object containers is likely to be when retrieving data from the event
data store. The sample code in Listing 7.5 shows how, once you have retrieved an object container from
the store you may iterate over its contents, just as with an STL vector.
Listing 7.5 Use of the ObjectVector templated class.
1: typedef ObjectVector MyTrackVector;
2: MyTrackVector *tracks;
3: MyTrackVector::iterator it;
4:
5: for( it = tracks->begin(); it != tracks->end(); it++ ) {
6: // Get the energy of the track and histogram it
7: double energy = (*it)->fourMomentum().e();
8: m_hEnergyDist->fill( energy, 1. );
9: }
The variable tracks is set to point to an object in the event data store of type:
ObjectVector with a dynamic cast (not shown above). An iterator (i.e. a pointer-like
object for looping over the contents of the container) is defined on line 3 and this is used within the loop
to point consecutively to each of the contained objects. In this case the objects contained within the
ObjectVector are of type “pointer to MyTrack”. The iterator returns each object in turn and in the
example, the energy of the object is used to fill a histogram.
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