addition, each primitive needs to access its associated features in lieu of a relational join 
table, so the features attribute variable was defined to hold a collection of associated 
features. Another variable, lastld , maintains the last primld used for each primitive class. 
This particular variable is defined as a class instance variable of TopologicalStructure, 
which allows each subclass to maintain a single, unique lastld value independently of the 
other subclasses. Therefore, the value of the last primld used for each primitive class can 
be easily stored and retrieved. 
Figure 5 shows several components of the topology framework. Figure 5A 
presents a partial class hierarchy of spatial feature classes. Each spatial feature class 
(bridge, buoy, etc.) would be represented by a subclass of PointFeature, LineFeature or 
AreaFeature. Each feature instance object holds onto its symbol or graphical representation 
(an instance of FeatureSymbol class, Figure 5B) which in turn holds a collection of one or 
more spatial primitives (Figure 5C) in the FeatureSymbol graphicElements attribute 
variable. Each feature class has been initialized to know what primitive class name 
(■defaultPrimClass ) to invoke when it becomes time to create its graphicElements. 
(A) (B) 
Figure 5. Linkages (Object Pointers) Among Features and Primitives 
Each primitive class has a specific collection for holding all its instances. As new 
features are created or imported, a set of existing primitive instances is scanned to prevent 
creation of duplicates. Each primitive class also maintains its topology-specific attribute 
variables, such as firstEdge for ConnectedNodePrim. These attribute variables hold onto 
pointers to other spatial primitive instances. For more detailed discussion, see (Chuns 
1995). 
5. FUNCTIONALITIES OF OVPF 
OVPF has several functionalities implemented easily using the object-oriented 
technology compared to the relational environment. With the previous sections as the 
basis, those functionalities and the property of object-oriented technology exploited to