GAT Object Model

The GAT API is an object oriented API which makes use of several primitives of object oriented programming such as encapsulation, inheritance, interfaces, polymorphism, ...C, however, doesn't naturally support any of these constructs. So, the introduction of these constructs in to the C version of GAT API in many cases required the reinvention of the wheel. However, you, the fearless application programmer, will never have to walk these minefields but will only reap the many benefits of this Sisyphean task.

The GAT object model consists of a set of ``classes'' arranged in a shallow ``inheritance'' tree, see figure .

Each GAT ``class'' consists of an opaque pointer, the details of which will never be of concern to the application programmer. Furthermore, as C does not natively support inheritance, the ``inheritance'' depicted in figure is actually a stand-in for an inheritance with language support. The means through which inheritance in actually implemented in GAT will never be of concern to the application programmer; however, the repercussions of this inheritance are of concern to the application programmer and are our next topic of discussion.

The class GATObject, from which all GAT classes inherit, posses a set of various functions, see figure .

where GATType is an enum whose values correspond to the various GAT types, GATObject_const is a const version of GATObject, GATResult indicates a function's completion status, GATBool is a boolean, and GATInterface is and enum whose values correspond to the various GAT interfaces. (Interfaces are a topic which we will examine in the next subsection.) Each subclass of this GATObject possess a set of ``corresponding'' functions. So, for example, the class GATTime has the functions depicted in figure .

Furthermore, for each subclass of GATObject there exists a set of utility functions which allow one to convert this subclass to a GATObject and from a GATObject. So, for example, in the case of GATTime this set of utility functions is given by

GATTime GATObject_ToGATTime(GATObject object)
GATObject GATTime_ToGATObject(GATTime derived)
GATTime_const GATObject_ToGATTime_const(GATObject_const object)
GATObject_const GATTime_ToGATObject_const(GATTime_const derived)

A class possessing these two properties, functions corresponding to the GATObject functions and the ability to convert to and from a GATObject, is said to inherit from GATObject. The full set of such GAT classes are depicted in figure .