To construct an abstract software model of a collection, we start by building the formal specification. The first component of this is the name of a data type - this is the type of objects that belong to the collection class. In C, we use typedef to define a new type which is a pointer to a structure:
Note that we are defining a pointer to a structure only; we have not specified details of the attributes of the structure. We are deliberately deferring this - the details of the implementation are irrelevant at this stage. We are only concerned with the abstract behaviour of the collection. In fact, as we will see later, we want to be able to substitute different data structures for the actual implementation of the collection, depending on our needs. The typedef declaration provides us with a C type (class in OO design parlance), collection. We can declare objects of type collection wherever needed. Although C forces us to reveal that the handle for objects of the class is a pointer, it is better to take an abstract view: we regard variables of type collection simply as handles to objects of the class and forget that the variables are actually C pointers.
