220:        PartNode* pNode = pHead;
221:        do
222:           (pNode->GetPart()->*func)();
223:        while (pNode = pNode->GetNext());
224:     }
225:   
226:     void PartsList::Insert(Part* pPart)
227:     {
228:        PartNode * pNode = new PartNode(pPart);
229:        PartNode * pCurrent = pHead;
230:        PartNode * pNext = 0;
231:   
232:        ULONG New =  pPart->GetPartNumber();
233:        ULONG Next = 0;
234:        itsCount++;
235:   
236:        if (!pHead)
237:        {
238:           pHead = pNode;
239:           return;
240:        }
241:   
242:        // if this one is smaller than head
243:        // this one is the new head
244:        if (pHead->GetPart()->GetPartNumber() > New)
245:        {
246:           pNode->SetNext(pHead);
247:           pHead = pNode;
248:           return;
249:        }
250:   
251:        for (;;)
252:        {
253:           // if there is no next, append this new one
254:           if (!pCurrent->GetNext())
255:           {
256:              pCurrent->SetNext(pNode);
257:              return;
258:           }
259:   
260:           // if this goes after this one and before the next
261:           // then insert it here, otherwise get the next
262:           pNext = pCurrent->GetNext();
263:           Next = pNext->GetPart()->GetPartNumber();
264:           if (Next > New)
265:           {
266:              pCurrent->SetNext(pNode);
267:              pNode->SetNext(pNext);
268:              return;
269:           }
270:           pCurrent = pNext;
271:        }
272:     }
273:   
274:     int main()
275:     {
276:        PartsList pl = PartsList::GetGlobalPartsList();
277:        Part * pPart = 0;
278:        ULONG PartNumber;
279:        USHORT value;
280:        ULONG choice;
281:   
282:        while (1)
283:        {
284:           cout << "(0)Quit (1)Car (2)Plane: ";
285:           cin >> choice;
286:   
287:           if (!choice)
288:              break;
289:   
290:           cout << "New PartNumber?: ";
291:           cin >>  PartNumber;
292:   
293:           if (choice == 1)
294:           {
295:              cout << "Model Year?: ";
296:              cin >> value;
297:              pPart = new CarPart(value,PartNumber);
298:           }
299:           else
300:           {
301:              cout << "Engine Number?: ";
302:              cin >> value;
303:              pPart = new AirPlanePart(value,PartNumber);
304:           }
305:   
306:           pl.Insert(pPart);
307:        }
308:        void (Part::*pFunc)()const = Part::Display;
309:        pl.Iterate(pFunc);
310:       return 0;
<TT>311: }</TT></FONT>
<FONT COLOR="#0066FF">
Output: (0)Quit (1)Car (2)Plane: 1
New PartNumber?: 2837
Model Year? 90
(0)Quit (1)Car (2)Plane: 2
New PartNumber?: 378
Engine Number?: 4938
(0)Quit (1)Car (2)Plane: 1
New PartNumber?: 4499
Model Year? 94
(0)Quit (1)Car (2)Plane: 1
New PartNumber?: 3000
Model Year? 93
(0)Quit (1)Car (2)Plane: 0

Part Number: 378
Engine No.: 4938

Part Number: 2837
Model Year: 90

Part Number: 3000
Model Year: 93

Part Number: 4499
Model Year: 94
</FONT></PRE>
<P><FONT COLOR="#000077"><B>Analysis: </B></FONT>The Week 2 in Review listing provides
a linked list implementation for <TT>Part</TT> objects. A linked list is a dynamic
data structure; that is, it is like an array but it is sized to fit as objects are
added and deleted.<BR>
This particular linked list is designed to hold objects of class <TT>Part</TT>, where
<TT>Part</TT> is an abstract data type serving as a base class to any objects with
a part number. In this example, <TT>Part</TT> has been subclassed into <TT>CarPart</TT>
and <TT>AirPlanePart</TT>.</P>
<P>Class <TT>Part</TT> is declared on lines 34-44, and consists of a part number
and some accessors. Presumably this class could be fleshed out to hold other important
information about the parts, such as what components they are used in, how many are
in stock, and so forth. <TT>Part</TT> is an abstract data type, enforced by the pure
virtual function <TT>Display()</TT>.</P>
<P>Note that <TT>Display()</TT> does have an implementation, on lines 48-51. It is
the designer's intention that derived classes will be forced to create their own
<TT>Display()</TT> method, but may chain up to this method as well.</P>
<P>Two simple derived classes, <TT>CarPart</TT> and <TT>AirPlanePart</TT>, are provided
on lines 55-67 and 77-89, respectively. Each provides an overridden <TT>Display()</TT>
method, which does in fact chain up to the base class <TT>Display()</TT> method.</P>
<P>The class <TT>PartNode</TT> serves as the interface between the <TT>Part</TT>
class and the <TT>PartList</TT> class. It contains a pointer to a part and a pointer
to the next node in the list. Its only methods are to get and set the next node in
the list and to return the <TT>Part</TT> to which it points.</P>
<P>The intelligence of the list is, appropriately, in the class <TT>PartsList</TT>,
whose declaration is on lines 140-160. <TT>PartsList</TT> keeps a pointer to the
first element in the list (<TT>pHead</TT>) and uses that to access all other methods
by walking the list. Walking the list means asking each node in the list for the
next node, until you reach a node whose next pointer is <TT>NULL</TT>.</P>
<P>This is only a partial implementation; a fully developed list would provide either
greater access to its first and last nodes, or would provide an iterator object,
which allows clients to easily walk the list.</P>
<P><TT>PartsList</TT> nonetheless provides a number of interesting methods, which
are listed in alphabetical order. This is often a good idea, as it makes finding
the functions easier.</P>
<P><TT>Find()</TT> takes a <TT>PartNumber</TT> and a <TT>ULONG</TT>. If the part
corresponding to <TT>PartNumber</TT> is found, it returns a pointer to the <TT>Part</TT>
and fills the <TT>ULONG</TT> with the position of that part in the list. If <TT>PartNumber</TT>
is not found, it returns <TT>NULL</TT>, and the position is meaningless.</P>
<P><TT>GetCount()</TT> returns the number of elements in the list. <TT>PartsList</TT>
keeps this number as a member variable, <TT>itsCount</TT>, though it could, of course,
compute this number by walking the list.</P>
<P><TT>GetFirst()</TT> returns a pointer to the first <TT>Part</TT> in the list,
or returns <TT>NULL</TT> if the list is empty.</P>
<P><TT>GetGlobalPartsList()</TT> returns a reference to the static member variable
<TT>GlobalPartsList</TT>. This is a static instance of this class; every program
with a <TT>PartsList</TT> also has one <TT>GlobalPartsList</TT>, though, of course,
it is free to make other <TT>PartsList</TT>s as well. A full implementation of this
idea would modify the constructor of <TT>Part</TT> to ensure that every part is created
on the <TT>GlobalPartsList</TT>.</P>
<P><TT>Insert</TT> takes a pointer to a <TT>Part</TT>, creates a <TT>PartNode</TT>
for it, and adds the <TT>Part</TT> to the list, ordered by <TT>PartNumber</TT>.</P>
<P><TT>Iterate</TT> takes a pointer to a member function of <TT>Part</TT>, which
takes no parameters, returns <TT>void</TT>, and is <TT>const</TT>. It calls that
function for every <TT>Part</TT> object in the list. In the example program this
is called on <TT>Display()</TT>, which is a virtual function, so the appropriate
<TT>Display()</TT> method will be called based on the runtime type of the <TT>Part</TT>
object called.</P>
<P><TT>Operator[]</TT> allows direct access to the <TT>Part</TT> at the offset provided.
Rudimentary bounds checking is provided; if the list is <TT>NULL</TT> or if the offset
requested is greater than the size of the list, <TT>NULL</TT> is returned as an error
condition.</P>
<P>Note that in a real program these comments on the functions would be written into
the class declaration.</P>
<P>The driver program is on lines 274-311. A pointer to <TT>PartsList</TT> is declared
on line 266 and initialized with <TT>GlobalPartsList</TT>. Note that <TT>GlobalPartsList</TT>
is initialized on line 162. This is necessary as the declaration of a static member
variable does not define it; definition must be done outside the declaration of the
class.</P>
<P>On lines 282-307, the user is repeatedly prompted to choose whether to enter a
car part or an airplane part. Depending on the choice the right value is requested,
and the appropriate part is created. Once created, the part is inserted into the
list on line 306.</P>
<P>The implementation for the <TT>Insert()</TT> method of <TT>PartsList</TT> is on
lines 226-272. When the first part number is entered, <TT>2837</TT>, a <TT>CarPart</TT>
with that part number and the model year <TT>90</TT> is created and passed in to
<TT>LinkedList::Insert()</TT>.</P>
<P>On line 228, a new <TT>PartNode</TT> is created with that part, and the variable
<TT>New</TT> is initialized with the part number. The <TT>PartsList</TT>'s <TT>itsCount</TT>
member variable is incremented on line 234.</P>
<P>On line 236, the test that <TT>pHead</TT> is <TT>NULL</TT> will evaluate <TT>TRUE</TT>.
Since this is the first node, it is true that the <TT>PartsList</TT>'s <TT>pHead</TT>
pointer has zero. Thus, on line 238, <TT>pHead</TT> is set to point to the new node
and this function returns.</P>
<P>The user is prompted to enter a second part, and this time an <TT>AirPlane</TT>
part with part number <TT>378</TT> and engine number <TT>4938</TT> is entered. Once
again <TT>PartsList::Insert()</TT> is called, and once again <TT>pNode</TT> is initialized
with the new node. The static member variable <TT>itsCount</TT> is incremented to
<TT>2</TT>, and <TT>pHead</TT> is tested. Since <TT>pHead</TT> was assigned last
time, it is no longer null and the test fails.</P>
<P>On line 244, the part number held by <TT>pHead</TT>, <TT>2837</TT>, is compared
against the current part number, <TT>378</TT>. Since the new one is smaller than
the one held by <TT>pHead</TT>, the new one must become the new head pointer, and
the test on line 244 is true.</P>
<P>On line 246, the new node is set to point to the node currently pointed to by
<TT>pHead</TT>. Note that this does not point the new node to <TT>pHead</TT>, but
rather to the node that <TT>pHead</TT> was pointing to! On line 247, <TT>pHead</TT>
is set to point to the new node.</P>
<P>The third time through the loop, the user enters the part number <TT>4499</TT>
for a <TT>Car</TT> with model year <TT>94</TT>. The counter is incremented and the
number this time is not less than the number pointed to by <TT>pHead</TT>, so the
<TT>for</TT> loop that begins on line 251 is entered.</P>
<P>The value pointed to by <TT>pHead</TT> is <TT>378</TT>. The value pointed to by
the second node is <TT>2837</TT>. The current value is <TT>4499</TT>. The pointer
<TT>pCurrent</TT> points to the same node as <TT>pHead</TT> and so has a next value;
<TT>pCurrent</TT> points to the second node, and so the test on line 254 fails.</P>
<P>The pointer <TT>pCurrent</TT> is set to point to the next node and the loop repeats.
This time the test on line 254 succeeds. There is no next item, so the current node
is told to point to the new node on line 256, and the insert is finished.</P>
<P>The fourth time through, the part number <TT>3000</TT> is entered. This proceeds
just like the previous iteration, but this time when the current node is pointing
to <TT>2837</TT> and the next node has <TT>4499</TT>, the test on line 264 returns
<TT>TRUE</TT> and the new node is inserted into position.</P>

<P>When the user finally presses 0, the test on line 287 evaluates <TT>true</TT>
and the <TT>while(1)</TT> loop breaks. On line 308, the member function <TT>Display()</TT>
is assigned to the pointer to member function <TT>pFunc</TT>. In a real program this
would be assigned dynamically, based on the user's choice of method.</P>

<P>The pointer to member function is passed to the <TT>PartsList</TT> <TT>Iterate()</TT>
method. On line 216, the <TT>Iterate()</TT> method ensures that the list is not empty.
Then, on lines 221-223, each <TT>Part</TT> on the list is called using the pointer
to member function. This calls the appropriate <TT>Display()</TT> method for the
<TT>Part</TT>, as shown in the output. <BR>

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