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In Review

The Week in Review program for Week 2 brings together many of the skills you've acquired over the past fortnight and produces a powerful program.

This demonstration of linked lists utilizes virtual functions, pure virtual functions, function overriding, polymorphism, public inheritance, function overloading, forever loops, pointers, references, and more.

The goal of this program is to create a linked list. The nodes on the list are designed to hold parts, as might be used in a factory. While this is not the final form of this program, it does make a good demonstration of a fairly advanced data structure. The code list is 311 lines. Try to analyze the code on your own before reading the analysis that follows the output.

Listing R2.1. Week 2 in Review listing.

    0:     // **************************************************
    1:     //
    2:     // Title:       Week 2 in Review
    3:     //
    4:     // File:       Week2
    5:     //
    6:     // Description:   Provide a linked list demonstration program
    7:     //
    8:     // Classes:      PART - holds part numbers and potentially other
    9:     //                     information about parts
    10:     //
    11:     //               PartNode - acts as a node in a PartsList
    12:     //
    13:     //               PartsList - provides the mechanisms for a linked list                                                            Âof parts
    14:     //
    15:     // Author:      Jesse Liberty (jl)
    16:     //
    17:     // Developed:   486/66 32mb RAM  MVC 1.5
    18:     //
    19:     // Target:      Platform independent
    20:     //
    21:     // Rev History:  9/94 - First release (jl)
    22:     //
    23:     // **************************************************
    24:
    25:     #include <iostream.h>
    26:
    27:     typedef unsigned long ULONG;
    28:     typedef unsigned short USHORT;
    29:
    30:
    31:     // **************** Part ************
    32:
    33:     // Abstract base class of parts
    34:     class Part
    35:     {
    36:     public:
    37:        Part():itsPartNumber(1) {}
    38:        Part(ULONG PartNumber):itsPartNumber(PartNumber){}
    39:        virtual ~Part(){};
    40:        ULONG GetPartNumber() const { return itsPartNumber; }
    41:        virtual void Display() const =0;  // must be overridden
    42:     private:
    43:        ULONG itsPartNumber;
    44:     };
    45:
    46:     // implementation of pure virtual function so that
    47:     // derived classes can chain up
    48:     void Part::Display() const
    49:     {
    50:         cout << "\nPart Number: " << itsPartNumber << endl;
    51:     }
    52:
    53:     // **************** Car Part ************
    54:
    55:     class CarPart : public Part
    56:     {
    57:     public:
    58:        CarPart():itsModelYear(94){}
    59:        CarPart(USHORT year, ULONG partNumber);
    60:        virtual void Display() const
    61:       {
    62:          Part::Display(); cout << "Model Year: ";
    63:          cout << itsModelYear << endl;
    64:       }
    65:     private:
    66:        USHORT itsModelYear;
    67:     };
    68:
    69:     CarPart::CarPart(USHORT year, ULONG partNumber):
    70:        itsModelYear(year),
    71:        Part(partNumber)
    72:     {}
    73:
    74:
    75:     // **************** AirPlane Part ************
    76:
    77:     class AirPlanePart : public Part
    78:     {
    79:     public:
    80:        AirPlanePart():itsEngineNumber(1){};
    81:        AirPlanePart(USHORT EngineNumber, ULONG PartNumber);
    82:        virtual void Display() const
    83:       {
    84:          Part::Display(); cout << "Engine No.: ";
    85:          cout << itsEngineNumber << endl;
    86:       }
    87:     private:
    88:        USHORT itsEngineNumber;
    89:     };
    90:
    91:     AirPlanePart::AirPlanePart(USHORT EngineNumber, ULONG PartNumber):
    92:        itsEngineNumber(EngineNumber),
    93:        Part(PartNumber)
    94:     {}
    95:
    96:     // **************** Part Node ************
    97:     class PartNode
    98:     {
    99:     public:
    100:        PartNode (Part*);
    101:        ~PartNode();
    102:        void SetNext(PartNode * node) { itsNext = node; }
    103:        PartNode * GetNext() const;
    104:        Part * GetPart() const;
    105:     private:
    106:        Part *itsPart;
    107:        PartNode * itsNext;
    108:     };
    109:
    110:     // PartNode Implementations...
    111:
    112:     PartNode::PartNode(Part* pPart):
    113:     itsPart(pPart),
    114:     itsNext(0)
    115:     {}
    116:
    117:     PartNode::~PartNode()
    118:     {
    119:        delete itsPart;
    120:        itsPart = 0;
    121:        delete itsNext;
    122:        itsNext = 0;
    123:     }
    124:
    125:     // Returns NULL if no next PartNode
    126:     PartNode * PartNode::GetNext() const
    127:     {
    128:           return itsNext;
    129:     }
    130:
    131:     Part * PartNode::GetPart() const
    132:     {
    133:        if (itsPart)
    134:           return itsPart;
    135:        else
    136:           return NULL; //error
    137:     }
    138:
    139:     // **************** Part List ************
    140:     class PartsList
    141:     {
    142:     public:
    143:        PartsList();
    144:        ~PartsList();
    145:        // needs copy constructor and operator equals!
    146:        Part*      Find(ULONG & position, ULONG PartNumber)  const;
    147:        ULONG      GetCount() const { return itsCount; }
    148:        Part*      GetFirst() const;
    149:        static     PartsList& GetGlobalPartsList()
    150:      {
    151:         return  GlobalPartsList;
    152:      }
    153:        void       Insert(Part *);
    154:        void       Iterate(void (Part::*f)()const) const;
    155:        Part*      operator[](ULONG) const;
    156:     private:
    157:        PartNode * pHead;
    158:        ULONG itsCount;
    159:        static PartsList GlobalPartsList;
    160:     };
    161:
    162:     PartsList PartsList::GlobalPartsList;
    163:
    164:     // Implementations for Lists...
    165:
    166:     PartsList::PartsList():
    167:        pHead(0),
    168:        itsCount(0)
    169:        {}
    170:
    171:     PartsList::~PartsList()
    172:     {
    173:        delete pHead;
    174:     }
    175:
    176:     Part*   PartsList::GetFirst() const
    177:     {
    178:        if (pHead)
    179:           return pHead->GetPart();
    180:        else
    181:           return NULL;  // error catch here
    182:     }
    183:
    184:     Part *  PartsList::operator[](ULONG offSet) const
    185:     {
    186:        PartNode* pNode = pHead;
    187:
    188:        if (!pHead)
    189:           return NULL; // error catch here
    190:
    191:        if (offSet > itsCount)
    192:           return NULL; // error
    193:
    194:        for (ULONG i=0;i<offSet; i++)
    195:           pNode = pNode->GetNext();
    196:
    197:       return   pNode->GetPart();
    198:     }
    199:
    200:     Part*   PartsList::Find(ULONG & position, ULONG PartNumber)  const
    201:     {
    202:        PartNode * pNode = 0;
    203:        for (pNode = pHead, position = 0;
    204:              pNode!=NULL;
    205:              pNode = pNode->GetNext(), position++)
    206:        {
    207:           if (pNode->GetPart()->GetPartNumber() == PartNumber)
    208:              break;
    209:        }
    210:        if (pNode == NULL)
    211:           return NULL;
    212:        else
    213:           return pNode->GetPart();
    214:     }
    215:
    216:     void PartsList::Iterate(void (Part::*func)()const) const
    217:     {
    218:        if (!pHead)
    219:           return;
    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;
    311: }

    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

Analysis: The Week 2 in Review listing provides a linked list implementation for `Part` 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.
This particular linked list is designed to hold objects of class `Part`, where `Part` is an abstract data type serving as a base class to any objects with a part number. In this example, `Part` has been subclassed into `CarPart` and `AirPlanePart`.

Class `Part` 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. `Part` is an abstract data type, enforced by the pure virtual function `Display()`.

Note that `Display()` does have an implementation, on lines 48-51. It is the designer's intention that derived classes will be forced to create their own `Display()` method, but may chain up to this method as well.

Two simple derived classes, `CarPart` and `AirPlanePart`, are provided on lines 55-67 and 77-89, respectively. Each provides an overridden `Display()` method, which does in fact chain up to the base class `Display()` method.

The class `PartNode` serves as the interface between the `Part` class and the `PartList` 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 `Part` to which it points.

The intelligence of the list is, appropriately, in the class `PartsList`, whose declaration is on lines 140-160. `PartsList` keeps a pointer to the first element in the list (`pHead`) 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 `NULL`.

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.

`PartsList` 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.

`Find()` takes a `PartNumber` and a `ULONG`. If the part corresponding to `PartNumber` is found, it returns a pointer to the `Part` and fills the `ULONG` with the position of that part in the list. If `PartNumber` is not found, it returns `NULL`, and the position is meaningless.

`GetCount()` returns the number of elements in the list. `PartsList` keeps this number as a member variable, `itsCount`, though it could, of course, compute this number by walking the list.

`GetFirst()` returns a pointer to the first `Part` in the list, or returns `NULL` if the list is empty.

`GetGlobalPartsList()` returns a reference to the static member variable `GlobalPartsList`. This is a static instance of this class; every program with a `PartsList` also has one `GlobalPartsList`, though, of course, it is free to make other `PartsList`s as well. A full implementation of this idea would modify the constructor of `Part` to ensure that every part is created on the `GlobalPartsList`.

`Insert` takes a pointer to a `Part`, creates a `PartNode` for it, and adds the `Part` to the list, ordered by `PartNumber`.

`Iterate` takes a pointer to a member function of `Part`, which takes no parameters, returns `void`, and is `const`. It calls that function for every `Part` object in the list. In the example program this is called on `Display()`, which is a virtual function, so the appropriate `Display()` method will be called based on the runtime type of the `Part` object called.

`Operator[]` allows direct access to the `Part` at the offset provided. Rudimentary bounds checking is provided; if the list is `NULL` or if the offset requested is greater than the size of the list, `NULL` is returned as an error condition.

Note that in a real program these comments on the functions would be written into the class declaration.

The driver program is on lines 274-311. A pointer to `PartsList` is declared on line 266 and initialized with `GlobalPartsList`. Note that `GlobalPartsList` 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.

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.

The implementation for the `Insert()` method of `PartsList` is on lines 226-272. When the first part number is entered, `2837`, a `CarPart` with that part number and the model year `90` is created and passed in to `LinkedList::Insert()`.

On line 228, a new `PartNode` is created with that part, and the variable `New` is initialized with the part number. The `PartsList`'s `itsCount` member variable is incremented on line 234.

On line 236, the test that `pHead` is `NULL` will evaluate `TRUE`. Since this is the first node, it is true that the `PartsList`'s `pHead` pointer has zero. Thus, on line 238, `pHead` is set to point to the new node and this function returns.

The user is prompted to enter a second part, and this time an `AirPlane` part with part number `378` and engine number `4938` is entered. Once again `PartsList::Insert()` is called, and once again `pNode` is initialized with the new node. The static member variable `itsCount` is incremented to `2`, and `pHead` is tested. Since `pHead` was assigned last time, it is no longer null and the test fails.

On line 244, the part number held by `pHead`, `2837`, is compared against the current part number, `378`. Since the new one is smaller than the one held by `pHead`, the new one must become the new head pointer, and the test on line 244 is true.

On line 246, the new node is set to point to the node currently pointed to by `pHead`. Note that this does not point the new node to `pHead`, but rather to the node that `pHead` was pointing to! On line 247, `pHead` is set to point to the new node.

The third time through the loop, the user enters the part number `4499` for a `Car` with model year `94`. The counter is incremented and the number this time is not less than the number pointed to by `pHead`, so the `for` loop that begins on line 251 is entered.

The value pointed to by `pHead` is `378`. The value pointed to by the second node is `2837`. The current value is `4499`. The pointer `pCurrent` points to the same node as `pHead` and so has a next value; `pCurrent` points to the second node, and so the test on line 254 fails.

The pointer `pCurrent` 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.

The fourth time through, the part number `3000` is entered. This proceeds just like the previous iteration, but this time when the current node is pointing to `2837` and the next node has `4499`, the test on line 264 returns `TRUE` and the new node is inserted into position.

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

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