Implementing Pure Virtual Functions in C++

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Typically, the pure virtual functions in an abstract base class are never implemented. Because no objects of that type are ever created, there is no reason to provide implementations, and the ADT works purely as the definition of an interface to objects which derive from it.

It is possible, however, to provide an implementation to a pure virtual function. The function can then be called by objects derived from the ADT, perhaps to provide common functionality to all the overridden functions. Listing below, this time with Shape as an ADT and with an implementation for the pure virtual function Draw(). The Circle class overrides Draw(), as it must, but it then chains up to the base class function for additional functionality.

In this example, the additional functionality is simply an additional message printed, but one can imagine that the base class provides a shared drawing mechanism, perhaps setting up a window that all derived classes will use.

Implementing pure virtual functions.

1:     //Implementing pure virtual functions
2:
3:     #include <iostream.h>
4:
5:     enum BOOL { FALSE, TRUE };
6:
7:     class Shape
8:     {
9:     public:
10:       Shape(){}
11:       ~Shape(){}
12:       virtual long GetArea() = 0; // error
13:       virtual long GetPerim()= 0;
14:       virtual void Draw() = 0;
15:    private:
16:    };
17:
18:     void Shape::Draw()
19:    {
20:       cout << "Abstract drawing mechanism!\n";
21:    }
22:
23:    class Circle : public Shape
24:    {
25:    public:
26:          Circle(int radius):itsRadius(radius){}
27:          ~Circle(){}
28:          long GetArea() { return 3 * itsRadius * itsRadius; }
29:          long GetPerim() { return 9 * itsRadius; }
30:          void Draw();
31:    private:
32:       int itsRadius;
33:       int itsCircumference;
34:    };
35:
36:    void Circle::Draw()
37:    {
38:       cout << "Circle drawing routine here!\n";
39:       Shape::Draw();
40:    }
41:
42: 
43:    class Rectangle : public Shape
44:    {
45:    public:
46:          Rectangle(int len, int width):
47:             itsLength(len), itsWidth(width){}
48:          ~Rectangle(){}
49:          long GetArea() { return itsLength * itsWidth; }
50:          long GetPerim() {return 2*itsLength + 2*itsWidth; }
51:          virtual int GetLength() { return itsLength; }
52:          virtual int GetWidth() { return itsWidth; }
53:          void Draw();
54:    private:
55:       int itsWidth;
56:       int itsLength;
57:    };
58:
59:    void Rectangle::Draw()
60:    {
61:       for (int i = 0; i<itsLength; i++)
62:       {
63:          for (int j = 0; j<itsWidth; j++)
64:             cout << "x ";
65:
66:          cout << "\n";
67:       }
68:       Shape::Draw();
69:    }
70:
71:
72:    class Square : public Rectangle
73:    {
74:    public:
75:          Square(int len);
76:          Square(int len, int width);
77:          ~Square(){}
78:          long GetPerim() {return 4 * GetLength();}
79:    };
80: 
81:    Square::Square(int len):
82:       Rectangle(len,len)
83:    {}
84:
85:    Square::Square(int len, int width):
86:       Rectangle(len,width)
87:
88:    {
89:       if (GetLength() != GetWidth())
90:          cout << "Error, not a square... a Rectangle??\n";
91:    }
92:
93:    int main()
94:    {
95:       int choice;
96:       BOOL fQuit = FALSE;
97:       Shape * sp;
98: 
99:       while (1)
100:      {
101:         cout << "(1)Circle (2)Rectangle (3)Square (0)Quit: ";
102:         cin >> choice;
103:
104:         switch (choice)
105:         {
106:            case 1: sp = new Circle(5);
107:            break;
108:            case 2: sp = new Rectangle(4,6);
109:            break;
110:            case 3: sp = new Square (5);
111:            break;
112:            default: fQuit = TRUE;
113:            break;
114:         }
115:         if (fQuit)
116:            break;
117:
118:         sp->Draw();
119:         cout << "\n";
120:      }
121:     return 0;
122: }

Output: (1)Circle (2)Rectangle (3)Square (0)Quit: 2
x x x x x x
x x x x x x
x x x x x x
x x x x x x
Abstract drawing mechanism!

(1)Circle (2)Rectangle (3)Square (0)Quit: 3
x x x x x
x x x x x
x x x x x
x x x x x
x x x x x
Abstract drawing mechanism!

(1)Circle (2)Rectangle (3)Square (0)Quit: 0

Analysis: On lines 7-16, the abstract data type Shape is declared, with all three of its accessor methods declared to be pure virtual. Note that this is not necessary. If any one were declared pure virtual, the class would have been an ADT.

The GetArea() and GetPerim() methods are not implemented, but Draw() is. Circle and Rectangle both override Draw(), and both chain up to the base method, taking advantage of shared functionality in the base class.


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