Object-Oriented Programming in C++: Constructors

Submitted by PhoeniciansSy on Wednesday, September 10, 2014 - 06:51.

Contents:

1. Constructors. 2. Default Constructors. 3. Constructors with Arguments. 4. Time Class with Constructors.

Constructors:

Constructor is a member function in the class, which has the following characteristics: 1. It has the same name as the class. 2. It doesn't return any type. 3. It is called every time an instance of a class is created. 3. It initialize the class members with initial values. We can have more than one constructor in a class, for example:
  1. class point {
  2.       double x,y;    // private data members
  3.     public:   // public methods
  4.       point (int x0,int y0);    // a constructor
  5.       point () { x = 0; y = 0;};   // a constructor
  6.       point (double alpha; doubler); // a constructor
  7.       void move (int dx,int dy);
  8.       void rotate (double alpha);
  9.       int distance (point p);
  10. }
And In the main function we can call them as follow:
  1. int main()
  2. {
  3.     point p1 (10,10), p2, p3 (pi /4, 2.5);
  4. }

Default Constructors:

Default Constructor is a constructor that can be called without having to provide any arguments. Like all member function, the implementation of the default constructor begins with '{' ends with '}', and its body is located between those brackets, for example:
  1. class point {
  2.       double x,y;
  3.    public:
  4.       point () { x = 0; y = 0;};  // default constructor
  5. }

Constructors with Arguments:

May the programmer wants to create objects from a class with initial values, then he can define constructors with arguments. For example:
  1. class point {
  2.       double x,y;
  3.    public:
  4.       point (int x0,int y0);    // a constructor
  5.      point (double alpha; double r); // a constructor
  6. }
And we can call the constructors in the above example as follow:
  1. point p1 (10, 20);  // call constructor with given arguments
  2. p3 (pi / 4, 2.5);   // call constructor with given arguments

Time Class with Constructors:

In the following example we re-implement Time class with constructors and default arguments:
  1. // Declaration of the Time class.
  2. class Time {
  3.    public:
  4.       Time( int h = 0, int m = 0,int s = 0 )  // default constructor
  5.       {
  6.          hour = h;
  7.          minute = m;
  8.          second = s;
  9.       }
  10.       void setTime( int h, int m, int s) // set hour, minute, second
  11.       {
  12.           hour = ( h >= 0 && h < 24 ) ? h : 0;
  13.           minute = ( m >= 0 && m < 60 ) ? m : 0;
  14.           second = ( s >= 0 && s < 60 ) ? s : 0;
  15.       }
  16.       void print24()  // print time in 24 format
  17.       {
  18.           cout << ( hour < 10 ? "0" : "" ) << hour << ":"
  19.           << ( minute < 10 ? "0" : "" ) << minute;
  20.       }
  21.       void print12()  // print time in 12 format
  22.       {
  23.           cout << ( ( hour == 0 || hour ==12 ) ? 12 : hour % 12 )
  24.           << ":" << ( minute < 10 ? "0" : "" ) << minute
  25.           << ":" << ( second < 10 ? "0" : "" ) << second
  26.           << ( hour < 12 ? " AM" : " PM" );
  27.       }
  28.    private:
  29.       int hour;  // 0 - 23
  30.       int minute;  // 0 - 59
  31.       int second;  // 0 - 59
  32. };
  33.  
  34. int main()
  35. {
  36.   Time t1,  // all arguments defaulted
  37.   t2(2),  // minute and second defaulted
  38.   t3(21, 34),  // second defaulted
  39.   t4(12, 25, 42), // all values specified
  40.   t5(27, 74, 99); // all bad values specified
  41.  
  42.   cout << "Constructed with:\n"
  43.   << "all arguments defaulted:\n ";
  44.   t1.print24();
  45.   cout << "\n ";
  46.   t1.print12();
  47.  
  48.   cout << "\nhourspecified; minute and second defaulted:"
  49.    << "\n ";
  50.   t2.print24();
  51.   cout << "\n ";
  52.   t2.print12();
  53.  
  54.   cout << "\nhourand minute specified; second defaulted:"
  55.   << "\n ";
  56.   t3.print24();
  57.   cout << "\n ";
  58.   t3.print24();
  59.  
  60.   cout << "\nhour, minute, and second specified:"
  61.    << "\n ";
  62.   t4.print24();
  63.   cout << "\n ";
  64.   t4.print12();
  65.  
  66.   cout << "\nall invalid values specified:"
  67.   << "\n ";
  68.   t5.print24();
  69.   cout << "\n ";
  70.   t5.print12();
  71.   cout <<endl;
  72.  
  73.   return 0;
  74. }
Note: You can find the full source code of this example in code.zip file.
Tags
C++
OOP