Object-Oriented Programming in C++: Constructors

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

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:

class point {
      double x,y;    // private data members
    public:   // public methods
      point (int x0,int y0);    // a constructor
      point () { x = 0; y = 0;};   // a constructor
      point (double alpha; doubler); // a constructor
      void move (int dx,int dy);
      void rotate (double alpha);
      int distance (point p);
}

And In the main function we can call them as follow:

int main()
{
    point p1 (10,10), p2, p3 (pi /4, 2.5);
}

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:

class point {
      double x,y;
   public:
      point () { x = 0; y = 0;};  // default constructor
}

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:

class point {
      double x,y; 
   public:
      point (int x0,int y0);    // a constructor
     point (double alpha; double r); // a constructor
}

And we can call the constructors in the above example as follow:

point p1 (10, 20);  // call constructor with given arguments
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:

// Declaration of the Time class.
class Time {
   public:
      Time( int h = 0, int m = 0,int s = 0 )  // default constructor
      {
         hour = h;
         minute = m;
         second = s;
      }
      void setTime( int h, int m, int s) // set hour, minute, second
      {
          hour = ( h >= 0 && h < 24 ) ? h : 0;
          minute = ( m >= 0 && m < 60 ) ? m : 0;
          second = ( s >= 0 && s < 60 ) ? s : 0;
      }
      void print24()  // print time in 24 format
      {
          cout << ( hour < 10 ? "0" : "" ) << hour << ":"
          << ( minute < 10 ? "0" : "" ) << minute;
      }
      void print12()  // print time in 12 format
      {
          cout << ( ( hour == 0 || hour ==12 ) ? 12 : hour % 12 )
          << ":" << ( minute < 10 ? "0" : "" ) << minute
          << ":" << ( second < 10 ? "0" : "" ) << second
          << ( hour < 12 ? " AM" : " PM" );
      }
   private:
      int hour;  // 0 - 23
      int minute;  // 0 - 59
      int second;  // 0 - 59
};
 
int main()
{
  Time t1,  // all arguments defaulted
  t2(2),  // minute and second defaulted
  t3(21, 34),  // second defaulted 
  t4(12, 25, 42), // all values specified
  t5(27, 74, 99); // all bad values specified
 
  cout << "Constructed with:\n"
  << "all arguments defaulted:\n ";
  t1.print24();
  cout << "\n ";
  t1.print12();
 
  cout << "\nhourspecified; minute and second defaulted:" 
   << "\n ";
  t2.print24();
  cout << "\n ";
  t2.print12();
 
  cout << "\nhourand minute specified; second defaulted:" 
  << "\n ";
  t3.print24();
  cout << "\n ";
  t3.print24();
 
  cout << "\nhour, minute, and second specified:" 
   << "\n ";
  t4.print24();
  cout << "\n ";
  t4.print12();
 
  cout << "\nall invalid values specified:" 
  << "\n ";
  t5.print24();
  cout << "\n ";
  t5.print12();
  cout <<endl;
 
  return 0;
}

Note: You can find the full source code of this example in code.zip file.