Parameterized

Parameterized Constructor

A default constructor does not have any parameter, but if you need, a constructor can have parameters. This helps you to assign initial value to an object at the time of its creation as shown in the following example:

#include <iostream>
 
using namespace std;
 
class Line {
   public:
      void setLength( double len );
      double getLength( void );
      Line(double len);  // This is the constructor
 
   private:
      double length;
};
 
// Member functions definitions including constructor
Line::Line( double len) {
   cout << "Object is being created, length = " << len << endl;
   length = len;
}
 
void Line::setLength( double len ) {
   length = len;
}
 
double Line::getLength( void ) {
   return length;
}

// Main function for the program
int main( ) {
   Line line(10.0);
 
   // get initially set length.
   cout << "Length of line : " << line.getLength() <<endl;
 
   // set line length again
   line.setLength(6.0); 
   cout << "Length of line : " << line.getLength() <<endl;
 
   return 0;
}

When the above code is compiled and executed, it produces the following result:

Object is being created, length = 10
Length of line : 10
Length of line : 6

Using Initialization Lists to Initialize Fields

In case of parameterized constructor, you can use following syntax to initialize the fields:

Line::Line( double len): length(len) {
   cout << "Object is being created, length = " << len << endl;
}

Above syntax is equal to the following syntax:

Line::Line( double len) {
   cout << "Object is being created, length = " << len << endl;
   length = len;
}

If for a class C, you have multiple fields X, Y, Z, etc., to be initialized, then use can use same syntax and separate the fields by comma as follows:

C::C( double a, double b, double c): X(a), Y(b), Z(c) {
   ....
}