Post By: Hanan Mannan
Contact Number: Pak (+92)-321-59-95-634
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D - Overloading

D allows you to specify more than one definition for a function name or an operator in the same scope, which is called function overloading and operator overloading respectively.

An overloaded declaration is a declaration that had been declared with the same name as a previously declared declaration in the same scope, except that both declarations have different arguments and obviously different definition (implementation).

When you call an overloaded function or operator, the compiler determines the most appropriate definition to use by comparing the argument types you used to call the function or operator with the parameter types specified in the definitions. The process of selecting the most appropriate overloaded function or operator is called overload resolution.

Function overloading

You can have multiple definitions for the same function name in the same scope. The definition of the function must differ from each other by the types and/or the number of arguments in the argument list. You can not overload function declarations that differ only by return type.

Following is the example where same function print() is being used to print different data types:

import std.stdio;
import std.string;
class printData
{
   public:
      void print(int i) {
         writeln("Printing int: ",i);
      }

      void print(double  f) {
        writeln("Printing float: ",f );
      }

      void print(string s) {
        writeln("Printing string: ",s);
      }
};

void main()
{
   printData pd = new printData();

   // Call print to print integer
   pd.print(5);
   // Call print to print float
   pd.print(500.263);
   // Call print to print character
   pd.print("Hello D");
}

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

Printing int: 5
Printing float: 500.263
Printing string: Hello D

Operators overloading

You can redefine or overload most of the built-in operators available in D. Thus a programmer can use operators with user-defined types as well.

Operators can be overloaded using string op followed by Add, Sub and so on based on the operator that is being overloaded. We can overload the operator + to add two boxes as shown below.

Box opAdd(Box b)
{
   Box box = new Box();
   box.length = this.length + b.length;
   box.breadth = this.breadth + b.breadth;
   box.height = this.height + b.height;
   return box;
}

Following is the example to show the concept of operator over loading using a member function. Here an object is passed as an argument whose properties will be accessed using this object, the object which will call this operator can be accessed using this operator as explained below:

import std.stdio;

class Box
{
   public:

      double getVolume()
      {
         return length * breadth * height;
      }
      void setLength( double len )
      {
         length = len;
      }

      void setBreadth( double bre )
      {
         breadth = bre;
      }

      void setHeight( double hei )
      {
         height = hei;
      }

      Box opAdd(Box b)
      {
         Box box = new Box();
         box.length = this.length + b.length;
         box.breadth = this.breadth + b.breadth;
         box.height = this.height + b.height;
         return box;
      }
   private:
      double length;      // Length of a box
      double breadth;     // Breadth of a box
      double height;      // Height of a box
};
// Main function for the program
void main( )
{
   Box box1 = new Box();    // Declare box1 of type Box
   Box box2 = new Box();    // Declare box2 of type Box
   Box box3 = new Box();    // Declare box3 of type Box
   double volume = 0.0;     // Store the volume of a box here

   // box 1 specification
   box1.setLength(6.0);
   box1.setBreadth(7.0);
   box1.setHeight(5.0);

   // box 2 specification
   box2.setLength(12.0);
   box2.setBreadth(13.0);
   box2.setHeight(10.0);

   // volume of box 1
   volume = box1.getVolume();
   writeln("Volume of Box1 : ", volume);

   // volume of box 2
   volume = box2.getVolume();
   writeln("Volume of Box2 : ", volume);

   // Add two object as follows:
   box3 = box1 + box2;

   // volume of box 3
   volume = box3.getVolume();
   writeln("Volume of Box3 : ", volume);

}

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

Volume of Box1 : 210
Volume of Box2 : 1560
Volume of Box3 : 5400