D - Encapsulation And D - Interfaces

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

All D programs are composed of the following two fundamental elements:

• Program statements (code): This is the part of a program that performs actions and they are called functions.
• Program data: The data is the information of the program which affected by the program functions.

Encapsulation is an Object Oriented Programming concept that binds together the data and functions that manipulate the data, and that keeps both safe from outside interference and misuse. Data encapsulation led to the important OOP concept of data hiding.

Data encapsulation is a mechanism of bundling the data, and the functions that use them and data abstraction is a mechanism of exposing only the interfaces and hiding the implementation details from the user.

D supports the properties of encapsulation and data hiding through the creation of user-defined types, called classes. We already have studied that a class can contain private, protected and publicmembers. By default, all items defined in a class are private. For example:

class Box
{
   public:
      double getVolume()
      {
         return length * breadth * height;
      }
   private:
      double length;      // Length of a box
      double breadth;     // Breadth of a box
      double height;      // Height of a box
};

The variables length, breadth, and height are private. This means that they can be accessed only by other members of the Box class, and not by any other part of your program. This is one way encapsulation is achieved.

To make parts of a class public (i.e., accessible to other parts of your program), you must declare them after the public keyword. All variables or functions defined after the public specifier are accessible by all other functions in your program.

Making one class a friend of another exposes the implementation details and reduces encapsulation. The ideal is to keep as many of the details of each class hidden from all other classes as possible.

Data Encapsulation Example:

Any D program where you implement a class with public and private members is an example of data encapsulation and data abstraction. Consider the following example:

import std.stdio;

class Adder{
   public:
      // constructor
      this(int i = 0)
      {
        total = i;
      }
      // interface to outside world
      void addNum(int number)
      {
          total += number;
      }
      // interface to outside world
      int getTotal()
      {
          return total;
      };
   private:
      // hidden data from outside world
      int total;
}
void main( )
{
   Adder a = new Adder();

   a.addNum(10);
   a.addNum(20);
   a.addNum(30);

   writeln("Total ",a.getTotal());
}

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

Total 60

Above class adds numbers together, and returns the sum. The public members addNum and getTotalare the interfaces to the outside world and a user needs to know them to use the class. The private member total is something that is hidden from the outside world, but is needed for the class to operate properly.

Designing Strategy:

Most of us have learned through bitter experience to make class members private by default unless we really need to expose them. That's just good encapsulation.

This wisdom is applied most frequently to data members, but it applies equally to all members, including virtual functions.

D - Interfaces

An interface is a way of forcing the classes that inherit from it to have to implement certain functions or variables. Functions must not be implemented in an interface because they are always implemented in the classes that inherit from the interface. An interface is created using the interface keyword instead of the class keyword even though the two are similar in a lot of ways. When you want to inherit from an interface and the class already inherits from another class then you need to separate the name of the class and the name of the interface with a comma.

Let us look at an simple example that explains the use of an interface.

import std.stdio;

// Base class
interface Shape
{
   public:
      void setWidth(int w);
      void setHeight(int h);
}

// Derived class
class Rectangle: Shape
{
   int width;
   int height;
   public:
      void setWidth(int w)
      {
         width = w;
      }
      void setHeight(int h)
      {
         height = h;
      }

      int getArea()
      {
         return (width * height);
      }
}

void main()
{
   Rectangle Rect = new Rectangle();

   Rect.setWidth(5);
   Rect.setHeight(7);

   // Print the area of the object.
   writeln("Total area: ", Rect.getArea());

}

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

Total area: 35

Interface with Final and Static Functions

An interface can have final and static method for which definitions should be included in interface itself. These functions cannot be over-riden by the derived class. A simple example is shown below.

import std.stdio;

// Base class
interface Shape
{
   public:
      void setWidth(int w);
      void setHeight(int h);
      static void myfunction1()
      {
         writeln("This is a static method");
      }
      final void myfunction2()
      {
         writeln("This is a final method");
      }
}

// Derived class
class Rectangle: Shape
{
   int width;
   int height;
   public:
      void setWidth(int w)
      {
         width = w;
      }
      void setHeight(int h)
      {
         height = h;
      }

      int getArea()
      {
         return (width * height);
      }
}

void main()
{
   Rectangle rect = new Rectangle();

   rect.setWidth(5);
   rect.setHeight(7);

   // Print the area of the object.
   writeln("Total area: ", rect.getArea());
   rect.myfunction1();
   rect.myfunction2();
}

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

Total area: 35
This is a static method
This is a final method