Classes are the central feature of D programming that supports object-oriented programming and are often called user-defined types.

A class is used to specify the form of an object and it combines data representation and methods for manipulating that data into one neat package. The data and functions within a class are called members of the class.

D Class Definitions

When you define a class, you define a blueprint for a data type. This does not actually define any data, but it defines what the class name means, that is, what an object of the class will consist of and what operations can be performed on such an object.

A class definition starts with the keyword class followed by the class name; and the class body, enclosed by a pair of curly braces. A class definition must be followed either by a semicolon or a list of declarations. For example, we defined the Box data type using the keyword class as follows −

class Box { 
   public: 
      double length;   // Length of a box 
      double breadth;  // Breadth of a box 
      double height;   // Height of a box 
}

The keyword public determines the access attributes of the members of the class that follow it. A public member can be accessed from outside the class anywhere within the scope of the class object. You can also specify the members of a class as private or protected which we will discuss in a sub-section.

Defining D Objects

A class provides the blueprints for objects, so basically an object is created from a class. You declare objects of a class with exactly the same sort of declaration that you declare variables of basic types. The following statements declare two objects of class Box −

Box Box1;          // Declare Box1 of type Box 
Box Box2;          // Declare Box2 of type Box 

Both of the objects Box1 and Box2 have their own copy of data members.

Accessing the Data Members

The public data members of objects of a class can be accessed using the direct member access operator (.). Let us try the following example to make the things clear −

import std.stdio;

class Box { 
   public: 
      double length;   // Length of a box 
      double breadth;  // Breadth of a box 
      double height;   // Height of a box 
}
  
void main() { 
   Box box1 = new Box();    // Declare Box1 of type Box 
   Box box2 = new Box();    // Declare Box2 of type Box 
   double volume = 0.0;     // Store the volume of a box here  
   
   // box 1 specification 
   box1.height = 5.0; 
   box1.length = 6.0; 
   box1.breadth = 7.0; 
   
   // box 2 specification 
   box2.height = 10.0; 
   box2.length = 12.0; 
   box2.breadth = 13.0;
   
   // volume of box 1 
   volume = box1.height * box1.length * box1.breadth; 
   writeln("Volume of Box1 : ",volume);
   
   // volume of box 2 
   volume = box2.height * box2.length * box2.breadth; 
   writeln("Volume of Box2 : ", volume); 
} 

Volume of Box1 : 210 
Volume of Box2 : 1560