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Field Guide

Creating Objects

You create an object using the new operator. This is much the same as we discussed in member functions in that it will do three things:

  1. Assigns space in memory for the object on the heap.
  2. Executes the constructor.
  3. Returns a reference to the new object’s location in memory.

The job of the constructor is to set the object’s initial state. That is, it should make sure that each of the object’s fields have a sensible value. It can do this by using literal values, values passed in through a parameter, calling constructors (i.e., if the field is of a class type), calling other methods (including methods from the class it is constructing), or a combination.

A constructor is often called as the expression within an assignment statement, so that the returned value can be stored in a variable. We can then use that variable to ask the object about things it knows, and to ask it to do things it can do.

In C

In C# every object is created using the keyword new, followed by the class name, then any arguments expected by the class’s constructor.

Example

For this example we will use the Greeting class from the classes concept. To add this class to your C# project, copy the class code into a file called “Greeting.cs” within a console project. We can now write a program that uses this class.

The following code creates a Greeting object, and uses it to greet the user.

public class Program
{
    public static void Main()
    {
        // myGreeting is a reference to a Greeting object
        Greeting myGreeting;
        // at this point there are no objects.


        // The next line creates an object and stores a reference to
        // it in myGreeting
        myGreeting = new Greeting("Hello World");
        string name;


        Write("Enter your name: ");
        name = Console.ReadLine();


        // We can now use the object to perform one of its responsibilities
        myGreeting.Greet(name);
    }
}