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CHAPTER 4 CLASSES: THE BASICS
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Creating Variables and Instances of a Class
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The class declaration is just the blueprint from which instances of the class are created. Once a class is declared, you can create instances of the class. Classes are reference types, which, as you will remember from the last chapter, means that they require memory for both the reference to the data and for the actual data. The reference to the data is stored in a variable of the class type. So, to create an instance of the class, you need to start by declaring a variable of the class type. If the variable is not initialized, its value is undefined. Figure 4-2 illustrates how to define the variable to hold the reference. At the top of the code on the left is a declaration for class Dealer. Below that is a declaration for class Program, which contains method Main. Main declares variable theDealer of type Dealer. Since the variable is uninitialized, its value is undefined, as shown on the right in the figure.
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Figure 4-2. Allocating memory for the reference of a class variable
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Allocating Memory for the Data
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Declaring the variable of the class type allocates the memory to hold the reference, but not the memory to hold the actual data of the class object. To allocate memory for the actual data, you use the new operator. The new operator allocates and initializes memory for an instance of any specified type. It allocates the memory from either the stack or the heap, depending on the type. Use the new operator to form an object-creation expression, which consists of the following: The keyword new. The name of the type of the instance for which memory is to be allocated. Matching parentheses, which might or might not include parameters. I ll discuss more about the possible parameters later. Keyword Parentheses are required. new TypeName ( ) Type If the memory allocated is for a reference type, the object-creation expression returns a reference to the allocated and initialized instance of the object in the heap. This is exactly what you need to allocate and initialize the memory to hold the class instance data. Use the new operator to create an object-creation expression, and assign the value returned by it to the class variable. Here s an example: Dealer theDealer; // Declare variable for the reference. theDealer = new Dealer(); // Allocate memory for the class object. Object-creation expression The code on the left in Figure 4-3 shows the new operator used to allocate memory and create an instance of class Dealer, which is then assigned to the class variable. The memory structure is illustrated in the figure, to the right of the code.
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Figure 4-3. Allocating memory for the data of a class variable
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Combining the Steps
The two steps can be combined by initializing the variable with the object-creation expression. Declare variable. Dealer theDealer = new Dealer(); // Declare and initialize. Initialize with an object-creation expression. In the case of local variables, but not fields, you can simplify the syntax a bit more by having the compiler infer the type in the declaration part on the left. But I ll cover that in the section on local variables in the next chapter.
CHAPTER 4 CLASSES: THE BASICS
Instance Members
A class declaration acts as a blueprint from which you can create as many instances of the class as you like. Instance members: Each instance of a class is a separate entity that has its own set of the data members, distinct from the other instances of the same class. These are called instance members since they are associated with an instance of the class. Static members: Instance members are the default, but you can also declare members that are associated with the class, rather than the instance. These are called static members, and they will be covered in 6. As an example of instance members, the following code shows the poker program with three instances of class Player. Figure 4-4 shows that each instance has a different value for the Name field. class Dealer { ... } class Player { string Name; ... } class Program { static void Main() { Dealer theDealer Player player1 = Player player2 = Player player3 = ... } } // Declare class // Declare class // Field
= new Dealer(); new Player(); new Player(); new Player();
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