Class Construction and Destruction
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Class Construction and Destruction |
Imagine you are writing a program for a business that sells flowers:
Consider the following program: using System;
namespace FlowerShop
{
public class Flower
{
public int Type;
public int Color;
public char Arrangement;
public double UnitPrice;
}
class Program
{
static int Main()
{
Flower flr = new Flower();
Console.WriteLine("Flower Type: {0}", flr.Type);
Console.WriteLine("Flower Color: {0}", flr.Color);
Console.WriteLine("Arrangement: {0}", flr.Arrangement);
Console.WriteLine("Price: {0:C}", flr.UnitPrice);
Console.WriteLine("");
return 0;
}
}
}
This would produce: Flower Type: 0 Flower Color: 0 Arrangement: Price: $0.00 Press any key to continue . . . If you declare a variable of a class in your program, when the program comes up, the compiler reserves enough memory space for each member of the class. The memory space reserved for each member variable is filled with an initial value based on its type. For a string object, the space would be left empty. For an integer type, the space would be filled with 0. A better way to take care of this type is to provide a value whose role would be to initialize the member variables with the values of your choice. A method that initializes an object can return any value but it is preferable to be of type void because its primary purpose is to reset the values. Since this method would give a starting value to all member variables that need to be initialized, it should have an equivalent argument for each of the member variables that it would initialize. Here is an example: namespace FlowerShop
{
public class Flower
{
public int Type;
public int Color;
public char Arrangement;
public double UnitPrice;
public void Initializer(int tp, int clr, char arng, double price)
{
}
}
}
The method initializer doesn't have to initialize all members of the class. For example, the previous execution of the program shows that the member variables that are of type string are initialized with empty strings. In such a case, you may not have to initialize such variables. To implement a method initializer, simply assign its argument to the corresponding member variable of the class. Here are examples: namespace FlowerShop
{
public class Flower
{
public int Type;
public int Color;
public char Arrangement;
public double UnitPrice;
public void Initializer(int tp, int clr, char arng, double price)
{
Type = tp;
Color = clr;
Arrangement = arng;
UnitPrice = price;
}
}
}
You can call a method initializer after declaring the instance of the class to give it initial values. Here is an example:
Using a method initializer, after initializing the object, you can use the values it holds as you see fit. A constructor is a special method that is created when the object comes to life. This particular method holds the same name as the class and it initializes the object whenever that object is created. When you create a class, if you don't declare a constructor, the compiler creates one for you; this is useful because it lets all other objects of the program know that the object exists. This compiler-created constructor is called the default constructor. If you want, you can create your own constructor. To create a constructor, declare a method that holds the same name as the class. Remember that the method must not return any value. Here is an example: namespace FlowerShop
{
public class Flower
{
Flower()
{
}
}
}
When you declare an instance of the class, whether you use that object or not, a constructor for the object is created. When an instance of a class has been declared, the default constructor is called, whether the object is used or not. This is illustrated in the following program: using System;
namespace FlowerShop
{
public class Flower
{
public int Type;
public int Color;
public char Arrangement;
public decimal UnitPrice;
public Flower()
{
Console.WriteLine("New Flower Order");
}
}
class Program
{
static int Main()
{
Flower flr = new Flower();
return 0;
}
}
}
This would produce: New Flower Order Press any key to continue... As you can see, even though the flr variable was not used, just its declaration was enough to signal it. You might find it sometimes convenient to create your own constructor because, whether you create an empty constructor or not, this does not negatively impact your program.
A constructor can be used to initialize the member variables of a class. As such, a constructor provides a valuable alternative to a method initializer, the type of method we saw earlier. To use a constructor to initialize the member variables of a class, provide as arguments the necessary variables that you intend to initialize. You don't have to initialize all member variables in the constructor, only those that need to be initialized. In fact, you should initialize only those members that you think the other objects would need when using this object. This means that your object may have fields that, either the external objects don't need to modify (or access) or the member variable(s) will be initialized later when called from the needed object(s). To implement a default constructor, you can just initialize the desired members of the class. For a member variable of a numeric type, you can just assign the desired constant to each. If the variable is a character, assign a single-quoted symbol to it. If the variable is a string, then assign a double-quoted value to the variable. Here are examples:
The default constructor is the favorite place to provide default values to the members of a class. Besides the default constructor, you can add as many constructors as you judge necessary. This feature of C# allows you to create various constructors for different reasons. This also means that the methods or constructors of a class can be overloaded. One of the rules of method overloading consists of having methods with different types of arguments. The most basic constructor you would create can use a single argument. When implementing a constructor that takes one argument, you should initialize the member that corresponds to the unique argument and initialize the other members with default values. Here is an example: using System;
namespace FlowerShop
{
public class Flower
{
public int Type;
public int Color;
public char Arrangement;
public decimal UnitPrice;
public Flower()
{
Type = 1;
Color = 1;
Arrangement = 'V';
UnitPrice = 0M;
}
public Flower(int tp)
{
Type = tp;
Color = 1;
Arrangement = 'V';
UnitPrice = 0M;
}
}
}
If you create a class with only one constructor as in the current example, when declaring an instance of the class, you must use that constructor: you cannot use the default constructor that doesn't take an argument. When declaring the variable, initialize it with a constructor with parentheses and provide the value(s) in the parentheses of the constructor. Here is an example: using System;
namespace FlowerShop
{
public class Flower
{
public string Type;
public string Color;
public string Arrangement;
public decimal UnitPrice;
public Flower()
{
Type = "";
Color = "Red";
Arrangement = "Basket";
UnitPrice = 35.95M;
}
public Flower(string tp)
{
Type = tp;
Color = "Red";
Arrangement = "Basket";
UnitPrice = 35.95M;
}
}
class Program
{
static int Main()
{
Flower flr = new Flower("Tulips");
Console.WriteLine("Flower Type: {0}", flr.Type);
Console.WriteLine("Flower Color: {0}", flr.Color);
Console.WriteLine("Arrangement: {0}", flr.Arrangement);
Console.WriteLine("Price: {0:C}", flr.UnitPrice);
Console.WriteLine("");
return 0;
}
}
}
This would produce: Flower Type: Tulips Flower Color: Red Arrangement: Basket Price: $35.95 Press any key to continue . . . In the same way, you can create different constructors for different initializations, although it would not be realistic to create a different constructor for each variable. If you create different constructors with different arguments to initialize (remember the rules of method overloading), when declaring the classes, make sure you initialize each instance with the right number of arguments; otherwise, the compiler would complain. If you create a class with only one constructor and that constructor has at least one argument, the default constructor would not be available anymore. If you want to access a default constructor of an object, you have two alternatives:
As opposed to a constructor, a destructor is called when a program has finished using an object. A destructor does the cleaning behind the scenes. Like the default constructor, the compiler always creates a default destructor if you don't create one. Unlike the constructor, the destructor cannot be overloaded. This means that, if you decide to create a destructor, you can have only one. Like the default constructor, a destructor also has the same name as its class. This time, the name of the destructor starts with a tilde "~". To create a destructor, type ~ followed by the name of the class. Here is an example:
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