Class Variables and Instance Variables

This diagram shows a Thing class (the rectangle) that contains a class variable, @@num_things, and an instance variable, @name. The three oval shapes represent “Thing objects”—that is, instances of the Thing class. When one of these objects assigns a value to its instance variable, @name, that value affects only the @name variable in the object itself. So here, each object has a different value for @name. But when an object assigns a value to the class variable, @@num_things, that value “lives inside” the Thing class and is shared by all instances of that class. Here @@num_things equals 3, and that is true for all the Thing objects.

Digging Deeper

Every class you create will descend from one or more other classes. Here I explain the fundamentals of the Ruby class hierarchy.

Superclasses

To understand how the super keyword works, take a look at the sample program super.rb. This contains five related classes. The Thing class is the ancestor of all the others, and from Thing descends Thing2, from Thing2 descends Thing3, from Thing3 descends Thing4, and from Thing4 descends Thing5.

super.rb

class Thing
    def initialize( aName, aDescription )
        @name = aName
        @description = aDescription
        puts("Thing.initialize: #{self.inspect}

")
    end

    def aMethod( aNewName )
        @name = aNewName
        puts("Thing.aMethod: #{self.inspect}

")
    end
end

class Thing2 < Thing
    def initialize( aName, aDescription )
        super
        @fulldescription = "This is #{@name}, which is #{@description}"
        puts("Thing2.initialize: #{self.inspect}

")
    end

    def aMethod( aNewName, aNewDescription )
        super( aNewName )
        puts("Thing2.aMethod: #{self.inspect}

")
    end
end

class Thing3 < Thing2
    def initialize( aName, aDescription, aValue )
        super( aName, aDescription )
        @value = aValue
        puts("Thing3.initialize: #{self.inspect}

")
    end

    def aMethod( aNewName, aNewDescription, aNewValue )
        super( aNewName, aNewDescription )
        @value = aNewValue
        puts("Thing3.aMethod: #{self.inspect}

")
    end
end

class Thing4 < Thing3
    def aMethod
        puts("Thing4.aMethod: #{self.inspect}

")
    end
end

class Thing5 < Thing4
end

Let’s take a closer look at the first three classes in this hierarchy: The Thing class has two instance variables, @name and @description. Thing2 also defines @fulldescription (a string that contains @name and @description); Thing3 adds yet another variable, @value.

These three classes each contain an initialize method that sets the values of the variables when a new object is created; they also each have a method named, rather inventively, aMethod, which changes the value of one or more variables. The descendant classes, Thing2 and Thing3, both use the super keyword in their methods.

At the bottom of this code unit I’ve written a “main” loop that executes when you run the program. Don’t worry about the syntax of this; you’ll be learning about loops in Chapter 5. I’ve added this loop so that you can easily run the different bits of code contained in the methods, test1 to test5. You can run the program in a command window and enter a number, 1 to 5, when prompted, or Q to quit. When you run it for the first time, type 1 at the prompt and press the enter key. This will run the test1 method containing these two lines of code:

t = Thing.new( "A Thing", "a lovely thing full of thinginess" )
t.aMethod( "A New Thing" )

The first line here creates and initializes a Thing object, and the second line calls its aMethod method. Because the Thing class doesn’t descend from anything special, nothing very new or interesting happens here. In fact, as with all Ruby classes, Thing descends from the Object class, which is the ancestor of all other classes (with the sole exception of the BasicObject class in Ruby 1.9, as explained later in this chapter). The output uses the inspect method to display the internal structure of the object when the Thing.initialize and Thing.aMethod methods are called. This is the result:

Thing.initialize: #<Thing:0x28e0290 @name="A Thing",
 @description="a lovely thing full of thinginess">
Thing.aMethod: #<Thing:0x28e0290 @name="A New Thing", @description="a
 lovely thing full of thinginess">

The inspect method can be used with all objects and is an invaluable debugging aid. Here, it shows a hexadecimal number, which identifies this specific object followed by the string values of the @name and @description variables.

Now enter 2 at the prompt to run test2, which contains the following code:

t2 = Thing2.new( "A Thing2", "a Thing2 thing of great beauty" )
t2.aMethod( "A New Thing2", "a new Thing2 description" )

This creates a Thing2 object, t2, and calls t2.aMethod. Look carefully at the output. You will see that even though t2 is a Thing2 object, it is the Thing class’s initialize method that is called first. And only then is the Thing2 class’s initialize called.

Thing.initialize: #<Thing2:0x2a410a0 @name="A Thing2",
 @description="a Thing2 thing of great beauty">

Thing2.initialize: #<Thing2:0x2a410a0 @name="A Thing2", @description="a Thing2
 thing of great beauty", @fulldescription="This is A Thing2, which is a Thing2
 thing of great beauty">

To understand why this is so, look at the code of the Thing2 class’s initialize method:

def initialize( aName, aDescription )
   super
   @fulldescription = "This is #{@name}, which is #{@description}"
   puts("Thing2.initialize: #{self.inspect}

")
end

This uses the super keyword to call the initialize method of Thing2’s ancestor, or superclass. The superclass of Thing2 is Thing, as you can see from its declaration:

class Thing2 < Thing

In Ruby, when the super keyword is used on its own (that is, without any arguments), it passes all the arguments from the current method (here Thing2.initialize) to a method with the same name in its superclass (here Thing.initialize). Alternatively, you can explicitly specify a list of arguments following super. So, in this case, the following code would have the same effect:

super( aName, aDescription )

Although it is permissible to use the super keyword all on its own, it is often preferable to explicitly specify the list of arguments to be passed to the superclass, for the sake of clarity. If you want to pass only a limited number of the arguments sent to the current method, an explicit argument list is necessary. Thing2’s aMethod, for example, passes only the aName argument to the initialize method of its superclass, Thing1:

super( aNewName )

This explains why the @description variable is not changed when Thing2.aMethod is called.

Now if you look at Thing3, you will see that this adds one more variable, @value. In its implementation of initialize, it passes the two arguments, aName and aDescription, to its superclass, Thing2. In its turn, as you’ve already seen, Thing2’s initialize method passes these same arguments to the initialize method of its superclass, Thing.

With the program running, enter 3 at the prompt to view the output. The following code will execute:

t3 = Thing3.new("A Thing3", "a Thing3 full of Thing and Thing2iness",500)
t3.aMethod( "A New Thing3", "and a new Thing3 description",1000)

Note how the flow of execution goes right up the hierarchy so that code in the initialize and aMethod methods of Thing execute before code in the matching methods of Thing2 and Thing3.

It is not obligatory to override a superclass’s methods as I have done in the examples so far. This is required only when you want to add some new behavior. Thing4 omits the initialize method but implements the aMethod method.

Enter 4 at the prompt to execute the following code:

t4 = Thing4.new( "A Thing4", "the nicest Thing4 you will ever see", 10 )
t4.aMethod

When you run it, notice that the first available initialize method is called when a Thing4 object is created. This happens to be Thing3.initialize, which, once again, also calls the initialize methods of its ancestor classes, Thing2 and Thing. However, the aMethod method implemented by Thing4 contains no call to its superclasses, so this executes right away, and the code in any other aMethod methods in the ancestor classes is ignored:

def aMethod
    puts("Thing4.aMethod: #{self.inspect}

")
end

Finally, Thing5 inherits from Thing4 and doesn’t introduce any new data or methods. Enter 5 at the prompt to execute the following:

t5 = Thing5.new( "A Thing5", "a very simple Thing5", 40 )
t5.aMethod

This time, you will see that the call to new causes Ruby to backtrack through the class hierarchy until it finds the first initialize method. This happens to belong to Thing3 (which also calls the initialize methods of Thing2 and Thing). The first implementation of aMethod, however, occurs in Thing4, and there are no calls to super, so that’s where the trail ends.

The Root of All Classes

As I mentioned earlier, all our Ruby classes will ultimately descend from the Object class. You may think of Object as the “root” or “base” class of the Ruby hierarchy. In Ruby 1.8 this is literally true—there are no classes from which Object itself descends. In Ruby 1.9, however, Object is derived from a new class called BasicObject. This new class was created to provide programmers with a very lightweight class—one that supplies only the bare minimum of methods for creating objects, testing equality, and manipulating special methods called singletons. (I’ll talk more about singletons in Chapter 7.)

The Ruby 1.9 Object class inherits the methods from BasicObject and adds a number of new methods of its own. BasicObject does not exist in Ruby 1.8, and the Object class supplies all the methods provided by the combination of BasicObject and Object in Ruby 1.9. Since all normal Ruby classes—both Ruby 1.8 and Ruby 1.9—descend from Object, you may generally think of Object as being the “root” of all other classes. Just bear in mind that in Ruby 1.9, the ultimate ancestor of all classes is BasicObject.

The root class itself has no superclass, and any attempt to locate its superclass will return nil. You can see this for yourself by running superclasses.rb. This calls the superclass method to climb up the class hierarchy from the Three class to the Object or BasicObject class. At each turn through the loop, the variable x is assigned the class of x’s immediate parent until x equals nil. Here class and superclass are methods that return references to Ruby classes rather than to objects created from those classes. The begin..until block is one of Ruby’s looping constructs, which you’ll look at in more detail in Chapter 5.

superclasses.rb

class One
end

class Two < One
end

class Three < Two
end

# Create ob as instance of class Three
# and display the class name
ob = Three.new
x = ob.class
puts( x )

# now climb back through the hierarchy to
# display all ancestor classes of ob
begin
    x = x.superclass
    puts(x.inspect)
end until x == nil

The previous code displays the following output:

Three
Two
One
Object
BasicObject    # Ruby 1.9 only!
nil

Constants Inside Classes

There may be times when you need to access constants (identifiers beginning with a capital letter, which are used to store nonchanging values) declared inside a class. Let’s assume you have this class:

classconsts.rb

class X
   A = 10

   class Y
   end
end

To access the constant A, you would need to use the special scope resolution operator :: like this:

X::A

Class names are constants, so this same operator gives you access to classes inside other classes. This makes it possible to create objects from “nested” classes such as class Y inside class X:

ob = X::Y.new

Partial Classes

In Ruby it is not obligatory to define a class all in one place. If you want, you can define a single class in separate parts of your program. When a class descends from a specific superclass, each subsequent partial (or open) class definition may optionally repeat the superclass in its definition using the < operator.

Here I create one class, A, and another that descends from it, B:

partial_classes.rb

class A
   def a
      puts( "a" )
   end
end

class B < A
   def ba1
      puts( "ba1" )
   end
end

class A
   def b
      puts( "b" )
   end
end

class B < A
   def ba2
      puts( "ba2" )
   end
end

Now, if I create a B object, all the methods of both A and B are available to it:

ob = B.new
ob.a
ob.b
ob.ba1
ob.ba2

You can also use partial class definitions to add features to Ruby’s standard classes such as Array:

class Array
   def gribbit
      puts( "gribbit" )
   end
end

This adds the gribbit method to the Array class so that the following code can now be executed:

[1,2,3].gribbit