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Class, which contains all the object things plus a list of methods and a reference to a superclass (which is itself another class). All method calls in Ruby nominate a receiver (which is by default self, the current object). Ruby finds the method to invoke by looking at the list of methods in the receiver's class. If it doesn't find the method there, it looks in the superclass, and then in the superclass's superclass, and so on. If the method cannot be found in the receiver's class or any of its ancestors, Ruby invokes the method method_missing on the original receiver.
And that's it---the entire explanation. On to the next chapter.
``But wait,'' you cry, ``I spent good money on this chapter. What about all this other stuff---singleton classes, class methods, and so on. How do they work?''
Good question.
lucille, the object's class, Guitar, and that class's superclass, Object. Notice how the object's class reference (called klass for historical reasons that really bug Andy) points to the class object, and how the super pointer from that class references the parent class.
| Figure not available... |
Guitar.strings(), it follows the same process as before: it goes to the receiver, class Guitar, follows the klass reference to class Guitar$'$, and finds the method.
Finally, note that an ``S'' has crept into the flags in class Guitar$'$. The classes that Ruby creates automatically are marked internally as singleton classes. Singleton classes are treated slightly differently within Ruby. The most obvious difference from the outside is that they are effectively invisible: they will never appear in a list of objects returned from methods such as Module#ancestors or ObjectSpace::each_object .
String objects. We then associate an anonymous class with one of them, overriding one of the methods in the object's base class and adding a new method.
a = "hello" | ||
b = a.dup | ||
| ||
class <<a | ||
def to_s | ||
"The value is '#{self}'" | ||
end | ||
def twoTimes | ||
self + self | ||
end | ||
end | ||
| ||
a.to_s |
» | "The value is 'hello'" |
a.twoTimes |
» | "hellohello" |
b.to_s |
» | "hello" |
class << obj'' notation, which basically says ``build me a new class just for object obj.'' We could also have written it as:
a = "hello" | ||
b = a.dup | ||
def a.to_s | ||
"The value is '#{self}'" | ||
end | ||
def a.twoTimes | ||
self + self | ||
end | ||
| ||
a.to_s |
» | "The value is 'hello'" |
a.twoTimes |
» | "hellohello" |
b.to_s |
» | "hello" |
a''. This gives us a strong hint about the Ruby implementation: a singleton class is created and inserted as a's direct class. a's original class, String, is made this singleton's superclass. The before and after pictures are shown in Figure 19.3 on page 242.
Ruby performs a slight optimization with these singleton classes. If an object's klass reference already points to a singleton class, a new one will not be created. This means that the first of the two method definitions in the previous example will create a singleton class, but the second will simply add a method to it.
| Figure not available... |
module SillyModule | ||
def hello | ||
"Hello." | ||
end | ||
end | ||
class SillyClass | ||
include SillyModule | ||
end | ||
s = SillyClass.new | ||
s.hello |
» | "Hello." |
module SillyModule | ||
def hello | ||
"Hi, there!" | ||
end | ||
end | ||
s.hello |
» | "Hi, there!" |
| Figure not available... |
class <<obj '', you can mix a module into an object using Object#extend . For example:
module Humor | ||
def tickle | ||
"hee, hee!" | ||
end | ||
end | ||
| ||
a = "Grouchy" | ||
a.extend Humor | ||
a.tickle |
» | "hee, hee!" |
extend. If you use it within a class definition, the module's methods become class methods.
module Humor | ||
def tickle | ||
"hee, hee!" | ||
end | ||
end | ||
| ||
class Grouchy | ||
include Humor | ||
extend Humor | ||
end | ||
| ||
Grouchy.tickle |
» | "hee, hee!" |
a = Grouchy.new | ||
a.tickle |
» | "hee, hee!" |
extend is equivalent to self.extend, so the methods are added to self, which in a class definition is the class itself.
class MediaPlayer include Tracing if $DEBUGGING if ::EXPORT_VERSION def decrypt(stream) raise "Decryption not available" end else def decrypt(stream) # ... end end end |
self must reference something. Let's find out what it is.
class Test
puts "Type of self = #{self.type}"
puts "Name of self = #{self.name}"
end
|
Type of self = Class Name of self = Test |
class Test
def Test.sayHello
puts "Hello from #{name}"
end
sayHello
end
|
Hello from Test |
Test.sayHello, and then call it in the body of the class definition. Within sayHello, we call name, an instance method of class Module. Because Module is an ancestor of Class, its instance methods can be called without an explicit receiver within a class definition.
In fact, many of the directives that you use when defining a class or module, things such as alias_method, attr, and public, are simply methods in class Module. This opens up some interesting possibilities---you can extend the functionality of class and module definitions by writing Ruby code. Let's look at a couple of examples.
As a first example, let's look at adding a basic documentation facility to modules and classes. This would allow us to associate a string with modules and classes that we write, a string that is accessible as the program is running. We'll choose a simple syntax.
class Example doc "This is a sample documentation string" # .. rest of class end |
doc available to any module or class, so we need to make it an instance method of class Module.
class Module
@@docs = Hash.new(nil)
def doc(str)
@@docs[self.name] = str
end
def Module::doc(aClass)
# If we're passed a class or module, convert to string
# ('<=' for classes checks for same class or subtype)
aClass = aClass.name if aClass.type <= Module
@@docs[aClass] || "No documentation for #{aClass}"
end
end
class Example
doc "This is a sample documentation string"
# .. rest of class
end
module Another
doc <<-edoc
And this is a documentation string
in a module
edoc
# rest of module
end
puts Module::doc(Example)
puts Module::doc("Another")
|
This is a sample documentation string And this is a documentation string in a module |
date module (described beginning on page 439). Say we have a class that represents some underlying quantity (in this case, a date). The class may have many attributes that present the same underlying date in different ways: as a Julian day number, as a string, as a [year, month, day] triple, and so on. Each value represents the same date and may involve a fairly complex calculation to derive. We therefore would like to calculate each attribute only once, when it is first accessed.
The manual way would be to add a test to each accessor:
class ExampleDate def initialize(dayNumber) @dayNumber = dayNumber end def asDayNumber @dayNumber end def asString unless @string # complex calculation @string = result end @string end def asYMD unless @ymd # another calculation @ymd = [ y, m, d ] end @ymd end # ... end |
once modifier for routines. We'd like to be able to write something like:
class ExampleDate def asDayNumber @dayNumber end def asString # complex calculation end def asYMD # another calculation [ y, m, d ] end once :asString, :asYMD end |
once as a directive by writing it as a class method of ExampleDate, but what should it look like internally? The trick is to have it rewrite the methods whose names it is passed. For each method, it creates an alias for the original code, then creates a new method with the same name. This new method does two things. First, it invokes the original method (using the alias) and stores the resulting value in an instance variable. Second, it redefines itself, so that on subsequent calls it simply returns the value of the instance variable directly. Here's Tadayoshi Funaba's code, slightly reformatted.
def ExampleDate.once(*ids)
for id in ids
module_eval <<-"end_eval"
alias_method :__#{id.to_i}__, #{id.inspect}
def #{id.id2name}(*args, &block)
def self.#{id.id2name}(*args, &block)
@__#{id.to_i}__
end
@__#{id.to_i}__ = __#{id.to_i}__(*args, &block)
end
end_eval
end
end
|
module_eval to execute a block of code in the context of the calling module (or, in this case, the calling class). The original method is renamed __nnn__, where the nnn part is the integer representation of the method name's symbol id. The code uses the same name for the caching instance variable. The bulk of the code is a method that dynamically redefines itself. Note that this redefinition uses the fact that methods may contain nested singleton method definitions, a clever trick.
Understand this code, and you'll be well on the way to true Ruby mastery.
However, we can take it further. Look in the date module, and you'll see method once written slightly differently.
class Date class << self def once(*ids) # ... end end # ... end |
class << self''. This defines a class based on the object self, and self happens to be the class object for Date. The result? Every method within the inner class definition is automatically a class method of Date.
The once feature is generally applicable---it should work for any class. If you took once and made it a private instance method of class Module, it would be available for use in any Ruby class.
Class object itself. When you say something such as String.new("gumby"), you're sending the message new to the object that is class String. But how does Ruby know to do this? After all, the receiver of a message should be an object reference, which implies that there must be a constant called ``String'' somewhere containing a reference to the String object.[It will be a constant, not a variable, because ``String'' starts with an uppercase letter.] And in fact, that's exactly what happens. All the built-in classes, along with the classes you define, have a corresponding global constant with the same name as the class. This is both straightforward and subtle. The subtlety comes from the fact that there are actually two things named (for example) String in the system. There's a constant that references an object of class String, and there's the object itself.
The fact that class names are just constants means that you can treat classes just like any other Ruby object: you can copy them, pass them to methods, and use them in expressions.
def factory(klass, *args) | ||
klass.new(*args) | ||
end | ||
| ||
factory(String, "Hello") |
» | "Hello" |
factory(Dir, ".") |
» | #<Dir:0x401b51bc> |
| ||
flag = true | ||
(flag ? Array : Hash)[1, 2, 3, 4] |
» | [1, 2, 3, 4] |
flag = false | ||
(flag ? Array : Hash)[1, 2, 3, 4] |
» | {1=>2, 3=>4} |
puts "Hello, World" |
"Hello, World" generates a Ruby String, so there's one object. We also know that the bare method call to puts is effectively the same as self.puts. But what is ``self''?
self.type |
» | Object |
Object, we can use all of Object's methods (including those mixed-in from Kernel) in function form. This explains why we can call Kernel methods such as puts at the top level (and indeed throughout Ruby): these methods are part of every object.
class Base def aMethod puts "Got here" end private :aMethod end class Derived1 < Base public :aMethod end class Derived2 < Base end |
aMethod in instances of class Derived1, but not via instances of Base or Derived2.
So how does Ruby pull off this feat of having one method with two different visibilities? Simply put, it cheats.
If a subclass changes the visibility of a method in a parent, Ruby effectively inserts a hidden proxy method in the subclass that invokes the original method using super. It then sets the visibility of that proxy to whatever you requested. This means that the code:
class Derived1 < Base public :aMethod end |
class Derived1 < Base def aMethod(*args) super end public :aMethod end |
super can access the parent's method regardless of its visibility, so the rewrite allows the subclass to override its parent's visibility rules. Pretty scary, eh?
Object#freeze . A frozen object may not be modified: you can't change its instance variables (directly or indirectly), you can't associate singleton methods with it, and, if it is a class or module, you can't add, delete, or modify its methods. Once frozen, an object stays frozen: there is no Object#thaw . You can test to see if an object is frozen using Object#frozen? .
What happens when you copy a frozen object? That depends on the method you use. If you call an object's clone method, the entire object state (including whether it is frozen) is copied to the new object. On the other hand, dup typically copies only the object's contents---the new copy will not inherit the frozen status.
str1 = "hello" | ||
str1.freeze |
» | "hello" |
str1.frozen? |
» | true |
str2 = str1.clone | ||
str2.frozen? |
» | true |
str3 = str1.dup | ||
str3.frozen? |
» | false |
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