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<title>Ruby FAQ:
Classes and modules
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<h2><a name="s6">6.
Classes and modules
</a></h2>
<h2><a name="ss6.1">6.1
Can a class definition be repeated?
</a></h2>
A class can be defined repeatedly and the definition is added to
the former definition. If a method is redefined, the former one
is overridden and lost.
<h2><a name="ss6.2">6.2
Is there a class variable?
</a></h2>
Ruby does not have a class variable. But container classes
(<code>Array</code>, <code>Hash</code>, etc) assigned to a class constant
can be used as a class variable.
<blockquote><code><pre>
class Foo
F = [0]
def foo
F[0] += 1
print F[0], "\n"
end
end
</pre></code></blockquote>
<h2><a name="ss6.3">6.3
What is a class instance variable?
</a></h2>
<blockquote><code><pre>
class Foo
@a = 123 # (1)
def foo
p @a # (2) ... nil not 123
end
end
</pre></code></blockquote>
(1) is a class instance variable, and (2) is an ordinary instance
variable. (2) belongs to an instance of the class <code>Foo</code>, and
(1) belongs to the class object <code>Foo</code>, which is an instance of
<code>Class</code> class.
<br><br>
There is no way to access the class instance variable from the instance
methods.
<br><br>
(2) is an uninitialized instance variable with the value <code>nil</code>.
<a name="singleton_method"></a><h2><a name="ss6.4">6.4
What is a singleton method?
</a></h2>
A singleton method is the proper method of a specified object.
<br><br>
It is used like this.
<blockquote><code><pre>
foo = Foo.new
def foo.hello
print "Hello\n"
end
foo.hello
</pre></code></blockquote>
It is useful when you want to add a method to an object but hesitate
to define a new subclass.
<br><br>
Those who are familiar with Java may think it similar to an anonymous
inner class.
<h2><a name="ss6.5">6.5
Is there a class method?
</a></h2>
A singleton method of a class is called a class method. A singleton
method is described as a proper method of an object, but there is a
characteristic in Ruby which is a metaclass, and every class has a
metaclass of the same name which is an instance of Class class.
A class method is defined in the metaclass.
<br><br>
Formally, a class method is a method whose receiver is a class.
<br><br>
Let's look at a singleton method of <code>Foo</code> which is an instance of
<code>Class</code>.
<blockquote><code><pre>
class Foo
def Foo.test
print "this is foo\n"
end
end
</pre></code></blockquote>
It is invoked this way.
<blockquote><code><pre>
Foo.test
</pre></code></blockquote>
This is a class method. Methods which are defined in <code>Class</code> can
be used as class methods for every class.
<h2><a name="ss6.6">6.6
What is a singleton class?
</a></h2>
A singleton class is a mechanism that enables various class operations
to an object.
<blockquote><code><pre>
class Foo
def hello
print "hello.\n"
end
end
foo = Foo.new
foo.hello
# -> hello.
class << foo
attr :name, TRUE
def hello
print "hello. I'm ", @name, ".\n"
end
end
foo.name = "Tom"
foo.hello
# -> hello. I'm Tom.
</pre></code></blockquote>
Wow, we can add anything to an object.
<br><br>
Here is a quiz for you.
<br><br>
<b>Q.</b> How can you make a class method private without using
<code>private_class_method</code>?
<br><br>
The answer is as follows.
<blockquote><code><pre>
class Foo
...
end
class << Foo
def class_method
print "class method\n"
end
private :class_method
end
Foo.class_method " # -> Error
</pre></code></blockquote>
There are two ways in defining a singleton method, one is to define
in a singleton class, and the other is to define directly using the
form def obj.method.
<br><br>
In a module, you can define a singleton method (at the same time
a private method) by making a method a module function.
<h2><a name="ss6.7">6.7
What is a module function?
</a></h2>
A method in a module which is defined as a singleton method as well as
a private method is called a module function. For example:
<blockquote><code><pre>
Math.sqrt(2)
</pre></code></blockquote>
Or can be used with <code>include</code>.
<blockquote><code><pre>
include Math
sqrt(2)
</pre></code></blockquote>
To make a method a module function, you invoke module_function method.
<blockquote><code><pre>
module_function :method_name
</pre></code></blockquote>
<h2><a name="ss6.8">6.8
What is the difference between a class and a module?
</a></h2>
A module cannot generate an instance. A class cannot be included.
<h2><a name="ss6.9">6.9
Does a module generate a subclass?
</a></h2>
A module is included in a class/module to mimic multiple inheritance
(Mix-in). This does not generate a subclass which is a direct
inheritance, but the class has is_a? relation with the module.
<h2><a name="ss6.10">6.10
What is the difference between defining a class method in the class definition and directly at the toplevel?
</a></h2>
In the class definition you can directly access a constant. At the
toplevel a constant is accessed using the class name dereference.
<h2><a name="ss6.11">6.11
What is the difference between <code>load</code> and <code>require</code>?
</a></h2>
Only a Ruby script (<code>*.rb</code>) is loaded and executed in <code>load</code>.
<br><br>
When <code>require</code>d, <code>*.o</code> file is also looked for. An already
<code>require</code>d file is never loaded again.
<br><br>
The loading path is different.
<h2><a name="ss6.12">6.12
What is the difference between <code>include</code> and <code>extend</code>?
</a></h2>
<code>include</code> is used to include a module to a class/module, and methods
in the module are called in function-style. <code>extend</code> is used to
include a module to an object(instance), and methods in the module
become singleton methods of the object.
<h2><a name="ss6.13">6.13
What does <code>self</code> mean?
</a></h2>
<code>self</code> means the object itself to which a method is applied.
A function form method call implies <code>self</code> as the receiver.
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