When to use self for Ruby Class - ruby

Can somebody help me distinguish When we create methods inside class << self block and when we define normal methods.
I saw somewhere code like this, but I don't know concisely the use cases of them
class Foo
def initialize
end
def bar
end
class << self
def foobar
end
end
end

The methods defined right inside a class block are instance methods:
class Foo
def bar
end
end
Methods defined within class << self inside a class block are class methods:
class Foo
class << self
def baz
end
end
end
Instance methods become available to any instance of a given class:
foo = Foo.new
foo.bar
Whereas class methods can be called directly on the class:
Foo.baz
Attempting to call instance methods on the class or vice versa results in an error:
Foo.bar #=> NoMethodError: undefined method `bar' for Foo:Class
foo.baz #=> NoMethodError: undefined method `baz' for #<Foo:0x00007ffe20055a20>
Another way to define class methods is by prefixing the method name with self.:
class Foo
def self.baz
end
end
You could also define them outside the class block, although this is rarely seen:
def Foo.baz
end
Or likewise:
class << Foo
def baz
end
end
Note that defining methods this way is not limited to classes. You can add methods to arbitrary objects, e.g.:
o = Object.new
def o.hello
"hello from o"
end
o.hello
#=> "hello from o"
Or via:
class << o
def hello
"hello from o"
end
end
Internally, these methods are added to the object's singleton class. It's a special purpose class to hold methods for just that instance:
o.singleton_class.instance_methods(false)
#=> [:hello]
For the Foo class above:
Foo.instance_methods(false) #=> [:bar]
Foo.singleton_class.instance_methods(false) #=> [:baz]
So technically, a class method is just an instance method defined on the class' singleton class.

You may need to read up on Ruby's instance and class methods.
But personally, I'd do
class Foo
class << self
def foobar
end
end
end
instead of
class Foo
def self.foobar
end
end
whenever I want to add some class level attributes, or make a method private etc as
class Foo
private
def self.foobar
end
end
wouldn't work the same as
class Foo
class << self
private
def foobar
end
end
end

Related

Is it possible to define class methods within `class_eval`?

I know it's possible to define instance methods using class_eval. Is it possible to define class methods within the context of class_eval?
Yes, it is possible:
class Foo
end
Foo.class_eval do
def self.bar
puts "I'm a class method defined using class_eval and self"
end
def baz
puts "I'm an instance method defined using class_eval without self"
end
end
Foo.bar # => "I'm a class method defined using class_eval and self"
foo = Foo.new
foo.baz # => "I'm an instance method defined using class_eval without self"
As far as I can tell, this is because within class_eval, self is the Foo class, and doing def Foo.bar would create a class method.
Foo.class_eval do
...
end
is identical to:
class Foo
...
end
We need Module#class_eval to operate on a variable that holds the name of a class. For example, if:
klass = Foo
you can write:
klass.class_eval do
...
end
whereas the keyword class demands a constant.
Here class_eval does two things:
it changes self to the value of klass (Foo); and then it
"opens" the value of klass (Foo) in the same way the keyword class does.

Can the Ruby 2.1 shorthand for private/protected be used for class methods?

In Ruby 2.1, def now returns a symbol
[1] pry(main)> def foo; end
=> :foo
One cool use case of this is that because private and protected are methods that take a symbol and make the method private, you can now create a private method like so:
private def foo
end
However, I can't get this to work with class methods. This code:
protected def self.baz
end
will error with: protected': undefined method 'baz' for class 'User' (NameError)".
Is there a way to get that working?
You can achieve that by using singleton class of your class:
class Foo
def self.baz
...
end
class << self
private :baz
end
end
or in a single attempt:
class Foo
class << self
private def baz
...
end
end
end
So, everything executed in a class << self block will be applied on a class level. Resulting in a private/protected class methods.
private is a method used to mark instance methods as private. The equivalent for class methods is private_class_method so the equivalent idiom would be the somewhat unwieldy and redundant:
private_class_method def self.foo
#...
end

How do I extend a Ruby module in class context?

I have the following Ruby script, in which class Foo includes module Baz, module Baz has a included hook to make Bar extended by the including class (Foo). I am just wondering why:
class << klass
extend Bar #this doesn't work. Why?
end
does not work while:
klass.extend(Bar) works.
Here is my code:
#! /usr/bin/env ruby
module Baz
def inst_method
puts "dude"
end
module Bar
def cls_method
puts "Yo"
end
end
class << self
def included klass
# klass.extend(Bar) this works, but why the other approach below doesn't?
class << klass
extend Bar #this doesn't work. Why?
def hello
puts "hello"
end
end
end
end
end
class Foo
include Baz
end
foo = Foo.new
foo.inst_method
Foo.hello
Foo.cls_method
Within the body of class << klass, self refers to the singleton class of klass, not klass itself, whereas in klass.extend(Bar), the receiver is klass itself. The difference comes from there.
class A
end
class << A
p self # => #<Class:A> # This is the singleton class of A, not A itself.
end
p A # => A # This is A itself.
And since you want to apply extend to klass, doing it within the body of class << klass does not work.
What you want is invoke the extend method on the class object (klass) not the singleton class (class << klass).
Therefore the following code doesn't work because you are invoking the extend method on the singleton class:
class << klass
extend Bar # doesn't work because self refers to the the singleton class of klass
def hello
puts "hello"
end
end

Ruby Class Methods vs. Methods in Eigenclasses

Are class methods and methods in the eigenclass (or metaclass) of that class just two ways to define one thing?
Otherwise, what are the differences?
class X
# class method
def self.a
"a"
end
# eigenclass method
class << self
def b
"b"
end
end
end
Do X.a and X.b behave differently in any way?
I recognize that I can overwrite or alias class methods by opening the eigenclass:
irb(main):031:0> class X; def self.a; "a"; end; end
=> nil
irb(main):032:0> class X; class << self; alias_method :b, :a; end; end
=> #<Class:X>
irb(main):033:0> X.a
=> "a"
irb(main):034:0> X.b
=> "a"
irb(main):035:0> class X; class << self; def a; "c"; end; end; end
=> nil
irb(main):036:0> X.a
=> "c"
The two methods are equivalent. The 'eigenclass' version is helpful for using the attr_* methods, for example:
class Foo
#instances = []
class << self;
attr_reader :instances
end
def initialize
self.class.instances << self
end
end
2.times{ Foo.new }
p Foo.instances
#=> [#<Foo:0x2a3f020>, #<Foo:0x2a1a5c0>]
You can also use define_singleton_method to create methods on the class:
Foo.define_singleton_method :bim do "bam!" end
In Ruby there really are no such things as class methods. Since everything is an object in Ruby (including classes), when you say def self.class_method, you are just really defining a singleton method on the instance of the class Class. So to answer your question, saying
class X
def self.a
puts "Hi"
end
class << self
def b
puts "there"
end
end
end
X.a # => Hi
X.b # => there
is two ways of saying the same thing. Both these methods are just singeton (eigen, meta, ghost, or whatever you want to call them) methods defined in the instance of your Class object, which in your example was X. This topic is part of metaprogramming, which is a fun topic, that if you have been using Ruby for a while, you should check out.
The Pragmatic Programmers have a great book on metaprogramming that you should definitely take a look at if you interested in the topic.
Yet another necromancer here to unearth this old question... One thing you might not be aware of is that marking a class method as private (using the private keyword instead of :private_class_method) is different than marking an eigenclass method as such. :
class Foo
class << self
def baz
puts "Eigenclass public method."
end
private
def qux
puts "Private method on eigenclass."
end
end
private
def self.bar
puts "Private class method."
end
end
Foo.bar
#=> Private class method.
Foo.baz
#=> Eigenclass public method.
Foo.qux
#=> NoMethodError: private method `qux' called for Foo:Class
# from (irb)
The following example will work how the previous one intends:
class Foo
class << self
def baz
puts "Eigen class public method."
end
private
def qux
puts "Private method on eigenclass."
end
end
def bar
puts "Private class method."
end
private_class_method :bar
end
Foo.bar
#=> NoMethodError: private method `bar' called for Foo:Class
# from (irb)
Foo.baz
#=> Eigen class public method.
Foo.qux
#=> NoMethodError: private method `qux' called for Foo:Class
# from (irb)
Most instance methods used in Ruby are global methods. That means they are available in all instances of the class on which they were defined. In contrast, a singleton method is implemented on a single object.
There is an apparent contradiction. Ruby stores methods in classes and all methods must be associated with a class. The object on which a singleton method is defined is not a class (it is an instance of a class). If only classes can store methods, how can an object store a singleton method? When a singleton method is created, Ruby automatically creates an anonymous class to store that method. These anonymous classes are called metaclasses, also known as singleton classes or eigenclasses. The singleton method is associated with the metaclass which, in turn, is associated with the object on which the singleton method was defined.
If multiple singleton methods are defined within a single object, they are all stored in the same metaclass.
class Zen
end
z1 = Zen.new
z2 = Zen.new
def z1.say_hello # Notice that the method name is prefixed with the object name
puts "Hello!"
end
z1.say_hello # Output: Hello!
z2.say_hello # Output: NoMethodError: undefined method `say_hello'…
In the above example, the say_hello method was defined within the z1 instance of the Zen class but not the z2 instance.
The following example shows a different way to define a singleton method, with the same result.
class Zen
end
z1 = Zen.new
z2 = Zen.new
class << z1
def say_hello
puts "Hello!"
end
end
z1.say_hello # Output: Hello!
z2.say_hello # Output: NoMethodError: undefined method `say_hello'…
In the above example, class << z1 changes the current self to point to the metaclass of the z1 object; then, it defines the say_hello method within the metaclass.
Both of the above examples serve to illustrate how singleton methods work. There is, however, an easier way to define a singleton method: using a built-in method called define_singleton_method.
class Zen
end
z1 = Zen.new
z2 = Zen.new
z1.define_singleton_method(:say_hello) { puts "Hello!" }
z1.say_hello # Output: Hello!
z2.say_hello # Output: NoMethodError: undefined method `say_hello'…
We learned earlier that classes are also objects (instances of the built-in class called Class). We also learned about class methods. Class methods are nothing more than singleton methods associated with a class object.
One more example:
class Zabuton
class << self
def stuff
puts "Stuffing zabuton…"
end
end
end
All objects may have metaclasses. That means classes can also have metaclasses. In the above example, class << self modifies self so it points to the metaclass of the Zabuton class. When a method is defined without an explicit receiver (the class/object on which the method will be defined), it is implicitly defined within the current scope, that is, the current value of self. Hence, the stuff method is defined within the metaclass of the Zabuton class. The above example is just another way to define a class method.
Read more at this post about Ruby Classes.

Remove/undef a class method

You can dynamically define a class method for a class like so:
class Foo
end
bar = %q{def bar() "bar!" end}
Foo.instance_eval(bar)
But how do you do the opposite: remove/undefine a class method? I suspect Module's remove_method and undef_method methods might be able to be used for this purpose, but all of the examples I've seen after Googling for hours have been for removing/undefining instance methods, not class methods. Or perhaps there's a syntax you can pass to instance_eval to do this as well.
Thanks in advance.
class Foo
def self.bar
puts "bar"
end
end
Foo.bar # => bar
class <<Foo
undef_method :bar
end
# or
class Foo
singleton_class.undef_method :bar
end
Foo.bar # => undefined method `bar' for Foo:Class (NoMethodError)
When you define a class method like Foo.bar, Ruby puts it Foo's singleton class. Ruby can't put it in Foo, because then it would be an instance method. Ruby creates Foo's singleton class, sets the superclass of the singleton class to Foo's superclass, and then sets Foo's superclass to the singleton class:
Foo -------------> Foo(singleton class) -------------> Object
super def bar super
There are a few ways to access the singleton class:
class <<Foo,
Foo.singleton_class,
class Foo; class << self which is commonly use to define class methods.
Note that we used undef_method, we could have used remove_method. The former prevents any call to the method, and the latter only removes the current method, having a fallback to the super method if existing. See Module#undef_method for more information.
This also works for me (not sure if there are differences between undef and remove_method):
class Foo
end
Foo.instance_eval do
def color
"green"
end
end
Foo.color # => "green"
Foo.instance_eval { undef :color }
Foo.color # => NoMethodError: undefined method `color' for Foo:Class
You can remove a method in two easy ways. The drastic
Module#undef_method( )
removes all methods, including the inherited ones. The kinder
Module#remove_method( )
removes the method from the receiver, but it
leaves inherited methods alone.
See below 2 simple example -
Example 1 using undef_method
class A
def x
puts "x from A class"
end
end
class B < A
def x
puts "x from B Class"
end
undef_method :x
end
obj = B.new
obj.x
result -
main.rb:15:in
': undefined methodx' for # (NoMethodError)
Example 2 using remove_method
class A
def x
puts "x from A class"
end
end
class B < A
def x
puts "x from B Class"
end
remove_method :x
end
obj = B.new
obj.x
Result -
$ruby main.rb
x from A class
I guess I can't comment on Adrian's answer because I don't have enough cred, but his answer helped me.
What I found: undef seems to completely remove the method from existence, while remove_method removes it from that class, but it will still be defined on superclasses or other modules that have been extened on this class, etc.
If you would like to remove method with name what calculate dinamically, you should use eigenclasses like:
class Foo
def self.bar
puts "bar"
end
end
name_of_method_to_remove = :bar
eigenclass = class << Foo; self; end
eigenclass.class_eval do
remove_method name_of_method_to_remove
end
this way is better than others answers, becouse here i used class_eval with block. As you now block see current namespace, so you could use your variables to remove methods dinamically
Object.send(:remove_const, :Foo)

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