I am reading Metaprogramming in Ruby book. In that book, when I was reading about scopegates, the following code was shown
my_var = "Success"
MyClass = Class.new do
puts "#{my_var} in the class definition"
define_method :my_method do
puts "#{my_var} in the method"
end
end
MyClass.new.my_method
=>Success in the class definition
Success in the method
Now when I do the execute in the following, I get an error saying undefined method my_method
MyClass.new
MyClass:0x00000100936a30
MyClass.my_method
NoMethodError: undefined method `my_method' for MyClass:Class
from (irb):11
from /usr/local/bin/irb:12:in `<main>'
Why my_method gets created, when it gets called as MyClass.new.my_method and not MyClass.my_method?
The define_method method adds an instance method to a class. When you call define_method in your anonymous class, your method is being added as an instance method. To add it as a class method, you have to add it to the metaclass.
The easiest way to do that is with the class << self syntax.
MyClass = Class.new do
class << self
define_method(:my_method) do
puts "class method"
end
end
end
MyClass.my_method # => "class method"
Related
class_eval and instance_eval are quite predictable in such cases like defining methods. I also understand the difference between class's instance and class's singleton (aka eigenclass).
BUT
I cannot figure out the only thing like following:
Let's say, for some strage purposes, we want make existing class to be singleton.
class A; end
class B; end
A.class_eval do
private :new
end
B.instance_eval do
private :new
end
in both cases got
NameError: undefined method 'new' for class
Did you mean? new
yes, I mean exactly this method.
Moreover, these two variants give the same result, like self points at class object in both cases
A.class_eval do
class << self
private :new
end
end
A.new
=> NoMethodError: private method 'new' called for A:Class
B.instance_eval do
class << self
private :new
end
end
B.new
=> NoMethodError: private method 'new' called for B:Class
How come? Can anybody shed the light on this?
Lets take a peek into what self is here:
class A
puts self.inspect
class << self
puts self.inspect
end
end
A.class_eval {
puts self.inspect
class << self
puts self.inspect
end
}
A.instance_eval{
puts self.inspect
class << self
puts self.inspect
end
}
We get the following output:
A
#<Class:A>
A
#<Class:A>
A
#<Class:A>
The class_eval method is defined for modules (and thus classes) and evaluates within the context of the module (class). The instance_eval method evaluates within the context of a BasicObject. It seems that in these cases the two (three actually) are the same thing.
However, I know for a fact that if methods are created inside the eval block that class_eval creates instance methods and instance_eval creates class methods. There is already an excellent posting for that observation:
In the following code, the module is extended which means the method hash_initialized is treated as a class method, or an instance method of the eigen class. This is what we need since hash_initialized is called in the context of the eigen class.
What i don't understand is if the context for the module is the eigen class, then define_method should create a instance method called "initialize" of the eigen class, or in other words, a class method of class Cheese. Don't we need an instance method "initialize" here?
module HashInitialized
def hash_initialized(*fields)
define_method(:initialize) do |h|
missing = fields - h.keys
raise Exception, "Not all fields set: #{missing}" if missing.any?
h.each do |k,v|
instance_variable_set("##{k}", v) if fields.include?(k)
end
end
end
end
class Cheese
extend HashInitialized
attr_accessor :color, :odor, :taste
hash_initialized :color, :odor, :taste
end
Calling extend technically puts the module in the lookup chain of the eigen object it is called on, which in this case is the same as the class object. So you are correct that the context for hash_initialized is the class. Additionally, you are correct that the context for define_method is the class. However, your final step is incorrect. When define_method is called in that context it defines an instance method, not a singleton method.
IOW, when you call define_method in a context, it defines the method in the same place that def would define it at that context.
When you encounter seeming conundrums such as this one, try salting your code with puts self statements:
module HashInitialized
puts "self when parsed=#{self}"
def hash_initialized(*fields)
puts "self within hash_initialized=#{self}"
define_method(:initialize) do |h|
missing = fields - h.keys
raise ArgumentError, "Not all fields set: #{missing}" if missing.any?
fields.each { |k| instance_variable_set("##{k}", h[k]) }
end
private :initialize
end
end
#-> self when parsed=HashInitialized
class Cheese
extend HashInitialized
attr_accessor :color, :odor, :taste
hash_initialized :color, :odor, :taste
end
#-> self within hash_initialized=Cheese
As you see, self is the class Cheese, not Cheese's singleton_class. Hence, the receiver for Module#define_method is Cheese, so the method obligingly creates the instance method initialize on Cheese.
Cheese.instance_methods(false)
#=> [:color, :color=, :odor, :odor=, :taste, :taste=]
initialize is not among the instance methods created on Cheese because I modified the code slightly to make it a private method:
Cheese.private_instance_methods(false)
#=> [:initialize]
I also slightly altered the code that assigns values to the instance variables, and made the type of exception more specific.
If appropriate, you could change your argument test to:
raise ArgumentError, "Fields #{fields} and keys #{h.keys} don't match" if
(fields-h.keys).any? || (h.keys-fields).any?
You may wish to have initialize create the assessors:
module HashInitialized
def hash_initialized(*fields)
define_method(:initialize) do |h|
missing = fields - h.keys
raise ArgumentError, "Not all fields set: #{missing}" if missing.any?
fields.each do |k|
instance_variable_set("##{k}", h[k])
self.class.singleton_class.send(:attr_accessor, k)
end
end
private :initialize
end
end
class Cheese
extend HashInitialized
hash_initialized :color, :odor, :taste
end
Cheese.new :color=>'blue', odor: 'whew!', taste: "wow!"
=> #<Cheese:0x007f97fa07d2a0 #color="blue", #odor="whew!", #taste="wow!">
I've made this clear by simplifying the example above and added a few printouts.
class Test
def self.define_something
define_method(:inside_class_method){puts "method defined inside a class method"}
puts "self inside class method "+self.to_s
proc = Proc.new{puts "method defined using send inside class method"}
self.send(:define_method, :define_using_send_inside_class_method, proc)
end
class << self
puts "self inside eigen class "+self.to_s
end
def test
puts "self inside of instance method "+self.to_s
end
puts "self outside of class method "+self.to_s
define_method(:outside_class_method){puts "method defined outside a class method"}
define_something
end
Test.new().inside_class_method
Test.new().outside_class_method
Test.new().test
Test.define_using_send_inside_class_method
This code produce the following outputs:
self inside eigen class #
self outside of class method Test
self inside class method Test
method defined inside a class method
method defined outside a class method
self inside of instance method #
test.rb:26:in <main>': undefined methoddefine_using_send_inside_class_method' for Test:Class (NoMethodError)
This code:
self.send(:define_method, :define_using_send_inside_class_method, proc)
It is also defining an instance method since it is called on self, and self is referring to the class Test.
If we need to define a class method, send needs to be called on the eigen class like this:
class << self
self.send(:define_method, :define_using_send_inside_class_method, proc)
end
Below is my snippet which i tried,
class Person
def test(arg)
self.class.instance_eval do
define_method(arg) {puts "this is a class method"}
end
end
end
irb(main):005:0> Person.new.test("foo")
=> #<Proc:0x9270b60#/home/kranthi/Desktop/method_check.rb:4 (lambda)>
irb(main):003:0> Person.foo
NoMethodError: undefined method `foo' for Person:Class
irb(main):007:0> Person.new.foo
this is a class method
=> nil
Here am adding a method to the Person class dynamically using instance_eval and define_method. But why is this behaving as instance method?.
Is that completely dependent on self?
Confused. Can anyone explain me or a reference link also appreciated.
It's because define_method defines instance method of the receiver. Your receiver (self) is Person class, so it defines instance method of Person class. You can achieve what you want accessing to Person's metaclass:
def test(arg)
class << self.class
define_method(arg) { puts 'this is a class method' }
end
end
I haven't tested it, but it should work.
I am writing a class method to create another class method. There seems to be some strangeness around how class_eval and instance_eval operate within the context of a class method. To illustrate:
class Test1
def self.add_foo
self.class_eval do # does what it says on the tin
define_method :foo do
puts "bar"
end
end
end
end
Test1.add_foo # creates new instance method, like I'd expect
Test1.new.foo # => "bar"
class Test2
def self.add_foo
self.instance_eval do # seems to do the same as "class_eval"
define_method :foo do
puts "bar"
end
end
end
end
Test2.add_foo # what is happening here?!
Test2.foo # => NoMethodError
Test2.new.foo # => "bar"
class Test3
def self.add_foo
(class << self; self; end).instance_eval do # call explicitly on metaclass
define_method :foo do
puts "bar"
end
end
end
end
Test3.add_foo # => creates new class method, as I'd expect
Test3.foo # => "bar"
My understanding is that class methods are instance methods defined on the metaclass of the class in question (Test2 in this case). Based on that logic, I would expect the receiver for the class method call add_foo to be the metaclass.
What is self referring to inside the Test2.add_foo method?
Why does calling instance_eval on this receiver object create an instance method?
The main difference between instance_eval and class_eval is that instance_eval works within the context of an instance, while class_eval works within the context of a class. I am not sure how familiar you are with Rails, but let's look at this for an example:
class Test3 < ActiveRecord::Base
end
t = Test3.first
t.class_eval { belongs_to :test_25 } #=> Defines a relationship to test_25 for this instance
t.test_25 #=> Method is defined (but fails because of how belongs_to works)
t2 = Test3.find(2)
t2.test_25 #=> NoMethodError
t.class.class_eval { belongs_to :another_test }
t.another_test #=> returns an instance of another_test (assuming relationship exists)
t2.another_test #=> same as t.another_test
t.class_eval { id } #=> NameError
t.instance_eval { id } #=> returns the id of the instance
t.instance_eval { belongs_to :your_mom } #=> NoMethodError
This happens because belongs_to is actually a method call that happens within the context of the class body, which you cannot call from an instance. When you try to call id with class_eval, it fails because id is a method defined on an instance, not in a class.
Defining methods with both class_eval and instance_eval work essentially the same when called against an instance. They will define a method only on the instance of the object it is called against.
t.class_eval do
def some_method
puts "Hi!"
end
end
t.instance_eval do
def another_method
puts "Hello!"
end
end
t.some_method #=> "Hi!"
t.another_method #=> "Hello!"
t2.some_method #=> NoMethodError
t2.another_method #=> NoMethodError
They differ, however, when dealing with the class.
t.class.class_eval do
def meow
puts "meow!"
end
end
t.class.instance_eval do
def bark
puts "woof!"
end
end
t.meow #=> meow!
t2.meow #=> meow!
t.bark #=> NoMethodError
t2.bark #=> NoMethodError
So where did bark go? It got defined on the instance of the class' singleton class. I'll explain more below. But for now:
t.class.bark #=> woof!
Test3.bark #=> woof!
So to answer your question about what self is referring to within the class body, you can construct a little test:
a = class Test4
def bar
puts "Now, I'm a #{self.inspect}"
end
def self.baz
puts "I'm a #{self.inspect}"
end
class << self
def foo
puts "I'm a #{self.inspect}"
end
def self.huh?
puts "Hmmm? indeed"
end
instance_eval do
define_method :razors do
puts "Sounds painful"
end
end
"But check this out, I'm a #{self.inspect}"
end
end
puts Test4.foo #=> "I'm a Test4"
puts Test4.baz #=> "I'm a Test4"
puts Test4.new.bar #=> Now I'm a #<Test4:0x007fa473358cd8>
puts a #=> But check this out, I'm a #<Class:Test4>
So what is happening here is that in the first puts statement above, we see that inspect is telling us that self within the context of a class method body is referring to the class Test4. In the second puts, we see the same thing, it's just defined differently (using the self.method_name notation for defining class methods). In the third, we see that self refers to an instance of Test4. The last one is a bit interesting because what we see is that self is referring to an instance of Class called Test4. That is because when you define a class, you're creating an object. Everything in Ruby is an object. This instance of object is called the metaclass or the eigenclass or singleton class.
You can access the eigenclass with the class << self idiom. While you're in there, you actually have access to the internals of the eigenclass. You can define instance methods inside of the eigenclass, which is congruent with calling self.method_name. But since you are within the context of the eigenclass, you can attach methods to the eigenclass' eigenclass.
Test4.huh? #=> NoMethodError
Test4.singleton_class.huh? #=> Hmmm? indeed
When you call instance_eval in the context of a method, you're really calling instance_eval on the class itself, which means that you are creating instance methods on Test4. What about where I called instance_eval inside of the eigenclass? It creates a method on the instance of Test4's eigenclass:
Test4.razors #=> Sounds painful
Hopefully, this clears up a few of your questions. I know that I have learned a few things typing out this answer!
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.