Overview
Class eval and Instance eval sometimes confuse me so I want to make an article to understand them better. The confusing part was their naming. they create exactly opposite things to what they mean by the class_eval and instance_eval. here are more clear examples: I
ClassName.class_eval = define an instance method
It’s used to define an instance method on a class object which applies on all the instances of the class.
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class MyClass
end
MyClass.class_eval do
def say_hello
"Hello!"
end
end
class = MyClass.new
class.say_hello # "Hello!"
ClassName.instance_eval = ` define a class method`
It’s used to define a class method that is associated with a class object but it’s not visible to instance objects of that class.
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class MyClass
end
MyClass.instance_eval do
def human?
true
end
end
MyClass.human? # true
Why it is so confusing?
This example of a instance_eval is similar, but evaluates code in the context of an instance instead of a class. This is confusing at first because class_eval creates instance methods and instance_eval creates class methods in this example. But there is a reason behind the insaneness.
class_eval is a method of the Module class. That is, the receiver is either a module or a class. Blocks you pass to class_eval are evaluated in the context of that class. When you define a method using the standard def keyword within a class, you define an instance method, and that’s where it happens.
On the other hand, instance_eval is a method of class Object, meaning that the receiver will be an object. Blocks you pass to instance_eval are evaluated in the context of that object. This means that Person.instance_eval is evaluated in the context of the Person object. Remember that a class name is simply a constant pointing to an instance of a class class. Because of this, defining a method in the context of a Class instance referenced by Person creates a class method for the Person class.
source: https://www.jimmycuadra.com/posts/metaprogramming-ruby-class-eval-and-instance-eval/
