class_eval
class_eval allows us to define a method within the class outside of its original definition and without reopening the class with the standard syntax. This could be useful when you want to add this method to is not known until runtime.
class Employee
end
# Employee is evaluated as a class
Employee.class_eval do
def say_hello
puts 'Hello'
end
end
e = Employee.new
e.say_hello
=> Hello
Remember that class_eval evaluates Employee as a class and say_hello method is added to it. So it becomes an instance method.
# String is evaluated as a class
String.class_eval do
def concat_hello
'Hello! ' + self
end
end
'Srinidhi'.concat_hello
=> "Hello! Srinidhi"
Again, class_eval evaluates String as a class. We added a method to String class to concat a string ‘Hello!’.
Note: String is a Ruby built-in class.
instance_eval
instance_eval effectively creates a singleton method for the object instance. That means that Student.instance_eval is evaluated in the context of the Student object.
class Student
end
# Student is evaluated as a instance
Student.instance_eval do
def name
puts 'I am a Student'
end
end
Student.name
=> I am a Student
In the above code, Student is evaluated as a instance. In other words, Student is no more a class, in the context of it’s method, it has become an Object or an instance. So name is the method of Student instance. Therefore it can be accessed as Student.name not as Student.new.name.
class Mentor
end
teacher = Mentor.new
teacher_1 = Mentor.new
# teacher is evaluated as a instance
teacher.instance_eval do
def evaluate
puts 'Yes I evaluate'
end
end
teacher.evaluate
=> Yes I evaluate
teacher_1.evaluate
NoMethodError: undefined method `evaluate' for #
In the above code, teacher is instance of class Mentor. instance_eval evaluates teacher as a instance, we add a method evaluate to that teacher instance only. Therefore teacher.evaluate works and teacher_1.evaluate throws an error.