1) eval , #{} and their difference.
eval is used to evaluate the string as expression.
#{} is used to delimit the expressions in a double-quoted string.
At first they both look same however there is a difference between both. Let me explain this with the help of an example.
Suppose we have a variableexp = "2*4"Now the output of eval(exp) will be 8Where as output of "#{exp}" will be 2*4
2) instance_eval, module_eval and class_eval
instance_eval can be used to access the instance variable of an object.
For example:-
class Test
def initialize
@variable = "hello"
end
end
obj = Test.new
obj.instance_eval do
puts @variable # Output-> hello
end
instance_eval can also be used for creating class methodsFor example:-
class Test
end
Test.instance_eval do
def hello
puts "hello world"
end
end
Test.hello # output => hello world
Like instance_eval, module_eval and class_eval are used to access class variable from outside a class.Both module_eval and class_eval operates on modules and class rather than their objects.
For Example:-
class Test
@@variable = "hello"
end
puts Test.class_eval(@@variable) # Output-> hello
module_eval and class_eval are also used to create new instance methods.For Example:-
class Test
end
Test.class_eval do
def hello
puts "hello world"
end
end
obj = Test.new
obj.hello # output => hello world
3) instance_variable_get and instance_variable_set
instance_variable_get, as the name suggests, is used to retrieve the value of any instance variable.
It takes the the variable name as symbol argument
For example:-
class Test
def initialize
@variable = "hello"
end
end
obj = Test.new
obj.instance_variable_get(:@variable) # Output => hello
instance_variable_set, as the name suggests, is used to set the value of any instance variable.It takes a symbol argument as variable name and second argument which is the variable value
For example:-
class Test
def initialize
@variable = "hello"
end
end
obj = Test.new
obj.instance_variable_set(:@variable,"hello world")
4) send methodsend method is used to call a method of the same name as specified as the first argument(which is in symbol format)
For Example:-
@variable = "test"
puts( @variable.send(:reverse)) # this will call the reverse method for that string
You can also pass the any number of arguments after specifying the first argument which is the method name.For Example:-
class Test
def sum (a,b)
return a+b
end
end
obj = Test.new
obj.send(:sum, 4 , 5) # Output =>9
5) define_method This method can be used to define new methods during run time.
For Example:-
class A
define_method("test") do ||
puts "hello"
end
end
b= A.new
b.test #output => hello
Another Example:-Here we are also passing some arguments to the newly created method
class Array define_method(:aNewMethod, lambda{ |*args| puts( args.inspect) } end [].aNewMethod([1,2,3]) #Ouput [1,2,3]6) remove_method
You can also remove the existing methods or method you created by using the remove_method.
For Example:-
puts("hello".reverse)
class String
remove_method(:reverse)
end
puts("hello".reverse) # Output => undefined method error!
Note: - If a method with the same name is defined for an ancestor of that class, the ancestor class method is not removed7) method_missing
There might be the times when you may try to execute a method which is not defined, this will throw an error and cause the program to exit. To handle this situation you can write your own method called method_missing with an argument to which the missing method name is assigned.
class Test
def method_missing (method_name)
puts("#{method_name} do not exist")
end
end
obj = Test.new
obj.new_method # Output = > new_method do not exist
8) Frozen Object
Ruby provides you a facility to freeze an object so that it cant be modified further. The object once frozen cannot be modified, if tried to do so it will raise a TypeError exception.
variable = "Hello"
variable << "world"
variable.freeze
variable << " !!!" # Error: "can't modify frozen string (TypeError)"
Please note once an object is frozen it cant be be undone.